https://wiki.haskell.org/api.php?action=feedcontributions&user=Kwantam&feedformat=atomHaskellWiki - User contributions [en]2019-12-14T15:41:20ZUser contributionsMediaWiki 1.27.4https://wiki.haskell.org/index.php?title=Template_Haskell&diff=39667Template Haskell2011-04-25T19:12:13Z<p>Kwantam: /* Template Haskell tutorials and papers */ added note about Google docs login issue under Bulat's tutorials.</p>
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<div>'''[http://www.haskell.org/th/ Template Haskell]''' is a [[GHC]] extension to Haskell that adds compile-time metaprogramming facilities. The original design can be found here: http://research.microsoft.com/~simonpj/papers/meta-haskell/ . It is [http://haskell.cs.yale.edu/ghc/docs/6.2/html/users_guide/template-haskell.html included] in GHC version 6. <br />
<br />
This page hopes to be a more central and organized repository of TH related things.<br />
<br />
=What is Template Haskell?=<br />
Template Haskell is an extension to Haskell 98 that allows you to do type-safe compile-time meta-programming, with Haskell both as the manipulating language and the language being manipulated. <br />
<br />
Intuitively Template Haskell provides new language features that allow us to convert back and forth between concrete syntax, i.e. what you would type when you write normal Haskell code, and abstract syntax trees. These abstract syntax trees are represented using Haskell datatypes and, at compile time, they can be manipulated by Haskell code. This allows you to reify (convert from concrete syntax to an abstract syntax tree) some code, transform it and splice it back in (convert back again), or even to produce completely new code and splice that in, while the compiler is compiling your module. <br />
<br />
For email about Template Haskell, use the [http://haskell.org/mailman/listinfo/glasgow-haskell-users GHC users mailing list]. It's worth joining if you start to use TH.<br />
<br />
= Template Haskell specification =<br />
<br />
Template Haskell is only documented rather informally at the moment. Here are the main resources:<br />
<br />
* [http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html The user manual section on Template Haskell]<br />
* [http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html#th-quasiquotation The user manual section on quasi-quotation], which is closely related to Template Haskell.<br />
* [http://research.microsoft.com/~simonpj/papers/meta-haskell/ The original Template Haskell paper]<br />
* [http://research.microsoft.com/~simonpj/tmp/notes2.ps Notes on Template Haskell version 2], which describes changes since the original paper. Section 8 describes the difficulty with pattern splices, which are therefore not implemented.<br />
* [http://haskell.org/ghc/docs/7.0-latest/html/libraries/template-haskell-2.5.0.0/Language-Haskell-TH.html The Template Haskell API]<br />
<br />
= Template Haskell tutorials and papers =<br />
<br />
* Bulat's tutorials:<br />
** [http://docs.google.com/uc?id=0B4BgTwf_ng_TM2MxZjJjZjctMTQ0OS00YzcwLWE5N2QtMDI0YzE4NGUwZDM3 [formerly /bz/thdoc.htm]]<br />
** [http://docs.google.com/uc?id=0B4BgTwf_ng_TOGJkZjM4ZTUtNGY5My00ZThhLTllNDQtYzJjMWJiMzJhZjNj [formerly /bz/th3.htm]]<br />
<br />
: One reader said "These docs are *brilliant* ! Exactly what I need to get an understanding of TH."<br />
<br />
<small>(Note: These documents are from [http://www.archive.org the Wayback machine] because the originals disappeared. They're public documents on Google docs, which shouldn't require logging in. However, if you're asked to sign in to view them, you're running into a known Google bug. You can fix it by browsing to [http://www.google.com Google], presumably gaining a cookie in the process.)</small><br />
<br />
* Mark Snyder's Template Haskell chapter on the Software Generation and Configuration Report<br />
** http://nix.cs.uu.nl/dist/courses/sgc-report-unstable-latest/manual/chunk-chapter/templatehaskell.html<br />
<br />
* A very short tutorial to understand the basics in 10 Minutes.<br />
** http://www.hyperedsoftware.com/blog/entries/first-stab-th.html<br />
<br />
* GHC Template Haskell documentation<br />
** http://www.haskell.org/ghc/docs/7.0.2/html/users_guide/template-haskell.html<br />
<br />
* Papers about Template Haskell<br />
<br />
** Template metaprogramming for Haskell, by Tim Sheard and Simon Peyton Jones, Oct 2002. [[http://haskell.org/wikiupload/c/ca/Meta-haskell.ps ps]]<br />
** Template Haskell: A Report From The Field, by Ian Lynagh, May 2003. [[http://haskell.org/wikiupload/2/24/Template_Haskell-A_Report_From_The_Field.ps ps]]<br />
** Unrolling and Simplifying Expressions with Template Haskell, by Ian Lynagh, December 2002. [[http://haskell.org/wikiupload/e/ed/Template-Haskell-Utils.ps ps]]<br />
** Automatic skeletons in Template Haskell, by Kevin Hammond, Jost Berthold and Rita Loogen, June 2003. [[http://haskell.org/wikiupload/6/69/AutoSkelPPL03.pdf pdf]]<br />
** Optimising Embedded DSLs using Template Haskell, by Sean Seefried, Manuel Chakravarty, Gabriele Keller, March 2004. [[http://haskell.org/wikiupload/b/b5/Seefried04th-pan.pdf pdf]]<br />
** Typing Template Haskell: Soft Types, by Ian Lynagh, August 2004. [[http://haskell.org/wikiupload/7/72/Typing_Template_Haskell_Soft_Types.ps ps]]<br />
<br />
= Other useful resources =<br />
<br />
* [http://www.haskell.org/th/ The old Template Haskell web page]. Would someone feel like moving this material into the HaskellWiki?<br />
* Old and probably not too useful for most but maybe... http://www.cse.unsw.edu.au/~chak/haskell/ghc/comm/exts/th.html<br />
*[http://web.comlab.ox.ac.uk/oucl/work/ian.lynagh/Fraskell/ Fraskell documentation] & explanation of how Template Haskell is used to vastly speed it up.<br />
*[[Quasiquotation]]<br />
Feel free to update our Wikipedia entry<br />
http://en.wikipedia.org/wiki/Template_Haskell<br />
<br />
= Projects =<br />
<br />
What are you doing/planning to do/have done with Template Haskell?<br />
<br />
* The [http://www.ict.kth.se/org/ict/ecs/sam/projects/forsyde/www ForSyDe methodology] is currently implemented as a Haskell-based DSL which makes extensive use of Template Haskell.<br />
<br />
* I have written a primitive (untyped) binding to the Objective-C runtime system on Mac OS X. It needs just TH, no "stub files" are created, no seperate utilities are required. Initial snapshot is at http://www.kfunigraz.ac.at/imawww/thaller/wolfgang/HOC020103.tar.bz2 -- WolfgangThaller<br />
<br />
* I am writing Template Greencard - a reimplementation of GreenCard using TH. Many bits work out really nicely. A few bits didn't work so nicely - once I get some time to think, I'll try to persuade the TH folk to make some changes to fix some of these. -- AlastairReid<br />
<br />
* I'm writing Hacanon - a framework for automatic generation of C++ bindings. Read "automated Template Greencard for C++" (-: Darcs repo: http://www.ScannedInAvian.org/repos/hacanon - You'll need gccxml (http://www.gccxml.org/) to compile the exmples. - 27 Dec Lemmih.<br />
<br />
* Following other FFI tools developers, I see some future for Template HSFFIG, especially when it comes to autogenerate peek and poke methods for structures defined in C; may be useful for implementation of certain network protocols such as X11 where layout of messages is provided as C structure/union declaration. - 16 Dec 2005 DimitryGolubovsky<br />
<br />
* I am using Template Haskell as a mechanism to get parsed, typechecked code into an Ajax based Haskell Equational Reasoning tool [[Haskell Equational Reasoning Assistant]], as well as simplify the specification of equational relationships between pieces of code. There was a quicktime movie of the tool being used on http://www.gill-warbington.com/home/andy/share/hera1.html - AndyGill <br />
<br />
* I am working on functional metaprogramming techniques to enhance programming reliability and productivity, by reusing much of the existing compiler technology. Template Haskell is especially interesting for me because it permits to check size information of structures by the compiler, provided this information is available at compile time. This approach is especially appropriate for hardware designs, where the structures are fixed before the circuit starts operating. See our metaprogramming web page at http://www.infosun.fmi.uni-passau.de/cl/metaprog/ -- ChristophHerrmann(http://www.cs.st-and.ac.uk/~ch)<br />
<br />
* I am using Template Haskell to do type safe database access. I initially [http://www.nabble.com/Using-Template-Haskell-to-make-type-safe-database-access-td17027286.html proposed this on haskell-cafe]. I connect to the database at compile-time and let the database do SQL parsing and type inference. The result from parsing and type inference is used to build a type safe database query which can executed at run-time. [[MetaHDBC | You can find the project page here]] -- [mailto:mads_lindstroem@yahoo.dk Mads Lindstrøm]<br />
<br />
= Utilities =<br />
<br />
Helper functions, debugging functions, or more involved code e.g. a monadic fold algebra for TH.Syntax.<br />
<br />
* http://www.haskell.org/pipermail/template-haskell/2003-September/000176.html<br />
<br />
= Known Bugs =<br />
<br />
Take a look at the [http://hackage.haskell.org/trac/ghc/query?status=new&status=assigned&status=reopened&component=Template+Haskell&order=priority open bugs against Template Haskell] on the GHC bug tracker.<br />
<br />
= Wish list =<br />
<br />
Things that Ian Lynagh (Igloo) mentioned in his paper ''Template Haskell: A Report From The Field'' in May 2003 (available [http://www.haskell.org/th/papers.html here]), by section:<br />
<br />
* Section 2 (curses)<br />
** The ability to splice names (into "foreign import" declarations, in particular)<br />
** The ability to add things to the export list from a splice(?)<br />
** The ability to use things defined at the toplevel of a module from splices in that same module (would require multi-stage compilation, as opposed to the current approach of expanding splices during typechecking)<br />
<br />
* Section 3 (deriving instances of classes)<br />
** <strike>First-class reification</strike> (the <hask>reify</hask> function)<br />
** A way to discover whether a data constructor was defined infix or prefix (which is necessary to derive instances for <hask>Read</hask> and <hask>Show</hask> as outlined in [http://www.haskell.org/onlinereport/derived.html The Haskell 98 Report: Specification of Derived Instances]) (if there is a way, [http://www-users.cs.york.ac.uk/~ndm/derive/ Derive] seems ignorant of it)<br />
** Type/context splicing (in <hask>instance</hask> headers in particular)<br />
<br />
* Section 4 (printf)<br />
** He says something to the effect that a pattern-matching form of the quotation brackets would be cool if it was expressive enough to be useful, but that this would be hard. (Don't expect this anytime soon.)<br />
<br />
* Section 5 (fraskell)<br />
** Type information for quoted code (so that simplification can be done safely even with overloaded operations, like, oh, <hask>(+)</hask>)<br />
<br />
* Section 6 (pan)<br />
** Type info again, and strictnes info too (this one seems a bit pie-in-the-sky...)<br />
<br />
(Please leave the implemented ones here, but crossed off.)<br />
<br />
Any other features that may be nice, and TH projects you'd want to see.<br />
<br />
* A TH tutorial (mainly a distillation and update of ''Template Meta-programming in Haskell'' at this point)<br />
* <strike>Write Haddock documentation for the Template Haskell library (http://hackage.haskell.org/trac/ghc/ticket/1576).</strike><br />
* Make `reify` on a class return a list of the instances of that class (http://www.haskell.org/pipermail/template-haskell/2005-December/000503.html). (See also [http://hackage.haskell.org/trac/ghc/ticket/1577 GHC ticket #1577].)<br />
* A set of simple examples on this wiki page<br />
* A TH T-shirt with new logo to wear at conferences<br />
* (Long-term) Unify Language.Haskell.Syntax with Language.Haskell.TH.Syntax so there's just one way to do things (http://hackage.haskell.org/package/haskell-src-meta does a one-way translation, for haskell-src-exts)<br />
<br />
---------------<br />
<br />
= Tips and Tricks =<br />
<br />
== What to do when you can't splice that there ==<br />
<br />
When you try to splice something into the middle of a template and find that you just can't, instead of getting frustrated about it, why not use the template to see what it would look like in longhand? <br />
<br />
First, an excerpt from a module of my own. I, by the way, am SamB.<br />
<haskell><br />
{-# OPTIONS_GHC -fglasgow-exts -fth #-}<br />
<br />
module MMixMemory where<br />
<br />
import Data.Int<br />
import Data.Word<br />
<br />
class (Integral int, Integral word)<br />
=> SignConversion int word | int -> word, word -> int where<br />
<br />
toSigned :: word -> int<br />
toSigned = fromIntegral<br />
toUnsigned :: int -> word<br />
toUnsigned = fromIntegral<br />
<br />
</haskell><br />
<br />
Say I want to find out what I need to do to splice in the types for an instance declaration for the SignConversion class, so that I can declare instances for Int8 with Word8 through Int64 with Word64. So, I start up good-ol' GHCi and do the following:<br />
<br />
<haskell><br />
$ ghci -fth -fglasgow-exts<br />
Prelude> :l MMixMemory<br />
*MMixMemory> :m +Language.Haskell.TH.Syntax<br />
*MMixMemory Language.Haskell.TH.Syntax> runQ [d| instance SignConversion Int Word where |] >>= print<br />
[InstanceD [] (AppT (AppT (ConT MMixMemory.SignConversion) (ConT GHC.Base.Int)) (ConT GHC.Word.Word)) []]<br />
</haskell><br />
<br />
== Why does <tt>runQ</tt> crash if I try to reify something? ==<br />
<br />
This program will fail with an error message when you run it:<br />
<haskell><br />
main = do info <- runQ (reify (mkName "Bool")) -- more hygenic is: (reify ''Bool)<br />
putStrLn (pprint info)<br />
</haskell><br />
Reason: <tt>reify</tt> consults the type environment, and that is not available at run-time. The type of <tt>reify</tt> is <br />
<haskell><br />
reify :: Quasi m => Q a -> m a<br />
</haskell><br />
The IO monad is a poor-man's instance of <tt>Quasi</tt>; it can allocate unique names and gather error messages, but it can't do <tt>reify</tt>. This error should really be caught statically.<br />
<br />
Instead, you can run the splice directly (ex. in ghci -XTemplateHaskell), as the following shows:<br />
<br />
<haskell><br />
GHCi> let tup = $(tupE $ take 4 $ cycle [ [| "hi" |] , [| 5 |] ])<br />
GHCi> :type tup<br />
tup :: ([Char], Integer, [Char], Integer)<br />
<br />
GHCi> tup<br />
("hi",5,"hi",5)<br />
<br />
GHCi> $(stringE . show =<< reify ''Int)<br />
"TyConI (DataD [] GHC.Types.Int [] [NormalC GHC.Types.I# [(NotStrict,ConT GHC.Prim.Int#)]] [])"<br />
</haskell><br />
<br />
Here's an [http://www.haskell.org/pipermail/glasgow-haskell-users/2006-August/010844.html email thread with more details].<br />
<br />
-----------------<br />
= Examples =<br />
== Tuples ==<br />
=== Select from a tuple ===<br />
<br />
An example to select an element from a tuple of arbitrary size. Taken from [http://www.haskell.org/th/papers/meta-haskell.ps this paper].<br />
<br />
Use like so:<br />
<br />
> $(sel 2 3) ('a','b','c')<br />
'b'<br />
> $(sel 3 4) ('a','b','c','d')<br />
'c'<br />
<br />
<br />
<haskell><br />
sel :: Int -> Int -> ExpQ<br />
sel i n = [| \x -> $(caseE [| x |] [alt]) |]<br />
where alt :: MatchQ<br />
alt = match pat (normalB rhs) []<br />
<br />
pat :: Pat<br />
pat = tupP (map varP as)<br />
<br />
rhs :: ExpQ<br />
rhs = varE(as !! (i -1)) -- !! is 0 based<br />
<br />
as :: [String]<br />
as = ["a" ++ show i | i <- [1..n] ]<br />
</haskell><br />
<br />
Alternately:<br />
<br />
<haskell><br />
sel' i n = lamE [pat] rhs<br />
where pat = tupP (map varP as)<br />
rhs = varE (as !! (i - 1))<br />
as = [ "a" ++ show j | j <- [1..n] ]<br />
</haskell><br />
<br />
=== Apply a function to the n'th element ===<br />
<br />
<haskell><br />
tmap i n = do<br />
f <- newName "f"<br />
as <- replicateM n (newName "a")<br />
lamE [varP f, tupP (map varP as)] $<br />
tupE [ if i == i'<br />
then [| $(varE f) $a |]<br />
else a<br />
| (a,i') <- map varE as `zip` [1..] ]<br />
</haskell><br />
<br />
Then tmap can be called as:<br />
<br />
> $(tmap 3 4) (+ 1) (1,2,3,4)<br />
(1,2,4,4)<br />
<br />
=== Convert the first n elements of a list to a tuple ===<br />
<br />
This example creates a tuple by extracting elemnts from a list. Taken from<br />
[http://www.xoltar.org/2003/aug/13/templateHaskellTupleSample.html www.xoltar.org]<br />
<br />
Use like so:<br />
<br />
> $(tuple 3) [1,2,3,4,5]<br />
(1,2,3)<br />
> $(tuple 2) [1,2]<br />
(1,2)<br />
<br />
<haskell><br />
tuple :: Int -> ExpQ<br />
tuple n = [|\list -> $(tupE (exprs [|list|])) |]<br />
where<br />
exprs list = [infixE (Just (list))<br />
(varE "!!")<br />
(Just (litE $ integerL (toInteger num)))<br />
| num <- [0..(n - 1)]]<br />
</haskell><br />
<br />
An alternative that has more informative errors (a failing pattern match failures give an exact location):<br />
<br />
<haskell><br />
tuple :: Int -> ExpQ<br />
tuple n = do<br />
ns <- replicateM n (newName "x")<br />
lamE [foldr (\x y -> conP '(:) [varP x,y]) wildP ns] (tupE $ map varE ns)<br />
</haskell><br />
<br />
=== Un-nest tuples ===<br />
Convert nested tuples like (a,(b,(c,()))) into (a,b,c) given the length to generate.<br />
<br />
<haskell><br />
unNest n = do<br />
vs <- replicateM n (newName "x")<br />
lamE [foldr (\a b -> tupP [varP a , b])<br />
(conP '() [])<br />
vs]<br />
(tupE (map varE vs))<br />
</haskell><br />
<br />
<br />
<br />
== [[Template Haskell/Marshall Data|Marshall a datatype to and from Dynamic]] ==<br />
This approach is an example of using template haskell to delay typechecking<br />
to be able to abstract out the repeated calls to fromDynamic:<br />
<br />
<haskell><br />
data T = T Int String Double<br />
<br />
toT :: [Dynamic] -> Maybe T<br />
toT [a,b,c] = do<br />
a' <- fromDynamic a<br />
b' <- fromDynamic b<br />
c' <- fromDynamic c<br />
return (T a' b' c')<br />
toT _ = Nothing<br />
</haskell><br />
<br />
== Printf ==<br />
This example taken from: http://haskell.cs.yale.edu/ghc/docs/6.0/html/users_guide/template-haskell.html<br />
<br />
Build it using a command similar to one of the following (depending on your environment):<br />
<br />
ghc/compiler/stage3/ghc-inplace --make -fglasgow-exts -package haskell-src main.hs -o main.exe<br />
ghc --make -fth Main.hs -o printfTest<br />
<br />
Main.hs:<br />
<br />
<haskell><br />
module Main where<br />
<br />
-- Import our template "pr"<br />
import Printf ( pr )<br />
<br />
-- The splice operator $ takes the Haskell source code<br />
-- generated at compile time by "pr" and splices it into<br />
-- the argument of "putStrLn".<br />
main = putStrLn ( $(pr "Hello") )<br />
</haskell><br />
<br />
Printf.hs:<br />
<br />
<haskell><br />
module Printf where<br />
<br />
-- Skeletal printf from the paper.<br />
-- It needs to be in a separate module to the one where<br />
-- you intend to use it.<br />
<br />
-- Import some Template Haskell syntax<br />
import Language.Haskell.TH.Syntax<br />
<br />
-- Describe a format string<br />
data Format = D | S | L String<br />
<br />
-- Parse a format string. This is left largely to you<br />
-- as we are here interested in building our first ever<br />
-- Template Haskell program and not in building printf.<br />
parse :: String -> [Format]<br />
parse s = [ L s ]<br />
<br />
-- Generate Haskell source code from a parsed representation<br />
-- of the format string. This code will be spliced into<br />
-- the module which calls "pr", at compile time.<br />
gen :: [Format] -> ExpQ<br />
gen [D] = [| \n -> show n |]<br />
gen [S] = [| \s -> s |]<br />
gen [L s] = stringE s<br />
<br />
-- Here we generate the Haskell code for the splice<br />
-- from an input format string.<br />
pr :: String -> ExpQ<br />
pr s = gen (parse s)<br />
</haskell><br />
<br />
== Handling Options with Templates ==<br />
A common idiom for treating a set of options, e.g. from GetOpt, is to define a datatype with all the flags and using a list over this datatype:<br />
<br />
<haskell><br />
data Options = B1 | B2 | V Integer<br />
<br />
options = [B1, V 3]<br />
</haskell><br />
<br />
While it's simple testing if a Boolean flag is set (simply use "elem"), it's harder to check if an option with an argument is set. It's even more tedious writing helper-functions to obtain the value from such an option since you have to explicitely "un-V" each. Here, Template Haskell can be (ab)used to reduce this a bit. The following example provides the module "OptionsTH" which can be reused regardless of the constructors in "Options". Let's start with showing how we'd like to be able to program. Notice that the resulting lists need some more treatment e.g. through "foldl".<br />
<br />
Options.hs:<br />
<br />
<haskell><br />
module Main where<br />
<br />
import OptionsTH<br />
import Language.Haskell.TH.Syntax<br />
<br />
data Options = B1 | B2 | V Int | S String deriving (Eq, Read, Show)<br />
<br />
options = [B1, V 3]<br />
<br />
main = do<br />
print foo -- test if B1 set: [True,False]<br />
print bar -- test if V present, w/o value: [False,True]<br />
print baz -- get value of V if available: [Nothing,Just 3]<br />
<br />
foo :: [Bool]<br />
-- Query constructor B1 which takes no arguments<br />
foo = map $(getopt (THNoArg (mkArg "B1" 0))) options<br />
<br />
bar :: [Bool]<br />
-- V is a unary constructor. Let mkArg generate the required<br />
-- wildcard-pattern "V _".<br />
bar = map $(getopt (THNoArg (mkArg "V" 1))) options<br />
<br />
-- Can't use a wildcard here!<br />
baz :: [(Maybe Int)]<br />
baz = map $(getopt (THArg (conP "V" [varP "x"]))) options<br />
</haskell><br />
<br />
OptionsTH.hs<br />
<br />
<haskell><br />
module OptionsTH where<br />
<br />
import Language.Haskell.TH.Syntax<br />
<br />
-- datatype for querying options:<br />
-- NoArg: Not interested in value (also applies to Boolean flags)<br />
-- Arg: Grep value of unary(!) constructor<br />
data Args = THNoArg Pat | THArg Pat<br />
<br />
getopt :: Args -> ExpQ<br />
getopt (THNoArg pat) = lamE [varP "y"] (caseE (varE "y") [cons0, cons1])<br />
where<br />
cons0 = match pat (normalB [| True |]) []<br />
cons1 = match wildP (normalB [| False |]) []<br />
<br />
-- bind "var" for later use!<br />
getopt (THArg pat@(ConP _ [VarP var])) = lamE [varP "y"] (caseE (varE "y") [cons0, cons1])<br />
where<br />
cons0 = match pat (normalB (appE [|Just|] (varE var))) []<br />
cons1 = match wildP (normalB [|Nothing|]) []<br />
<br />
mkArg :: String -> Int -> Pat<br />
mkArg k c = conP k (replicate c wildP)<br />
</haskell><br />
<br />
While the example might look contrived for the Boolean options which could have been tested much easier, it shows how both types of arguments can be treated in a similar way.<br />
<br />
=== Limitations ===<br />
<tt>getopt (THArg pat)</tt> is only able to treat unary constructors. See the pattern-binding: It matches exactly a single VarP.<br />
<br />
=== Improvements ===<br />
The following reduces things even a bit more, though I still don't know if I like it. It only works since <tt>c</tt> is either 0 or 1.<br />
<br />
<haskell><br />
mkArg k c = conP k (replicate c (varP "x"))<br />
<br />
baz = map $(getopt (THArg (mkArg "V" 1)))<br />
</haskell><br />
-- VolkerStolz<br />
<br />
== Generic constructor for records ==<br />
<br />
I have a large number of record types like this, of different length:<br />
<br />
<haskell><br />
data PGD = PGD {<br />
pgdXUnitBase :: !Word8,<br />
pgdYUnitBase :: !Word8,<br />
pgdXLUnitsperUnitBase :: !Word16<br />
}<br />
</haskell><br />
<br />
Currently I use GHC's Binary module to read them from files; it can handle<br />
types like <tt>(Word8, (Word8, Word16))</tt>, but there was no easy way to generate<br />
the correct amount of "uncurry" calls for automatically grabbing each element.<br />
<br />
With Template Haskell, the instance declarations are now written as such:<br />
<br />
<haskell><br />
instance Binary PGD where<br />
get bh = do a <- get bh ; return $ $(constrRecord PGD) a<br />
</haskell><br />
<br />
Here the trick lies in constrRecord, which is defined as:<br />
<br />
<haskell><br />
constrRecord x = reify exp where<br />
reify = \(Just r) -> appE r $ conE $ last args<br />
exp = foldl (dot) uncur $ replicate terms uncur<br />
terms = ((length args) `div` 2) - 2<br />
dot x y = (Just $ infixE x (varE ".") y)<br />
uncur = (Just [|uncurry|])<br />
args = words . show $ typeOf x<br />
</haskell><br />
<br />
-- AutrijusTang<br />
<br />
== 'generic' zipWith ==<br />
A generalization of zipWith to almost any data. Demonstrates the ability to do dynamic binding with TH splices (note 'dyn').<br />
<br />
<haskell><br />
zipCons :: Name -> Int -> [String] -> ExpQ<br />
zipCons tyName ways functions = do<br />
let countFields :: Con -> (Name,Int)<br />
countFields x = case x of<br />
NormalC n (length -> fields) -> (n, fields)<br />
RecC n (length -> fields) -> (n,fields)<br />
InfixC _ n _ -> (n,2)<br />
ForallC _ _ ct -> countFields ct<br />
<br />
TyConI (DataD _ _ _ [countFields -> (c,n)] _) <- reify tyName<br />
when (n /= length functions) $ fail "wrong number of functions named"<br />
vs <- replicateM ways $ replicateM n $ newName "x"<br />
lamE (map (conP c . map varP) vs) $<br />
foldl (\con (vs,f) -><br />
con `appE`<br />
foldl appE<br />
(dyn f)<br />
(map varE vs))<br />
(conE c)<br />
(transpose vs `zip` functions)<br />
</haskell><br />
<br />
This example uses whichever '+' is in scope when the expression is spliced:<br />
<br />
<haskell><br />
:type $(zipCons ''(,,,) 2 (replicate 4 "+"))<br />
<br />
$(zipCons ''(,,,) 2 (replicate 4 "+"))<br />
:: (Num t, Num t1, Num t2, Num t3) =><br />
(t, t1, t2, t3) -> (t, t1, t2, t3) -> (t, t1, t2, t3)<br />
</haskell><br />
<br />
<br />
==[[Template haskell/Instance deriving example|Instance deriving example]]==<br />
An example using a 'deriving function' to generate a method instance <br />
per constructor of a type. The deriving function provides the body of the<br />
method.<br />
<br />
Note that this example assumes that the functions of the class take a parameter that is the same type as instance is parameterized with. <br />
<br />
The message [http://www.haskell.org/pipermail/template-haskell/2006-August/000581.html email message] contains the full source ([http://www.iist.unu.edu/~vs/haskell/TH_render.hs extracted file]).<br />
<br />
== [[Quasiquotation|QuasiQuoters]] ==<br />
New in ghc-6.10 is -XQuasiQuotes, which allows one to extend ghc's syntax from library code. Quite a few examples are given in [http://hackage.haskell.org/package/haskell-src-meta haskell-src-meta].<br />
<br />
=== Similarity with splices ===<br />
<br />
Quasiquoters used in expression contexts (those using the ''quoteExp'') behave to a first approximation like regular TH splices:<br />
<br />
<haskell><br />
simpleQQ = QuasiQuoter { quoteExp = stringE } -- in another module<br />
<br />
[$simpleQQ| a b c d |] == $(quoteExp simpleQQ " a b c d ")<br />
</haskell><br />
<br />
[[Category:Language extensions]]</div>Kwantamhttps://wiki.haskell.org/index.php?title=Template_Haskell&diff=39650Template Haskell2011-04-22T19:12:32Z<p>Kwantam: /* Template Haskell tutorials and papers */ re-hosted the Bulat tutorials on Google docs</p>
<hr />
<div>'''[http://www.haskell.org/th/ Template Haskell]''' is a [[GHC]] extension to Haskell that adds compile-time metaprogramming facilities. The original design can be found here: http://research.microsoft.com/~simonpj/papers/meta-haskell/ . It is [http://haskell.cs.yale.edu/ghc/docs/6.2/html/users_guide/template-haskell.html included] in GHC version 6. <br />
<br />
This page hopes to be a more central and organized repository of TH related things.<br />
<br />
=What is Template Haskell?=<br />
Template Haskell is an extension to Haskell 98 that allows you to do type-safe compile-time meta-programming, with Haskell both as the manipulating language and the language being manipulated. <br />
<br />
Intuitively Template Haskell provides new language features that allow us to convert back and forth between concrete syntax, i.e. what you would type when you write normal Haskell code, and abstract syntax trees. These abstract syntax trees are represented using Haskell datatypes and, at compile time, they can be manipulated by Haskell code. This allows you to reify (convert from concrete syntax to an abstract syntax tree) some code, transform it and splice it back in (convert back again), or even to produce completely new code and splice that in, while the compiler is compiling your module. <br />
<br />
For email about Template Haskell, use the [http://haskell.org/mailman/listinfo/glasgow-haskell-users GHC users mailing list]. It's worth joining if you start to use TH.<br />
<br />
= Template Haskell specification =<br />
<br />
Template Haskell is only documented rather informally at the moment. Here are the main resources:<br />
<br />
* [http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html The user manual section on Template Haskell]<br />
* [http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html#th-quasiquotation The user manual section on quasi-quotation], which is closely related to Template Haskell.<br />
* [http://research.microsoft.com/~simonpj/papers/meta-haskell/ The original Template Haskell paper]<br />
* [http://research.microsoft.com/~simonpj/tmp/notes2.ps Notes on Template Haskell version 2], which describes changes since the original paper. Section 8 describes the difficulty with pattern splices, which are therefore not implemented.<br />
* [http://haskell.org/ghc/docs/7.0-latest/html/libraries/template-haskell-2.5.0.0/Language-Haskell-TH.html The Template Haskell API]<br />
<br />
= Template Haskell tutorials and papers =<br />
<br />
* Bulat's tutorials:<br />
** [http://docs.google.com/uc?id=0B4BgTwf_ng_TM2MxZjJjZjctMTQ0OS00YzcwLWE5N2QtMDI0YzE4NGUwZDM3 [formerly /bz/thdoc.htm]]<br />
** [http://docs.google.com/uc?id=0B4BgTwf_ng_TOGJkZjM4ZTUtNGY5My00ZThhLTllNDQtYzJjMWJiMzJhZjNj [formerly /bz/th3.htm]]<br />
<br />
: One reader said "These docs are *brilliant* ! Exactly what I need to get an understanding of TH."<br />
<br />
* Mark Snyder's Template Haskell chapter on the Software Generation and Configuration Report<br />
** http://nix.cs.uu.nl/dist/courses/sgc-report-unstable-latest/manual/chunk-chapter/templatehaskell.html<br />
<br />
* A very short tutorial to understand the basics in 10 Minutes.<br />
** http://www.hyperedsoftware.com/blog/entries/first-stab-th.html<br />
<br />
* GHC Template Haskell documentation<br />
** http://www.haskell.org/ghc/docs/7.0.2/html/users_guide/template-haskell.html<br />
<br />
* Papers about Template Haskell<br />
<br />
** Template metaprogramming for Haskell, by Tim Sheard and Simon Peyton Jones, Oct 2002. [http://haskell.org/wikiupload/c/ca/Meta-haskell.ps [ps]] <br />
** Template Haskell: A Report From The Field, by Ian Lynagh, May 2003. [http://haskell.org/wikiupload/2/24/Template_Haskell-A_Report_From_The_Field.ps [ps]]<br />
** Unrolling and Simplifying Expressions with Template Haskell, by Ian Lynagh, December 2002. [http://haskell.org/wikiupload/e/ed/Template-Haskell-Utils.ps [ps]]<br />
** Automatic skeletons in Template Haskell, by Kevin Hammond, Jost Berthold and Rita Loogen, June 2003. [http://haskell.org/wikiupload/6/69/AutoSkelPPL03.pdf [pdf]]<br />
** Optimising Embedded DSLs using Template Haskell, by Sean Seefried, Manuel Chakravarty, Gabriele Keller, March 2004. [http://haskell.org/wikiupload/b/b5/Seefried04th-pan.pdf [pdf]]<br />
** Typing Template Haskell: Soft Types, by Ian Lynagh, August 2004. [http://haskell.org/wikiupload/7/72/Typing_Template_Haskell_Soft_Types.ps [ps]]<br />
<br />
= Other useful resources =<br />
<br />
* [http://www.haskell.org/th/ The old Template Haskell web page]. Would someone feel like moving this material into the HaskellWiki?<br />
* Old and probably not too useful for most but maybe... http://www.cse.unsw.edu.au/~chak/haskell/ghc/comm/exts/th.html<br />
*[http://web.comlab.ox.ac.uk/oucl/work/ian.lynagh/Fraskell/ Fraskell documentation] & explanation of how Template Haskell is used to vastly speed it up.<br />
*[[Quasiquotation]]<br />
Feel free to update our Wikipedia entry<br />
http://en.wikipedia.org/wiki/Template_Haskell<br />
<br />
= Projects =<br />
<br />
What are you doing/planning to do/have done with Template Haskell?<br />
<br />
* The [http://www.ict.kth.se/org/ict/ecs/sam/projects/forsyde/www ForSyDe methodology] is currently implemented as a Haskell-based DSL which makes extensive use of Template Haskell.<br />
<br />
* I have written a primitive (untyped) binding to the Objective-C runtime system on Mac OS X. It needs just TH, no "stub files" are created, no seperate utilities are required. Initial snapshot is at http://www.kfunigraz.ac.at/imawww/thaller/wolfgang/HOC020103.tar.bz2 -- WolfgangThaller<br />
<br />
* I am writing Template Greencard - a reimplementation of GreenCard using TH. Many bits work out really nicely. A few bits didn't work so nicely - once I get some time to think, I'll try to persuade the TH folk to make some changes to fix some of these. -- AlastairReid<br />
<br />
* I'm writing Hacanon - a framework for automatic generation of C++ bindings. Read "automated Template Greencard for C++" (-: Darcs repo: http://www.ScannedInAvian.org/repos/hacanon - You'll need gccxml (http://www.gccxml.org/) to compile the exmples. - 27 Dec Lemmih.<br />
<br />
* Following other FFI tools developers, I see some future for Template HSFFIG, especially when it comes to autogenerate peek and poke methods for structures defined in C; may be useful for implementation of certain network protocols such as X11 where layout of messages is provided as C structure/union declaration. - 16 Dec 2005 DimitryGolubovsky<br />
<br />
* I am using Template Haskell as a mechanism to get parsed, typechecked code into an Ajax based Haskell Equational Reasoning tool [[Haskell Equational Reasoning Assistant]], as well as simplify the specification of equational relationships between pieces of code. There was a quicktime movie of the tool being used on http://www.gill-warbington.com/home/andy/share/hera1.html - AndyGill <br />
<br />
* I am working on functional metaprogramming techniques to enhance programming reliability and productivity, by reusing much of the existing compiler technology. Template Haskell is especially interesting for me because it permits to check size information of structures by the compiler, provided this information is available at compile time. This approach is especially appropriate for hardware designs, where the structures are fixed before the circuit starts operating. See our metaprogramming web page at http://www.infosun.fmi.uni-passau.de/cl/metaprog/ -- ChristophHerrmann(http://www.cs.st-and.ac.uk/~ch)<br />
<br />
* I am using Template Haskell to do type safe database access. I initially [http://www.nabble.com/Using-Template-Haskell-to-make-type-safe-database-access-td17027286.html proposed this on haskell-cafe]. I connect to the database at compile-time and let the database do SQL parsing and type inference. The result from parsing and type inference is used to build a type safe database query which can executed at run-time. [[MetaHDBC | You can find the project page here]] -- [mailto:mads_lindstroem@yahoo.dk Mads Lindstrøm]<br />
<br />
= Utilities =<br />
<br />
Helper functions, debugging functions, or more involved code e.g. a monadic fold algebra for TH.Syntax.<br />
<br />
* http://www.haskell.org/pipermail/template-haskell/2003-September/000176.html<br />
<br />
= Known Bugs =<br />
<br />
Take a look at the [http://hackage.haskell.org/trac/ghc/query?status=new&status=assigned&status=reopened&component=Template+Haskell&order=priority open bugs against Template Haskell] on the GHC bug tracker.<br />
<br />
= Wish list =<br />
<br />
Things that Ian Lynagh (Igloo) mentioned in his paper ''Template Haskell: A Report From The Field'' in May 2003 (available [http://www.haskell.org/th/papers.html here]), by section:<br />
<br />
* Section 2 (curses)<br />
** The ability to splice names (into "foreign import" declarations, in particular)<br />
** The ability to add things to the export list from a splice(?)<br />
** The ability to use things defined at the toplevel of a module from splices in that same module (would require multi-stage compilation, as opposed to the current approach of expanding splices during typechecking)<br />
<br />
* Section 3 (deriving instances of classes)<br />
** <strike>First-class reification</strike> (the <hask>reify</hask> function)<br />
** A way to discover whether a data constructor was defined infix or prefix (which is necessary to derive instances for <hask>Read</hask> and <hask>Show</hask> as outlined in [http://www.haskell.org/onlinereport/derived.html The Haskell 98 Report: Specification of Derived Instances]) (if there is a way, [http://www-users.cs.york.ac.uk/~ndm/derive/ Derive] seems ignorant of it)<br />
** Type/context splicing (in <hask>instance</hask> headers in particular)<br />
<br />
* Section 4 (printf)<br />
** He says something to the effect that a pattern-matching form of the quotation brackets would be cool if it was expressive enough to be useful, but that this would be hard. (Don't expect this anytime soon.)<br />
<br />
* Section 5 (fraskell)<br />
** Type information for quoted code (so that simplification can be done safely even with overloaded operations, like, oh, <hask>(+)</hask>)<br />
<br />
* Section 6 (pan)<br />
** Type info again, and strictnes info too (this one seems a bit pie-in-the-sky...)<br />
<br />
(Please leave the implemented ones here, but crossed off.)<br />
<br />
Any other features that may be nice, and TH projects you'd want to see.<br />
<br />
* A TH tutorial (mainly a distillation and update of ''Template Meta-programming in Haskell'' at this point)<br />
* <strike>Write Haddock documentation for the Template Haskell library (http://hackage.haskell.org/trac/ghc/ticket/1576).</strike><br />
* Make `reify` on a class return a list of the instances of that class (http://www.haskell.org/pipermail/template-haskell/2005-December/000503.html). (See also [http://hackage.haskell.org/trac/ghc/ticket/1577 GHC ticket #1577].)<br />
* A set of simple examples on this wiki page<br />
* A TH T-shirt with new logo to wear at conferences<br />
* (Long-term) Unify Language.Haskell.Syntax with Language.Haskell.TH.Syntax so there's just one way to do things (http://hackage.haskell.org/package/haskell-src-meta does a one-way translation, for haskell-src-exts)<br />
<br />
---------------<br />
<br />
= Tips and Tricks =<br />
<br />
== What to do when you can't splice that there ==<br />
<br />
When you try to splice something into the middle of a template and find that you just can't, instead of getting frustrated about it, why not use the template to see what it would look like in longhand? <br />
<br />
First, an excerpt from a module of my own. I, by the way, am SamB.<br />
<haskell><br />
{-# OPTIONS_GHC -fglasgow-exts -fth #-}<br />
<br />
module MMixMemory where<br />
<br />
import Data.Int<br />
import Data.Word<br />
<br />
class (Integral int, Integral word)<br />
=> SignConversion int word | int -> word, word -> int where<br />
<br />
toSigned :: word -> int<br />
toSigned = fromIntegral<br />
toUnsigned :: int -> word<br />
toUnsigned = fromIntegral<br />
<br />
</haskell><br />
<br />
Say I want to find out what I need to do to splice in the types for an instance declaration for the SignConversion class, so that I can declare instances for Int8 with Word8 through Int64 with Word64. So, I start up good-ol' GHCi and do the following:<br />
<br />
<haskell><br />
$ ghci -fth -fglasgow-exts<br />
Prelude> :l MMixMemory<br />
*MMixMemory> :m +Language.Haskell.TH.Syntax<br />
*MMixMemory Language.Haskell.TH.Syntax> runQ [d| instance SignConversion Int Word where |] >>= print<br />
[InstanceD [] (AppT (AppT (ConT MMixMemory.SignConversion) (ConT GHC.Base.Int)) (ConT GHC.Word.Word)) []]<br />
</haskell><br />
<br />
== Why does <tt>runQ</tt> crash if I try to reify something? ==<br />
<br />
This program will fail with an error message when you run it:<br />
<haskell><br />
main = do info <- runQ (reify (mkName "Bool")) -- more hygenic is: (reify ''Bool)<br />
putStrLn (pprint info)<br />
</haskell><br />
Reason: <tt>reify</tt> consults the type environment, and that is not available at run-time. The type of <tt>reify</tt> is <br />
<haskell><br />
reify :: Quasi m => Q a -> m a<br />
</haskell><br />
The IO monad is a poor-man's instance of <tt>Quasi</tt>; it can allocate unique names and gather error messages, but it can't do <tt>reify</tt>. This error should really be caught statically.<br />
<br />
Instead, you can run the splice directly (ex. in ghci -XTemplateHaskell), as the following shows:<br />
<br />
<haskell><br />
GHCi> let tup = $(tupE $ take 4 $ cycle [ [| "hi" |] , [| 5 |] ])<br />
GHCi> :type tup<br />
tup :: ([Char], Integer, [Char], Integer)<br />
<br />
GHCi> tup<br />
("hi",5,"hi",5)<br />
<br />
GHCi> $(stringE . show =<< reify ''Int)<br />
"TyConI (DataD [] GHC.Types.Int [] [NormalC GHC.Types.I# [(NotStrict,ConT GHC.Prim.Int#)]] [])"<br />
</haskell><br />
<br />
Here's an [http://www.haskell.org/pipermail/glasgow-haskell-users/2006-August/010844.html email thread with more details].<br />
<br />
-----------------<br />
= Examples =<br />
== Tuples ==<br />
=== Select from a tuple ===<br />
<br />
An example to select an element from a tuple of arbitrary size. Taken from [http://www.haskell.org/th/papers/meta-haskell.ps this paper].<br />
<br />
Use like so:<br />
<br />
> $(sel 2 3) ('a','b','c')<br />
'b'<br />
> $(sel 3 4) ('a','b','c','d')<br />
'c'<br />
<br />
<br />
<haskell><br />
sel :: Int -> Int -> ExpQ<br />
sel i n = [| \x -> $(caseE [| x |] [alt]) |]<br />
where alt :: MatchQ<br />
alt = match pat (normalB rhs) []<br />
<br />
pat :: Pat<br />
pat = tupP (map varP as)<br />
<br />
rhs :: ExpQ<br />
rhs = varE(as !! (i -1)) -- !! is 0 based<br />
<br />
as :: [String]<br />
as = ["a" ++ show i | i <- [1..n] ]<br />
</haskell><br />
<br />
Alternately:<br />
<br />
<haskell><br />
sel' i n = lamE [pat] rhs<br />
where pat = tupP (map varP as)<br />
rhs = varE (as !! (i - 1))<br />
as = [ "a" ++ show j | j <- [1..n] ]<br />
</haskell><br />
<br />
=== Apply a function to the n'th element ===<br />
<br />
<haskell><br />
tmap i n = do<br />
f <- newName "f"<br />
as <- replicateM n (newName "a")<br />
lamE [varP f, tupP (map varP as)] $<br />
tupE [ if i == i'<br />
then [| $(varE f) $a |]<br />
else a<br />
| (a,i') <- map varE as `zip` [1..] ]<br />
</haskell><br />
<br />
Then tmap can be called as:<br />
<br />
> $(tmap 3 4) (+ 1) (1,2,3,4)<br />
(1,2,4,4)<br />
<br />
=== Convert the first n elements of a list to a tuple ===<br />
<br />
This example creates a tuple by extracting elemnts from a list. Taken from<br />
[http://www.xoltar.org/2003/aug/13/templateHaskellTupleSample.html www.xoltar.org]<br />
<br />
Use like so:<br />
<br />
> $(tuple 3) [1,2,3,4,5]<br />
(1,2,3)<br />
> $(tuple 2) [1,2]<br />
(1,2)<br />
<br />
<haskell><br />
tuple :: Int -> ExpQ<br />
tuple n = [|\list -> $(tupE (exprs [|list|])) |]<br />
where<br />
exprs list = [infixE (Just (list))<br />
(varE "!!")<br />
(Just (litE $ integerL (toInteger num)))<br />
| num <- [0..(n - 1)]]<br />
</haskell><br />
<br />
An alternative that has more informative errors (a failing pattern match failures give an exact location):<br />
<br />
<haskell><br />
tuple :: Int -> ExpQ<br />
tuple n = do<br />
ns <- replicateM n (newName "x")<br />
lamE [foldr (\x y -> conP '(:) [varP x,y]) wildP ns] (tupE $ map varE ns)<br />
</haskell><br />
<br />
=== Un-nest tuples ===<br />
Convert nested tuples like (a,(b,(c,()))) into (a,b,c) given the length to generate.<br />
<br />
<haskell><br />
unNest n = do<br />
vs <- replicateM n (newName "x")<br />
lamE [foldr (\a b -> tupP [varP a , b])<br />
(conP '() [])<br />
vs]<br />
(tupE (map varE vs))<br />
</haskell><br />
<br />
<br />
<br />
== [[Template Haskell/Marshall Data|Marshall a datatype to and from Dynamic]] ==<br />
This approach is an example of using template haskell to delay typechecking<br />
to be able to abstract out the repeated calls to fromDynamic:<br />
<br />
<haskell><br />
data T = T Int String Double<br />
<br />
toT :: [Dynamic] -> Maybe T<br />
toT [a,b,c] = do<br />
a' <- fromDynamic a<br />
b' <- fromDynamic b<br />
c' <- fromDynamic c<br />
return (T a' b' c')<br />
toT _ = Nothing<br />
</haskell><br />
<br />
== Printf ==<br />
This example taken from: http://haskell.cs.yale.edu/ghc/docs/6.0/html/users_guide/template-haskell.html<br />
<br />
Build it using a command similar to one of the following (depending on your environment):<br />
<br />
ghc/compiler/stage3/ghc-inplace --make -fglasgow-exts -package haskell-src main.hs -o main.exe<br />
ghc --make -fth Main.hs -o printfTest<br />
<br />
Main.hs:<br />
<br />
<haskell><br />
module Main where<br />
<br />
-- Import our template "pr"<br />
import Printf ( pr )<br />
<br />
-- The splice operator $ takes the Haskell source code<br />
-- generated at compile time by "pr" and splices it into<br />
-- the argument of "putStrLn".<br />
main = putStrLn ( $(pr "Hello") )<br />
</haskell><br />
<br />
Printf.hs:<br />
<br />
<haskell><br />
module Printf where<br />
<br />
-- Skeletal printf from the paper.<br />
-- It needs to be in a separate module to the one where<br />
-- you intend to use it.<br />
<br />
-- Import some Template Haskell syntax<br />
import Language.Haskell.TH.Syntax<br />
<br />
-- Describe a format string<br />
data Format = D | S | L String<br />
<br />
-- Parse a format string. This is left largely to you<br />
-- as we are here interested in building our first ever<br />
-- Template Haskell program and not in building printf.<br />
parse :: String -> [Format]<br />
parse s = [ L s ]<br />
<br />
-- Generate Haskell source code from a parsed representation<br />
-- of the format string. This code will be spliced into<br />
-- the module which calls "pr", at compile time.<br />
gen :: [Format] -> ExpQ<br />
gen [D] = [| \n -> show n |]<br />
gen [S] = [| \s -> s |]<br />
gen [L s] = stringE s<br />
<br />
-- Here we generate the Haskell code for the splice<br />
-- from an input format string.<br />
pr :: String -> ExpQ<br />
pr s = gen (parse s)<br />
</haskell><br />
<br />
== Handling Options with Templates ==<br />
A common idiom for treating a set of options, e.g. from GetOpt, is to define a datatype with all the flags and using a list over this datatype:<br />
<br />
<haskell><br />
data Options = B1 | B2 | V Integer<br />
<br />
options = [B1, V 3]<br />
</haskell><br />
<br />
While it's simple testing if a Boolean flag is set (simply use "elem"), it's harder to check if an option with an argument is set. It's even more tedious writing helper-functions to obtain the value from such an option since you have to explicitely "un-V" each. Here, Template Haskell can be (ab)used to reduce this a bit. The following example provides the module "OptionsTH" which can be reused regardless of the constructors in "Options". Let's start with showing how we'd like to be able to program. Notice that the resulting lists need some more treatment e.g. through "foldl".<br />
<br />
Options.hs:<br />
<br />
<haskell><br />
module Main where<br />
<br />
import OptionsTH<br />
import Language.Haskell.TH.Syntax<br />
<br />
data Options = B1 | B2 | V Int | S String deriving (Eq, Read, Show)<br />
<br />
options = [B1, V 3]<br />
<br />
main = do<br />
print foo -- test if B1 set: [True,False]<br />
print bar -- test if V present, w/o value: [False,True]<br />
print baz -- get value of V if available: [Nothing,Just 3]<br />
<br />
foo :: [Bool]<br />
-- Query constructor B1 which takes no arguments<br />
foo = map $(getopt (THNoArg (mkArg "B1" 0))) options<br />
<br />
bar :: [Bool]<br />
-- V is a unary constructor. Let mkArg generate the required<br />
-- wildcard-pattern "V _".<br />
bar = map $(getopt (THNoArg (mkArg "V" 1))) options<br />
<br />
-- Can't use a wildcard here!<br />
baz :: [(Maybe Int)]<br />
baz = map $(getopt (THArg (conP "V" [varP "x"]))) options<br />
</haskell><br />
<br />
OptionsTH.hs<br />
<br />
<haskell><br />
module OptionsTH where<br />
<br />
import Language.Haskell.TH.Syntax<br />
<br />
-- datatype for querying options:<br />
-- NoArg: Not interested in value (also applies to Boolean flags)<br />
-- Arg: Grep value of unary(!) constructor<br />
data Args = THNoArg Pat | THArg Pat<br />
<br />
getopt :: Args -> ExpQ<br />
getopt (THNoArg pat) = lamE [varP "y"] (caseE (varE "y") [cons0, cons1])<br />
where<br />
cons0 = match pat (normalB [| True |]) []<br />
cons1 = match wildP (normalB [| False |]) []<br />
<br />
-- bind "var" for later use!<br />
getopt (THArg pat@(ConP _ [VarP var])) = lamE [varP "y"] (caseE (varE "y") [cons0, cons1])<br />
where<br />
cons0 = match pat (normalB (appE [|Just|] (varE var))) []<br />
cons1 = match wildP (normalB [|Nothing|]) []<br />
<br />
mkArg :: String -> Int -> Pat<br />
mkArg k c = conP k (replicate c wildP)<br />
</haskell><br />
<br />
While the example might look contrived for the Boolean options which could have been tested much easier, it shows how both types of arguments can be treated in a similar way.<br />
<br />
=== Limitations ===<br />
<tt>getopt (THArg pat)</tt> is only able to treat unary constructors. See the pattern-binding: It matches exactly a single VarP.<br />
<br />
=== Improvements ===<br />
The following reduces things even a bit more, though I still don't know if I like it. It only works since <tt>c</tt> is either 0 or 1.<br />
<br />
<haskell><br />
mkArg k c = conP k (replicate c (varP "x"))<br />
<br />
baz = map $(getopt (THArg (mkArg "V" 1)))<br />
</haskell><br />
-- VolkerStolz<br />
<br />
== Generic constructor for records ==<br />
<br />
I have a large number of record types like this, of different length:<br />
<br />
<haskell><br />
data PGD = PGD {<br />
pgdXUnitBase :: !Word8,<br />
pgdYUnitBase :: !Word8,<br />
pgdXLUnitsperUnitBase :: !Word16<br />
}<br />
</haskell><br />
<br />
Currently I use GHC's Binary module to read them from files; it can handle<br />
types like <tt>(Word8, (Word8, Word16))</tt>, but there was no easy way to generate<br />
the correct amount of "uncurry" calls for automatically grabbing each element.<br />
<br />
With Template Haskell, the instance declarations are now written as such:<br />
<br />
<haskell><br />
instance Binary PGD where<br />
get bh = do a <- get bh ; return $ $(constrRecord PGD) a<br />
</haskell><br />
<br />
Here the trick lies in constrRecord, which is defined as:<br />
<br />
<haskell><br />
constrRecord x = reify exp where<br />
reify = \(Just r) -> appE r $ conE $ last args<br />
exp = foldl (dot) uncur $ replicate terms uncur<br />
terms = ((length args) `div` 2) - 2<br />
dot x y = (Just $ infixE x (varE ".") y)<br />
uncur = (Just [|uncurry|])<br />
args = words . show $ typeOf x<br />
</haskell><br />
<br />
-- AutrijusTang<br />
<br />
== 'generic' zipWith ==<br />
A generalization of zipWith to almost any data. Demonstrates the ability to do dynamic binding with TH splices (note 'dyn').<br />
<br />
<haskell><br />
zipCons :: Name -> Int -> [String] -> ExpQ<br />
zipCons tyName ways functions = do<br />
let countFields :: Con -> (Name,Int)<br />
countFields x = case x of<br />
NormalC n (length -> fields) -> (n, fields)<br />
RecC n (length -> fields) -> (n,fields)<br />
InfixC _ n _ -> (n,2)<br />
ForallC _ _ ct -> countFields ct<br />
<br />
TyConI (DataD _ _ _ [countFields -> (c,n)] _) <- reify tyName<br />
when (n /= length functions) $ fail "wrong number of functions named"<br />
vs <- replicateM ways $ replicateM n $ newName "x"<br />
lamE (map (conP c . map varP) vs) $<br />
foldl (\con (vs,f) -><br />
con `appE`<br />
foldl appE<br />
(dyn f)<br />
(map varE vs))<br />
(conE c)<br />
(transpose vs `zip` functions)<br />
</haskell><br />
<br />
This example uses whichever '+' is in scope when the expression is spliced:<br />
<br />
<haskell><br />
:type $(zipCons ''(,,,) 2 (replicate 4 "+"))<br />
<br />
$(zipCons ''(,,,) 2 (replicate 4 "+"))<br />
:: (Num t, Num t1, Num t2, Num t3) =><br />
(t, t1, t2, t3) -> (t, t1, t2, t3) -> (t, t1, t2, t3)<br />
</haskell><br />
<br />
<br />
==[[Template haskell/Instance deriving example|Instance deriving example]]==<br />
An example using a 'deriving function' to generate a method instance <br />
per constructor of a type. The deriving function provides the body of the<br />
method.<br />
<br />
Note that this example assumes that the functions of the class take a parameter that is the same type as instance is parameterized with. <br />
<br />
The message [http://www.haskell.org/pipermail/template-haskell/2006-August/000581.html email message] contains the full source ([http://www.iist.unu.edu/~vs/haskell/TH_render.hs extracted file]).<br />
<br />
== [[Quasiquotation|QuasiQuoters]] ==<br />
New in ghc-6.10 is -XQuasiQuotes, which allows one to extend ghc's syntax from library code. Quite a few examples are given in [http://hackage.haskell.org/package/haskell-src-meta haskell-src-meta].<br />
<br />
=== Similarity with splices ===<br />
<br />
Quasiquoters used in expression contexts (those using the ''quoteExp'') behave to a first approximation like regular TH splices:<br />
<br />
<haskell><br />
simpleQQ = QuasiQuoter { quoteExp = stringE } -- in another module<br />
<br />
[$simpleQQ| a b c d |] == $(quoteExp simpleQQ " a b c d ")<br />
</haskell><br />
<br />
[[Category:Language extensions]]</div>Kwantamhttps://wiki.haskell.org/index.php?title=Template_Haskell&diff=39649Template Haskell2011-04-22T18:52:36Z<p>Kwantam: /* Template Haskell tutorials and papers */ Fixed links in the the papers section, added link to GHC TH doc</p>
<hr />
<div>'''[http://www.haskell.org/th/ Template Haskell]''' is a [[GHC]] extension to Haskell that adds compile-time metaprogramming facilities. The original design can be found here: http://research.microsoft.com/~simonpj/papers/meta-haskell/ . It is [http://haskell.cs.yale.edu/ghc/docs/6.2/html/users_guide/template-haskell.html included] in GHC version 6. <br />
<br />
This page hopes to be a more central and organized repository of TH related things.<br />
<br />
=What is Template Haskell?=<br />
Template Haskell is an extension to Haskell 98 that allows you to do type-safe compile-time meta-programming, with Haskell both as the manipulating language and the language being manipulated. <br />
<br />
Intuitively Template Haskell provides new language features that allow us to convert back and forth between concrete syntax, i.e. what you would type when you write normal Haskell code, and abstract syntax trees. These abstract syntax trees are represented using Haskell datatypes and, at compile time, they can be manipulated by Haskell code. This allows you to reify (convert from concrete syntax to an abstract syntax tree) some code, transform it and splice it back in (convert back again), or even to produce completely new code and splice that in, while the compiler is compiling your module. <br />
<br />
For email about Template Haskell, use the [http://haskell.org/mailman/listinfo/glasgow-haskell-users GHC users mailing list]. It's worth joining if you start to use TH.<br />
<br />
= Template Haskell specification =<br />
<br />
Template Haskell is only documented rather informally at the moment. Here are the main resources:<br />
<br />
* [http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html The user manual section on Template Haskell]<br />
* [http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html#th-quasiquotation The user manual section on quasi-quotation], which is closely related to Template Haskell.<br />
* [http://research.microsoft.com/~simonpj/papers/meta-haskell/ The original Template Haskell paper]<br />
* [http://research.microsoft.com/~simonpj/tmp/notes2.ps Notes on Template Haskell version 2], which describes changes since the original paper. Section 8 describes the difficulty with pattern splices, which are therefore not implemented.<br />
* [http://haskell.org/ghc/docs/7.0-latest/html/libraries/template-haskell-2.5.0.0/Language-Haskell-TH.html The Template Haskell API]<br />
<br />
= Template Haskell tutorials and papers =<br />
<br />
* Bulat's tutorials:<br />
** http://www.haskell.org/bz/thdoc.htm<br />
** http://www.haskell.org/bz/th3.htm<br />
: One reader said "These docs are *brilliant* ! Exactly what I need to get an understanding of TH."<br />
<br />
* Mark Snyder's Template Haskell chapter on the Software Generation and Configuration Report<br />
** http://nix.cs.uu.nl/dist/courses/sgc-report-unstable-latest/manual/chunk-chapter/templatehaskell.html<br />
<br />
* A very short tutorial to understand the basics in 10 Minutes.<br />
** http://www.hyperedsoftware.com/blog/entries/first-stab-th.html<br />
<br />
* GHC Template Haskell documentation<br />
** http://www.haskell.org/ghc/docs/7.0.2/html/users_guide/template-haskell.html<br />
<br />
* Papers about Template Haskell<br />
<br />
** Template metaprogramming for Haskell, by Tim Sheard and Simon Peyton Jones, Oct 2002. [http://haskell.org/wikiupload/c/ca/Meta-haskell.ps [ps]] <br />
** Template Haskell: A Report From The Field, by Ian Lynagh, May 2003. [http://haskell.org/wikiupload/2/24/Template_Haskell-A_Report_From_The_Field.ps [ps]]<br />
** Unrolling and Simplifying Expressions with Template Haskell, by Ian Lynagh, December 2002. [http://haskell.org/wikiupload/e/ed/Template-Haskell-Utils.ps [ps]]<br />
** Automatic skeletons in Template Haskell, by Kevin Hammond, Jost Berthold and Rita Loogen, June 2003. [http://haskell.org/wikiupload/6/69/AutoSkelPPL03.pdf [pdf]]<br />
** Optimising Embedded DSLs using Template Haskell, by Sean Seefried, Manuel Chakravarty, Gabriele Keller, March 2004. [http://haskell.org/wikiupload/b/b5/Seefried04th-pan.pdf [pdf]]<br />
** Typing Template Haskell: Soft Types, by Ian Lynagh, August 2004. [http://haskell.org/wikiupload/7/72/Typing_Template_Haskell_Soft_Types.ps [ps]]<br />
<br />
= Other useful resources =<br />
<br />
* [http://www.haskell.org/th/ The old Template Haskell web page]. Would someone feel like moving this material into the HaskellWiki?<br />
* Old and probably not too useful for most but maybe... http://www.cse.unsw.edu.au/~chak/haskell/ghc/comm/exts/th.html<br />
*[http://web.comlab.ox.ac.uk/oucl/work/ian.lynagh/Fraskell/ Fraskell documentation] & explanation of how Template Haskell is used to vastly speed it up.<br />
*[[Quasiquotation]]<br />
Feel free to update our Wikipedia entry<br />
http://en.wikipedia.org/wiki/Template_Haskell<br />
<br />
= Projects =<br />
<br />
What are you doing/planning to do/have done with Template Haskell?<br />
<br />
* The [http://www.ict.kth.se/org/ict/ecs/sam/projects/forsyde/www ForSyDe methodology] is currently implemented as a Haskell-based DSL which makes extensive use of Template Haskell.<br />
<br />
* I have written a primitive (untyped) binding to the Objective-C runtime system on Mac OS X. It needs just TH, no "stub files" are created, no seperate utilities are required. Initial snapshot is at http://www.kfunigraz.ac.at/imawww/thaller/wolfgang/HOC020103.tar.bz2 -- WolfgangThaller<br />
<br />
* I am writing Template Greencard - a reimplementation of GreenCard using TH. Many bits work out really nicely. A few bits didn't work so nicely - once I get some time to think, I'll try to persuade the TH folk to make some changes to fix some of these. -- AlastairReid<br />
<br />
* I'm writing Hacanon - a framework for automatic generation of C++ bindings. Read "automated Template Greencard for C++" (-: Darcs repo: http://www.ScannedInAvian.org/repos/hacanon - You'll need gccxml (http://www.gccxml.org/) to compile the exmples. - 27 Dec Lemmih.<br />
<br />
* Following other FFI tools developers, I see some future for Template HSFFIG, especially when it comes to autogenerate peek and poke methods for structures defined in C; may be useful for implementation of certain network protocols such as X11 where layout of messages is provided as C structure/union declaration. - 16 Dec 2005 DimitryGolubovsky<br />
<br />
* I am using Template Haskell as a mechanism to get parsed, typechecked code into an Ajax based Haskell Equational Reasoning tool [[Haskell Equational Reasoning Assistant]], as well as simplify the specification of equational relationships between pieces of code. There was a quicktime movie of the tool being used on http://www.gill-warbington.com/home/andy/share/hera1.html - AndyGill <br />
<br />
* I am working on functional metaprogramming techniques to enhance programming reliability and productivity, by reusing much of the existing compiler technology. Template Haskell is especially interesting for me because it permits to check size information of structures by the compiler, provided this information is available at compile time. This approach is especially appropriate for hardware designs, where the structures are fixed before the circuit starts operating. See our metaprogramming web page at http://www.infosun.fmi.uni-passau.de/cl/metaprog/ -- ChristophHerrmann(http://www.cs.st-and.ac.uk/~ch)<br />
<br />
* I am using Template Haskell to do type safe database access. I initially [http://www.nabble.com/Using-Template-Haskell-to-make-type-safe-database-access-td17027286.html proposed this on haskell-cafe]. I connect to the database at compile-time and let the database do SQL parsing and type inference. The result from parsing and type inference is used to build a type safe database query which can executed at run-time. [[MetaHDBC | You can find the project page here]] -- [mailto:mads_lindstroem@yahoo.dk Mads Lindstrøm]<br />
<br />
= Utilities =<br />
<br />
Helper functions, debugging functions, or more involved code e.g. a monadic fold algebra for TH.Syntax.<br />
<br />
* http://www.haskell.org/pipermail/template-haskell/2003-September/000176.html<br />
<br />
= Known Bugs =<br />
<br />
Take a look at the [http://hackage.haskell.org/trac/ghc/query?status=new&status=assigned&status=reopened&component=Template+Haskell&order=priority open bugs against Template Haskell] on the GHC bug tracker.<br />
<br />
= Wish list =<br />
<br />
Things that Ian Lynagh (Igloo) mentioned in his paper ''Template Haskell: A Report From The Field'' in May 2003 (available [http://www.haskell.org/th/papers.html here]), by section:<br />
<br />
* Section 2 (curses)<br />
** The ability to splice names (into "foreign import" declarations, in particular)<br />
** The ability to add things to the export list from a splice(?)<br />
** The ability to use things defined at the toplevel of a module from splices in that same module (would require multi-stage compilation, as opposed to the current approach of expanding splices during typechecking)<br />
<br />
* Section 3 (deriving instances of classes)<br />
** <strike>First-class reification</strike> (the <hask>reify</hask> function)<br />
** A way to discover whether a data constructor was defined infix or prefix (which is necessary to derive instances for <hask>Read</hask> and <hask>Show</hask> as outlined in [http://www.haskell.org/onlinereport/derived.html The Haskell 98 Report: Specification of Derived Instances]) (if there is a way, [http://www-users.cs.york.ac.uk/~ndm/derive/ Derive] seems ignorant of it)<br />
** Type/context splicing (in <hask>instance</hask> headers in particular)<br />
<br />
* Section 4 (printf)<br />
** He says something to the effect that a pattern-matching form of the quotation brackets would be cool if it was expressive enough to be useful, but that this would be hard. (Don't expect this anytime soon.)<br />
<br />
* Section 5 (fraskell)<br />
** Type information for quoted code (so that simplification can be done safely even with overloaded operations, like, oh, <hask>(+)</hask>)<br />
<br />
* Section 6 (pan)<br />
** Type info again, and strictnes info too (this one seems a bit pie-in-the-sky...)<br />
<br />
(Please leave the implemented ones here, but crossed off.)<br />
<br />
Any other features that may be nice, and TH projects you'd want to see.<br />
<br />
* A TH tutorial (mainly a distillation and update of ''Template Meta-programming in Haskell'' at this point)<br />
* <strike>Write Haddock documentation for the Template Haskell library (http://hackage.haskell.org/trac/ghc/ticket/1576).</strike><br />
* Make `reify` on a class return a list of the instances of that class (http://www.haskell.org/pipermail/template-haskell/2005-December/000503.html). (See also [http://hackage.haskell.org/trac/ghc/ticket/1577 GHC ticket #1577].)<br />
* A set of simple examples on this wiki page<br />
* A TH T-shirt with new logo to wear at conferences<br />
* (Long-term) Unify Language.Haskell.Syntax with Language.Haskell.TH.Syntax so there's just one way to do things (http://hackage.haskell.org/package/haskell-src-meta does a one-way translation, for haskell-src-exts)<br />
<br />
---------------<br />
<br />
= Tips and Tricks =<br />
<br />
== What to do when you can't splice that there ==<br />
<br />
When you try to splice something into the middle of a template and find that you just can't, instead of getting frustrated about it, why not use the template to see what it would look like in longhand? <br />
<br />
First, an excerpt from a module of my own. I, by the way, am SamB.<br />
<haskell><br />
{-# OPTIONS_GHC -fglasgow-exts -fth #-}<br />
<br />
module MMixMemory where<br />
<br />
import Data.Int<br />
import Data.Word<br />
<br />
class (Integral int, Integral word)<br />
=> SignConversion int word | int -> word, word -> int where<br />
<br />
toSigned :: word -> int<br />
toSigned = fromIntegral<br />
toUnsigned :: int -> word<br />
toUnsigned = fromIntegral<br />
<br />
</haskell><br />
<br />
Say I want to find out what I need to do to splice in the types for an instance declaration for the SignConversion class, so that I can declare instances for Int8 with Word8 through Int64 with Word64. So, I start up good-ol' GHCi and do the following:<br />
<br />
<haskell><br />
$ ghci -fth -fglasgow-exts<br />
Prelude> :l MMixMemory<br />
*MMixMemory> :m +Language.Haskell.TH.Syntax<br />
*MMixMemory Language.Haskell.TH.Syntax> runQ [d| instance SignConversion Int Word where |] >>= print<br />
[InstanceD [] (AppT (AppT (ConT MMixMemory.SignConversion) (ConT GHC.Base.Int)) (ConT GHC.Word.Word)) []]<br />
</haskell><br />
<br />
== Why does <tt>runQ</tt> crash if I try to reify something? ==<br />
<br />
This program will fail with an error message when you run it:<br />
<haskell><br />
main = do info <- runQ (reify (mkName "Bool")) -- more hygenic is: (reify ''Bool)<br />
putStrLn (pprint info)<br />
</haskell><br />
Reason: <tt>reify</tt> consults the type environment, and that is not available at run-time. The type of <tt>reify</tt> is <br />
<haskell><br />
reify :: Quasi m => Q a -> m a<br />
</haskell><br />
The IO monad is a poor-man's instance of <tt>Quasi</tt>; it can allocate unique names and gather error messages, but it can't do <tt>reify</tt>. This error should really be caught statically.<br />
<br />
Instead, you can run the splice directly (ex. in ghci -XTemplateHaskell), as the following shows:<br />
<br />
<haskell><br />
GHCi> let tup = $(tupE $ take 4 $ cycle [ [| "hi" |] , [| 5 |] ])<br />
GHCi> :type tup<br />
tup :: ([Char], Integer, [Char], Integer)<br />
<br />
GHCi> tup<br />
("hi",5,"hi",5)<br />
<br />
GHCi> $(stringE . show =<< reify ''Int)<br />
"TyConI (DataD [] GHC.Types.Int [] [NormalC GHC.Types.I# [(NotStrict,ConT GHC.Prim.Int#)]] [])"<br />
</haskell><br />
<br />
Here's an [http://www.haskell.org/pipermail/glasgow-haskell-users/2006-August/010844.html email thread with more details].<br />
<br />
-----------------<br />
= Examples =<br />
== Tuples ==<br />
=== Select from a tuple ===<br />
<br />
An example to select an element from a tuple of arbitrary size. Taken from [http://www.haskell.org/th/papers/meta-haskell.ps this paper].<br />
<br />
Use like so:<br />
<br />
> $(sel 2 3) ('a','b','c')<br />
'b'<br />
> $(sel 3 4) ('a','b','c','d')<br />
'c'<br />
<br />
<br />
<haskell><br />
sel :: Int -> Int -> ExpQ<br />
sel i n = [| \x -> $(caseE [| x |] [alt]) |]<br />
where alt :: MatchQ<br />
alt = match pat (normalB rhs) []<br />
<br />
pat :: Pat<br />
pat = tupP (map varP as)<br />
<br />
rhs :: ExpQ<br />
rhs = varE(as !! (i -1)) -- !! is 0 based<br />
<br />
as :: [String]<br />
as = ["a" ++ show i | i <- [1..n] ]<br />
</haskell><br />
<br />
Alternately:<br />
<br />
<haskell><br />
sel' i n = lamE [pat] rhs<br />
where pat = tupP (map varP as)<br />
rhs = varE (as !! (i - 1))<br />
as = [ "a" ++ show j | j <- [1..n] ]<br />
</haskell><br />
<br />
=== Apply a function to the n'th element ===<br />
<br />
<haskell><br />
tmap i n = do<br />
f <- newName "f"<br />
as <- replicateM n (newName "a")<br />
lamE [varP f, tupP (map varP as)] $<br />
tupE [ if i == i'<br />
then [| $(varE f) $a |]<br />
else a<br />
| (a,i') <- map varE as `zip` [1..] ]<br />
</haskell><br />
<br />
Then tmap can be called as:<br />
<br />
> $(tmap 3 4) (+ 1) (1,2,3,4)<br />
(1,2,4,4)<br />
<br />
=== Convert the first n elements of a list to a tuple ===<br />
<br />
This example creates a tuple by extracting elemnts from a list. Taken from<br />
[http://www.xoltar.org/2003/aug/13/templateHaskellTupleSample.html www.xoltar.org]<br />
<br />
Use like so:<br />
<br />
> $(tuple 3) [1,2,3,4,5]<br />
(1,2,3)<br />
> $(tuple 2) [1,2]<br />
(1,2)<br />
<br />
<haskell><br />
tuple :: Int -> ExpQ<br />
tuple n = [|\list -> $(tupE (exprs [|list|])) |]<br />
where<br />
exprs list = [infixE (Just (list))<br />
(varE "!!")<br />
(Just (litE $ integerL (toInteger num)))<br />
| num <- [0..(n - 1)]]<br />
</haskell><br />
<br />
An alternative that has more informative errors (a failing pattern match failures give an exact location):<br />
<br />
<haskell><br />
tuple :: Int -> ExpQ<br />
tuple n = do<br />
ns <- replicateM n (newName "x")<br />
lamE [foldr (\x y -> conP '(:) [varP x,y]) wildP ns] (tupE $ map varE ns)<br />
</haskell><br />
<br />
=== Un-nest tuples ===<br />
Convert nested tuples like (a,(b,(c,()))) into (a,b,c) given the length to generate.<br />
<br />
<haskell><br />
unNest n = do<br />
vs <- replicateM n (newName "x")<br />
lamE [foldr (\a b -> tupP [varP a , b])<br />
(conP '() [])<br />
vs]<br />
(tupE (map varE vs))<br />
</haskell><br />
<br />
<br />
<br />
== [[Template Haskell/Marshall Data|Marshall a datatype to and from Dynamic]] ==<br />
This approach is an example of using template haskell to delay typechecking<br />
to be able to abstract out the repeated calls to fromDynamic:<br />
<br />
<haskell><br />
data T = T Int String Double<br />
<br />
toT :: [Dynamic] -> Maybe T<br />
toT [a,b,c] = do<br />
a' <- fromDynamic a<br />
b' <- fromDynamic b<br />
c' <- fromDynamic c<br />
return (T a' b' c')<br />
toT _ = Nothing<br />
</haskell><br />
<br />
== Printf ==<br />
This example taken from: http://haskell.cs.yale.edu/ghc/docs/6.0/html/users_guide/template-haskell.html<br />
<br />
Build it using a command similar to one of the following (depending on your environment):<br />
<br />
ghc/compiler/stage3/ghc-inplace --make -fglasgow-exts -package haskell-src main.hs -o main.exe<br />
ghc --make -fth Main.hs -o printfTest<br />
<br />
Main.hs:<br />
<br />
<haskell><br />
module Main where<br />
<br />
-- Import our template "pr"<br />
import Printf ( pr )<br />
<br />
-- The splice operator $ takes the Haskell source code<br />
-- generated at compile time by "pr" and splices it into<br />
-- the argument of "putStrLn".<br />
main = putStrLn ( $(pr "Hello") )<br />
</haskell><br />
<br />
Printf.hs:<br />
<br />
<haskell><br />
module Printf where<br />
<br />
-- Skeletal printf from the paper.<br />
-- It needs to be in a separate module to the one where<br />
-- you intend to use it.<br />
<br />
-- Import some Template Haskell syntax<br />
import Language.Haskell.TH.Syntax<br />
<br />
-- Describe a format string<br />
data Format = D | S | L String<br />
<br />
-- Parse a format string. This is left largely to you<br />
-- as we are here interested in building our first ever<br />
-- Template Haskell program and not in building printf.<br />
parse :: String -> [Format]<br />
parse s = [ L s ]<br />
<br />
-- Generate Haskell source code from a parsed representation<br />
-- of the format string. This code will be spliced into<br />
-- the module which calls "pr", at compile time.<br />
gen :: [Format] -> ExpQ<br />
gen [D] = [| \n -> show n |]<br />
gen [S] = [| \s -> s |]<br />
gen [L s] = stringE s<br />
<br />
-- Here we generate the Haskell code for the splice<br />
-- from an input format string.<br />
pr :: String -> ExpQ<br />
pr s = gen (parse s)<br />
</haskell><br />
<br />
== Handling Options with Templates ==<br />
A common idiom for treating a set of options, e.g. from GetOpt, is to define a datatype with all the flags and using a list over this datatype:<br />
<br />
<haskell><br />
data Options = B1 | B2 | V Integer<br />
<br />
options = [B1, V 3]<br />
</haskell><br />
<br />
While it's simple testing if a Boolean flag is set (simply use "elem"), it's harder to check if an option with an argument is set. It's even more tedious writing helper-functions to obtain the value from such an option since you have to explicitely "un-V" each. Here, Template Haskell can be (ab)used to reduce this a bit. The following example provides the module "OptionsTH" which can be reused regardless of the constructors in "Options". Let's start with showing how we'd like to be able to program. Notice that the resulting lists need some more treatment e.g. through "foldl".<br />
<br />
Options.hs:<br />
<br />
<haskell><br />
module Main where<br />
<br />
import OptionsTH<br />
import Language.Haskell.TH.Syntax<br />
<br />
data Options = B1 | B2 | V Int | S String deriving (Eq, Read, Show)<br />
<br />
options = [B1, V 3]<br />
<br />
main = do<br />
print foo -- test if B1 set: [True,False]<br />
print bar -- test if V present, w/o value: [False,True]<br />
print baz -- get value of V if available: [Nothing,Just 3]<br />
<br />
foo :: [Bool]<br />
-- Query constructor B1 which takes no arguments<br />
foo = map $(getopt (THNoArg (mkArg "B1" 0))) options<br />
<br />
bar :: [Bool]<br />
-- V is a unary constructor. Let mkArg generate the required<br />
-- wildcard-pattern "V _".<br />
bar = map $(getopt (THNoArg (mkArg "V" 1))) options<br />
<br />
-- Can't use a wildcard here!<br />
baz :: [(Maybe Int)]<br />
baz = map $(getopt (THArg (conP "V" [varP "x"]))) options<br />
</haskell><br />
<br />
OptionsTH.hs<br />
<br />
<haskell><br />
module OptionsTH where<br />
<br />
import Language.Haskell.TH.Syntax<br />
<br />
-- datatype for querying options:<br />
-- NoArg: Not interested in value (also applies to Boolean flags)<br />
-- Arg: Grep value of unary(!) constructor<br />
data Args = THNoArg Pat | THArg Pat<br />
<br />
getopt :: Args -> ExpQ<br />
getopt (THNoArg pat) = lamE [varP "y"] (caseE (varE "y") [cons0, cons1])<br />
where<br />
cons0 = match pat (normalB [| True |]) []<br />
cons1 = match wildP (normalB [| False |]) []<br />
<br />
-- bind "var" for later use!<br />
getopt (THArg pat@(ConP _ [VarP var])) = lamE [varP "y"] (caseE (varE "y") [cons0, cons1])<br />
where<br />
cons0 = match pat (normalB (appE [|Just|] (varE var))) []<br />
cons1 = match wildP (normalB [|Nothing|]) []<br />
<br />
mkArg :: String -> Int -> Pat<br />
mkArg k c = conP k (replicate c wildP)<br />
</haskell><br />
<br />
While the example might look contrived for the Boolean options which could have been tested much easier, it shows how both types of arguments can be treated in a similar way.<br />
<br />
=== Limitations ===<br />
<tt>getopt (THArg pat)</tt> is only able to treat unary constructors. See the pattern-binding: It matches exactly a single VarP.<br />
<br />
=== Improvements ===<br />
The following reduces things even a bit more, though I still don't know if I like it. It only works since <tt>c</tt> is either 0 or 1.<br />
<br />
<haskell><br />
mkArg k c = conP k (replicate c (varP "x"))<br />
<br />
baz = map $(getopt (THArg (mkArg "V" 1)))<br />
</haskell><br />
-- VolkerStolz<br />
<br />
== Generic constructor for records ==<br />
<br />
I have a large number of record types like this, of different length:<br />
<br />
<haskell><br />
data PGD = PGD {<br />
pgdXUnitBase :: !Word8,<br />
pgdYUnitBase :: !Word8,<br />
pgdXLUnitsperUnitBase :: !Word16<br />
}<br />
</haskell><br />
<br />
Currently I use GHC's Binary module to read them from files; it can handle<br />
types like <tt>(Word8, (Word8, Word16))</tt>, but there was no easy way to generate<br />
the correct amount of "uncurry" calls for automatically grabbing each element.<br />
<br />
With Template Haskell, the instance declarations are now written as such:<br />
<br />
<haskell><br />
instance Binary PGD where<br />
get bh = do a <- get bh ; return $ $(constrRecord PGD) a<br />
</haskell><br />
<br />
Here the trick lies in constrRecord, which is defined as:<br />
<br />
<haskell><br />
constrRecord x = reify exp where<br />
reify = \(Just r) -> appE r $ conE $ last args<br />
exp = foldl (dot) uncur $ replicate terms uncur<br />
terms = ((length args) `div` 2) - 2<br />
dot x y = (Just $ infixE x (varE ".") y)<br />
uncur = (Just [|uncurry|])<br />
args = words . show $ typeOf x<br />
</haskell><br />
<br />
-- AutrijusTang<br />
<br />
== 'generic' zipWith ==<br />
A generalization of zipWith to almost any data. Demonstrates the ability to do dynamic binding with TH splices (note 'dyn').<br />
<br />
<haskell><br />
zipCons :: Name -> Int -> [String] -> ExpQ<br />
zipCons tyName ways functions = do<br />
let countFields :: Con -> (Name,Int)<br />
countFields x = case x of<br />
NormalC n (length -> fields) -> (n, fields)<br />
RecC n (length -> fields) -> (n,fields)<br />
InfixC _ n _ -> (n,2)<br />
ForallC _ _ ct -> countFields ct<br />
<br />
TyConI (DataD _ _ _ [countFields -> (c,n)] _) <- reify tyName<br />
when (n /= length functions) $ fail "wrong number of functions named"<br />
vs <- replicateM ways $ replicateM n $ newName "x"<br />
lamE (map (conP c . map varP) vs) $<br />
foldl (\con (vs,f) -><br />
con `appE`<br />
foldl appE<br />
(dyn f)<br />
(map varE vs))<br />
(conE c)<br />
(transpose vs `zip` functions)<br />
</haskell><br />
<br />
This example uses whichever '+' is in scope when the expression is spliced:<br />
<br />
<haskell><br />
:type $(zipCons ''(,,,) 2 (replicate 4 "+"))<br />
<br />
$(zipCons ''(,,,) 2 (replicate 4 "+"))<br />
:: (Num t, Num t1, Num t2, Num t3) =><br />
(t, t1, t2, t3) -> (t, t1, t2, t3) -> (t, t1, t2, t3)<br />
</haskell><br />
<br />
<br />
==[[Template haskell/Instance deriving example|Instance deriving example]]==<br />
An example using a 'deriving function' to generate a method instance <br />
per constructor of a type. The deriving function provides the body of the<br />
method.<br />
<br />
Note that this example assumes that the functions of the class take a parameter that is the same type as instance is parameterized with. <br />
<br />
The message [http://www.haskell.org/pipermail/template-haskell/2006-August/000581.html email message] contains the full source ([http://www.iist.unu.edu/~vs/haskell/TH_render.hs extracted file]).<br />
<br />
== [[Quasiquotation|QuasiQuoters]] ==<br />
New in ghc-6.10 is -XQuasiQuotes, which allows one to extend ghc's syntax from library code. Quite a few examples are given in [http://hackage.haskell.org/package/haskell-src-meta haskell-src-meta].<br />
<br />
=== Similarity with splices ===<br />
<br />
Quasiquoters used in expression contexts (those using the ''quoteExp'') behave to a first approximation like regular TH splices:<br />
<br />
<haskell><br />
simpleQQ = QuasiQuoter { quoteExp = stringE } -- in another module<br />
<br />
[$simpleQQ| a b c d |] == $(quoteExp simpleQQ " a b c d ")<br />
</haskell><br />
<br />
[[Category:Language extensions]]</div>Kwantamhttps://wiki.haskell.org/index.php?title=File:Typing_Template_Haskell_Soft_Types.ps&diff=39645File:Typing Template Haskell Soft Types.ps2011-04-22T15:25:33Z<p>Kwantam: Typing Template Haskell: Soft Types by Ian Lynagh</p>
<hr />
<div>Typing Template Haskell: Soft Types by Ian Lynagh</div>Kwantamhttps://wiki.haskell.org/index.php?title=File:Seefried04th-pan.pdf&diff=39644File:Seefried04th-pan.pdf2011-04-22T15:23:47Z<p>Kwantam: Optimising Embedded DSLs using Template Haskell by Seefried, Chakravarty, and Keller</p>
<hr />
<div>Optimising Embedded DSLs using Template Haskell by Seefried, Chakravarty, and Keller</div>Kwantamhttps://wiki.haskell.org/index.php?title=File:AutoSkelPPL03.pdf&diff=39643File:AutoSkelPPL03.pdf2011-04-22T15:02:52Z<p>Kwantam: Automatic Skeletons in Template Haskell by Hammond, Berthold, and Loogen</p>
<hr />
<div>Automatic Skeletons in Template Haskell by Hammond, Berthold, and Loogen</div>Kwantamhttps://wiki.haskell.org/index.php?title=File:Template-Haskell-Utils.ps&diff=39642File:Template-Haskell-Utils.ps2011-04-22T14:59:03Z<p>Kwantam: Unrolling and Simplifying Expressions with Template Haskell by Ian Lynagh</p>
<hr />
<div>Unrolling and Simplifying Expressions with Template Haskell by Ian Lynagh</div>Kwantamhttps://wiki.haskell.org/index.php?title=File:Template_Haskell-A_Report_From_The_Field.ps&diff=39641File:Template Haskell-A Report From The Field.ps2011-04-22T14:55:04Z<p>Kwantam: Template Haskell: A Report from the Field by Ian Lynagh</p>
<hr />
<div>Template Haskell: A Report from the Field by Ian Lynagh</div>Kwantamhttps://wiki.haskell.org/index.php?title=File:Meta-haskell.ps&diff=39639File:Meta-haskell.ps2011-04-22T14:46:24Z<p>Kwantam: Template Meta-programming for Haskell by Sheard and Jones</p>
<hr />
<div>Template Meta-programming for Haskell by Sheard and Jones</div>Kwantamhttps://wiki.haskell.org/index.php?title=Applications_and_libraries/Games&diff=30115Applications and libraries/Games2009-09-20T07:33:41Z<p>Kwantam: reordered the list to be closer to alphabetical; added my own contribution, TriHs, a Gtk2Hs+Cairo Tetris implementation.</p>
<hr />
<div>{{LibrariesPage}}<br />
<br />
== Applications ==<br />
<br />
;Bridge<br />
:David Roundy originally developed a Bridge game in Haskell and wrote a versioning system for this project. However as time went by, the versioning project [[Darcs]] became the more important part of the development and the bridge game was stalled.<br />
<br />
; [http://www.maths.tcd.ie/~icecube/2008/11/endless-cavern/ Endless Cavern]: A 2D procedurally-generated cave exploration game.<br />
<br />
;[http://sourceforge.net/projects/fooengine/?abmode=1 Foo]<br />
:Foo (abbreviation from football) is a playing machine of [http://en.wikipedia.org/wiki/Paper_Soccer Paper Soccer], a pencil and paper game for two players. It contains a simple interface using HOpenGL library and provides many playing algorithms.<br />
<br />
;[[Frag]]<br />
:Frag is a 3D first person shooting game written in Haskell, by Mun Hon Cheong. It uses Yampa, Quake 3 BSP level format and OpenGL. It is licensed under the GPL.<br />
<br />
;[http://www.informatik.uni-bremen.de/~cxl/lehre/pi3.ws01/asteroids/ Haskell in Space]<br />
:An asteroid like game<br />
<br />
;[http://web.comlab.ox.ac.uk/oucl/work/ian.lynagh/Hetris/ Hetris]<br />
:ASCII tetris in Haskell<br />
<br />
;[http://www.cs.mu.oz.au/~bjpop/code/HInvaders.tar.gz hinvaders]<br />
:A simple ANSI-graphics space invaders written entirely in Haskell 98. See also [http://hackage.haskell.org/packages/archive/pkg-list.html#cat:Game hackage] and [http://joyful.com/darcsweb/darcsweb.cgi?r=hinvaders;a=summary darcs].<br />
<br />
;[http://www.cse.unsw.edu.au/~dons/code/contrib/hsChess hsChess]<br />
:Chess AI engine<br />
<br />
;[http://mu.org/~mux/LambdaChess/ LambdaChess]<br />
:GTK chess client<br />
<br />
;[http://www.scannedinavian.com/~shae/mage-1.0pre35.tar.gz Mage] (tar.gz)<br />
:Nethack clone written in Haskell<br />
<br />
;[http://hackage.haskell.org/package/MazesOfMonad MazesOfMonad]<br />
:Role-Playing Game (influenced by Nethack), complete and fully playable. Console mode only.<br />
<br />
;[http://www.geocities.jp/takascience/haskell/monadius_en.html Monadius]<br />
:Monadius is a shoot 'em up with the selection bar power-up system for Windows, written in Haskell (now on [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Monadius-0.9.20071203 Hackage])<br />
<br />
;[http://d.hatena.ne.jp/authorNari/20080422/1208880928 Nario]<br />
:A Super Mario clone, using an SDL binding different from the one in Hackage: [http://fxp.hp.infoseek.co.jp/haskell/HSDL/arc/HSDL-0.2.0.zip]<br />
<br />
;[http://berlinbrowndev.blogspot.com/2007/09/octane-mech-opengl-haskell-based-mech.html Octane Mech]<br />
:Octane Mech, OpenGL Haskell based mech game<br />
<br />
;[http://haskell-tetris.pbwiki.com/Main OpenGL Tetris]<br />
:Tetris in Haskell with OpenGL<br />
<br />
;[http://www24.brinkster.com/srineet/para/para.html Paratrooper]<br />
:Paratrooper is a simple action game that runs on Windows and is written in literate Haskell.<br />
<br />
;[http://roguestar.downstairspeople.org Roguestar]<br />
:Roguestar is a science fiction adventure role playing game using Haskell and OpenGL.<br />
<br />
;[http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Shu-thing-1.0.20071203 Shu-thing]<br />
:A 2-D vector graphics upwards-scrolling keyboard-controlled shooter. You shoot the enemies while dodging their bullets until you reach and defeat the enemy.<br />
<br />
;[http://web.jfet.org/~kwantam/TriHs.tar.gz TriHs] (tar.gz)<br />
:A 1- or 2-player Tetris game using Gtk2Hs and Cairo.<br />
<br />
;[[wxAsteroids]]<br />
:Your space ship enters an asteroid belt, try to avoid collisions! wxAsteroids is based on wxHaskell.<br />
<br />
;[http://xiangqiboard.blogspot.com/2007/12/gnuxiangqi-angekndigt.html Xiangqiboard]<br />
:An implementation of xiangqi for Unix, using gtk2hs + cairo<br />
<br />
<br />
== Libraries ==<br />
<br />
;[http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Hipmunk Hipmunk]<br />
:Hipmunk: A Haskell binding for [http://wiki.slembcke.net/main/published/Chipmunk Chipmunk]. Chipmunk is a fast, simple, portable, 2D physics engine. It is completely self-contained. See also [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HipmunkPlayground HipmunkPlayground]: a simple OpenGL program that allows you to see some of Hipmunk's functions in action.<br />
<br />
;[[Hpysics]]<br />
:Hpysics is a physics engine written using Data Parallel Haskell during Google Summer of Code 2008.<br />
<br />
;[http://www.cin.ufpe.br/~haskell/fungen FunGEn - a game engine for Haskell]<br />
:FunGEn (Functional Game Engine) is a 2D platform-independent game engine implemented in and for Haskell, using HOpenGL. It is intended to help game programmers in the game development process, in a faster and automated way.<br />
:Note, that this is released in 2002 and currently will not compile.<br />
<br />
;[http://projects.haskell.org/game-tree/ game-tree - searching dynamic game trees]<br />
:game-tree is a purely functional library for searching game trees - useful for zero-sum two player games.<br />
== Further reading ==<br />
<br />
=== Blog articles ===<br />
* [http://prog21.dadgum.com/23.html Purely Functional Retrogames]<br />
<br />
* [http://prog21.dadgum.com/36.html Accidentally Introducing Side Effects into Purely Functional Code]<br />
<br />
=== Other ===<br />
* [http://www.londonhug.net/2007/09/24/better-video-for-games-in-haskell/ Games in Haskell, the video]<br />
:Matthew Sackman and Tristan Allwood on developing games with OpenGL in Haskell<br />
<br />
[[Category:Games|*]]<br />
[[Category:Applications]]<br />
[[Category:Libraries]]</div>Kwantamhttps://wiki.haskell.org/index.php?title=Merchandise&diff=28810Merchandise2009-06-30T16:35:32Z<p>Kwantam: updated links in the CafePress section; broke out "Making new merch" into new section; added new Thompson-Wheeler logo T store link under CafePress</p>
<hr />
<div>= T-shirts, Mugs, and Mouse Pads =<br />
<br />
== Spreadshirt ==<br />
<br />
Make your own tshirt on spreadshirt:<br />
<br />
[http://cache.spreadshirt.com/image.php?type=image&partner_id=317036&product_id=3992586&img_id=1&size=huge&bgcolor_images=white http://galois.com/~dons/images/cabal-tshirt.png]<br />
<br />
*[http://www.spreadshirt.com/us/US/There-is-no-Cabal/Products-64/Marketplace/Products/detail/article/3612542 Cabal tshirts]<br />
<br />
[http://87154.spreadshirt.com/us/US/Shop/Article/Index/article/2125373 http://galois.com/~dons/images/xmonad-tshirt.png]<br />
<br />
*[http://87154.spreadshirt.com/us/US/Shop/Article/Index/article/2125373 The famous xmonad tshirt], "purely functional".<br />
<br />
There is a <b>new shop on spreadshirt.net</b> offering a (growing) variety of products and (more or less original) designs, including a clean logo variant: [http://409517.spreadshirt.net The gleemerch Haskell geekware shop], non-profit of course. A few examples:<br />
<br />
[http://409517.spreadshirt.net http://hpsg.fu-berlin.de/~rsling/img/haskell-shirt.jpeg] [http://409517.spreadshirt.net http://hpsg.fu-berlin.de/~rsling/img/haskell-bag.jpeg]<br />
<br />
== CafePress ==<br />
<br />
=== New logo ===<br />
<br />
* [http://www.cafepress.com/twhask T-shirts, stickers, and magnets] using the new [[Thompson-Wheeler logo]] (in a blue and green scheme that should reproduce well on light and dark backgrounds) are now available at cost on CafePress. Please email [mailto:rsw@jfet.org rsw] with problems or item requests.<br />
<br />
=== Older logos ===<br />
<br />
These time-tested, hacker-approved shirts (plus mugs, mouse pads, and more) are based on older logo designs. Wear them with pride.<br />
<br />
Thanks to Fritz Ruehr, Conal Elliott, Andrew Cooke and Wouter Swierstra for design work. Send mail to [mailto:fruehr@willamette.edu Fritz Ruehr] with any merchandise quality or other issues for products from CafePress below, except as noted.<br />
<br />
See the [http://www.willamette.edu/~fruehr/logos/intro.html logos page] for more details. The following stores are available (the merchandise is organized into separate "stores" for historical/logistical reasons):<br />
<br />
*[http://www.cafepress.com/haskell Original haskell.org logo] (Pan image by Conal Elliott on back).<br />
*[http://www.cafepress.com/haskellnewlogo New Haskell logo] (words on front, circle back)<br />
*[http://www.cafepress.com/purehaskell "purity" logo] (w/ "Pure Fun" on back)<br />
*[http://www.cafepress.com/classyhaskell Alternate "classy" logo] (feature list on back)<br />
*[http://www.cafepress.com/dysfunc Dysfunctional programming] (nothing on back) <br />
*[http://www.cafepress.com/statictypes Strong, static types] (typing judgement on back)<br />
*[http://www.cafepress.com/highordertyped HOT (Higher-Order & Typed)] (lambda on back; available in shirts, boxers, thongs, barbecue apron or mug)<br />
*[http://www.cafepress.com/doitmonad Do it in a Monad] (monad laws on back)<br />
*[http://www.cafepress.com/generichaskell Generic Haskell logo] (no back yet)<br />
*[http://www.cafepress.com/pancito Pancito "bugs" artwork by Andrew Cooke] (no back yet)<br />
*[http://www.cafepress.com/TheMonadReader The Monad.Reader logo] (logo on front; slogan on back optional; design and CafePress shop by Wouter Swierstra)<br />
*[http://www.cafepress.com/haskore Haskore t-shirt] (Puttin' the "funk" back in functional programming; temporal media laws on back)<br />
*[http://www.cafepress.com/monadpure "Do it in a Monad ... and remain pure!"] (various shirts with logo on front, monad laws on back)<br />
*[http://www.cafepress.com/haskellgirls infant and toddler girls' wear] (with dancing bear logo; no back yet)<br />
*[http://www.cafepress.com/haskellboys infant and toddler boys' wear] (with juggling bear logo; no back yet)<br />
*[http://www.cafepress.com/haskellhackers Haskell Hackers] t-shirts in the much-requested black color with red and white design. Fritz notes (paraphrased) with regard to this shirt:<br />
<ul><ul>Historically there have been some issues with printing on dark colors as noted by the CafePress people, but our Test Marketing Focus Panel tells me that the design looks good and wears and washes well. Black shirts are a bit pricier, but as always, we sell the shirts at cost. I wouldn't recommend the other (non-black) colors; <i>if you order the shirt (or any other merchandise I've created) and have quality problems, please let me (Fritz) know right away.</i></ul></ul><br />
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== Making new merchandise ==<br />
<br />
If you would like to make other merchandise available, add links here. (You may also mail your ideas/sketches to [mailto:fruehr@willamette.edu Fritz Ruehr], who can provide graphic design work or CafePress preparation work.) <i>If you link in merchandise that is sold above cost, please mark it as such.</i> You need not restrict yourself to CafePress as a manufacturer/distributor. <br />
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= Other =<br />
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== Stamps (Germany) ==<br />
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You can attach your favorite logo to an [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/internetmarke online stamp].<br />
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[[Category:Community]]</div>Kwantam