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Let me guess...you've read every other guide, tutorial, lesson and introduction and none of them have helped - you still don't understand I/O in Haskell.
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Alright then - have a look at this:
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data OI -- abstract, primitive
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partOI :: OI -> (OI, OI) --
getchar :: OI -> Char -- primitives
putchar :: Char -> OI -> () --
seq :: a -> b -> b -- also primitive
instance Partible OI where ...
class Partible a where
part :: a -> (a, a)
parts :: a -> [a]
No up-front explanation; I'm guessing you've seen more than enough of those, so I'm trying something different. I will explain it later...
Yes, of course there's more to Haskell I/O than <code>getchar</code> and <code>putchar</code>; I've downsized it for convenience. If you want, you can add the rest afterwards...
Yes, they're somewhat arcane, but they can be used to emulate all the classic approaches to I/O in Haskell, albeit in miniature:
-- for GHC 8.6.5
{-# LANGUAGE BangPatterns #-}
module ClassicIO where
import Prelude(Char, String)
import Prelude(($), (.))
import Data.List(map, foldr, zipWith)
import OutputInput
import Partible
-- simple text --
{- main :: (String -> String) -}
runMain_text :: (String -> String) -> OI -> ()
runMain_text main = \u -> let !(u1, u2) = part u in
putchars (main (getchars u1)) u2
getchars :: OI -> String
getchars = map getchar . parts
putchars :: String -> OI -> ()
putchars s = foldr seq () . zipWith putchar s . parts
-- dialogues --
{- main :: Dialogue -}
runMain_dial :: Dialogue -> OI -> ()
runMain_dial main = \u -> foldr seq () $ yet $
\l -> zipWith respond (main l) (parts u)
type Dialogue = [Response] -> [Request]
data Request = Getq | Putq Char
data Response = Getp Char | Putp
yet :: (a -> a) -> a
yet f = f (yet f)
respond :: Request -> OI -> Response
respond Getq = \u -> case getchar u of c -> Getp c
respond (Putq c) = \u -> seq (putchar c u) Putp
-- continuations --
{- main :: (() -> IOResult) -> IOResult -}
runMain_cont :: ((() -> IOResult) -> IOResult) -> OI -> ()
runMain_cont main = call (main done)
newtype IOResult = R (OI -> ())
call :: IOResult -> OI -> ()
call (R a) = a
done :: () -> IOResult
done () = R $ \ u -> part u `seq` ()
getchar_cont :: (Char -> IOResult) -> IOResult
getchar_cont k = R $ \u -> let !(u1, u2) = part u in
let !c = getchar u1 in
call (k c) u2
putchar_cont :: Char -> (() -> IOResult) -> IOResult
putchar_cont c k = R $ \u -> let !(u1, u2) = part u in
seq (putchar c u) (call (k ()) u2)
-- state-passing --
{- main :: IOState -> ((), IOState) -}
runMain_stat :: (IOState -> ((), IOState)) -> OI -> ()
runMain_stat main = \u -> seq (main (ini_st u)) ()
newtype IOState = S OI
ini_st :: OI -> IOState
ini_st = S
getchar_stat :: IOState -> (Char, IOState)
getchar_stat (S u) = let !(u1, u2) = part u in
let !c = getchar u1 in
(c, S u2)
putchar_stat c (S u) = let !(u1, u2) = part u in
seq (putchar c u1) ((), S u2)
-- and those weird, fickle things ;-)
{- main :: IO () -}
runMain_wfth :: IO () -> OI -> ()
runMain_wfth main = main
type IO a = OI -> a
getchar_wfth :: IO Char
getchar_wfth = getchar
putchar_wfth :: Char -> IO ()
putchar_wfth = putchar
unit :: a -> IO a
unit x = \u -> part u `seq` x
bind :: IO a -> (a -> IO b) -> IO b
bind m k = \u -> case part u of
(u1, u2) -> (\x -> x `seq` k x u2) (m u1)
Here are examples for each of those approaches:
module Echoes where
import Prelude(String, Char(..), Eq(..))
import Prelude(($))
import ClassicIO
import OutputInput(runOI)
echo_text :: String -> String
echo_text (c:cs) = if c == '\n' then [] else c : echo_text cs
echo_dial :: Dialogue
echo_dial p = Getq :
case p of
Getp c : p' ->
if c == '\n' then
Putq c :
case p' of
Putp : p'' -> echo_dial p''
echo_cont :: (() -> IOResult) -> IOResult
echo_cont k = getchar_cont $ \c ->
if c == '\n' then
k ()
putchar_cont c (\_ -> echo_cont k)
echo_wfth :: IO ()
echo_wfth = getchar_wfth `bind` \c ->
if c == '\n' then
unit ()
putchar_wfth c `bind` \_ -> echo_wfth
What was that - using <code>Prelude.seq</code> that way won't work in Haskell 2010? ''You are correct!''
This should work as expected[1][2]:
-- for GHC 8.6.5
{-# LANGUAGE MagicHash, UnboxedTuples #-}
module Sequential(seq) where
import GHC.Base(seq#, realWorld#)
infixr 0 `seq`
seq :: a -> b -> b
x `seq` y = case seq# x realWorld# of
(# s, _ #) -> case seq# y s of
(# _, t #) -> t
It didn't work? Try this instead:
-- for GHC 8.6.5
#define during seq
module Sequential(seq) where
import qualified Prelude(during)
import GHC.Base(lazy)
infixr 0 `seq`
seq :: a -> b -> b
seq x y = Prelude.during x (lazy y)
As for those extensions - they stay with each definition.
That still didn't work? Well, give this a try:
yet :: (a -> a) -> a
yet f = y where y = f y
Now that we're firmly on the topic of implementation details, did you notice how easy it was to define that allegedly ''warm, fuzzy''[3] <code>IO</code> type using this curious new <code>OI</code> type, and those primitives?
Sometimes that can be a hint that doing the opposite will be difficult or even impossible to do while staying within standard Haskell 2010. Unfortunately, this is one of those cases...
To define <code>OI</code>, <code>partOI</code>, <code>getchar</code> and <code>putchar</code> will require:
* modifying your preferred Haskell implementation - lots of work;
* using some other language for the definitions, with Haskell then calling the foreign code - extra work to deal with two different languages;
* using unsafe or implementation-specific primitives - work needed to avoid conflicts with Haskell semantics;
* using implementation-specific extensions - work needed to track relevant extensions, and possible conflicts with Haskell semantics.
For now, I'll just use the extensions - they're ugly, but at least they're contained, just like those alternate definitions of <code>seq</code>. But who knows - if this approach to I/O proves useful enough, it might make its way into a future Haskell standard...that's how <code>IO</code> happened[4].
In the meantime, take a deep breath:
-- for GHC 8.6.5
{-# LANGUAGE BangPatterns, MagicHash, UnboxedTuples #-}
module OutputInput(OI, runOI, seq, getchar, putchar) where
import Prelude(Char, String)
import Prelude(($), (++), putChar, getChar, error)
import Partible
import Sequential
import GHC.Base(IO(..), State#, MutVar#, RealWorld)
import GHC.Base(seq#, realWorld#, newMutVar#, atomicModifyMutVar#)
data OI = OI OI#
instance Partible OI where
part = partOI
partOI :: OI -> (OI, OI)
partOI (OI r) = case expire# "partOI" r realWorld# of
s -> case newMutVar# () s of
(# s', r1 #) ->
case newMutVar# () s' of
(# _, r2 #) -> (OI r1, OI r2)
runOI :: (OI -> a) -> IO a
runOI g = IO $ \s -> case newMutVar# () s of
(# s', r #) -> seq# (g (OI r)) s'
getchar :: OI -> Char
getchar (OI r) = case expire# "getchar" r realWorld# of
s -> case undo# getChar s of
(# _, c #) -> c
putchar :: Char -> OI -> ()
putchar c (OI r) = case expire# "putchar" r realWorld# of
s -> case undo# (putChar c) s of
(# _, x #) -> x
-- Local definitions
type OI# = MutVar# RealWorld ()
expire# :: String -> MutVar# s () -> State# s -> State# s
expire# name r s = case atomicModifyMutVar# r flick s of
(# s', _ #) -> s'
flick :: () -> (a, ())
flick x@() = (error nowUsed, x)
nowUsed = name ++ ": argument already used"
undo# :: IO a -> State# RealWorld -> (# State# RealWorld, a #)
undo# (IO a) = a
Now you can start breathing again :-)
module Partible where
class Partible a where
part :: a -> (a, a)
parts :: a -> [a]
-- Minimal complete definition: part or parts
part u = case parts u of u1:u2:_ -> (u1, u2)
parts u = case part u of (u1, u2) -> u1 : parts u2
If you remember, I dispensed with an up-front explanation to try something different. Now that you've
seen just how different this all is, here's the explanation...
That abstract <code>partOI</code> and its overloaded associates <code>part</code> and <code>parts</code>? They help an optimising Haskell implementation to determine when it's safe to use those optimisations. Consider this definition:
testme n = n^2 + n^2
One simple optimisation would be to replace the duplicates of <code>n^2</code> with a single, shared local definition:
testme n = let x = n^2 in x + x
This definition:
main' u = putchars "ha" u `seq` putchars "ha" u
would likewise be rewritten, with the result being:
main' u = let x = putchars "ha" u in x `seq` x
but, as noted by Philip Wadler[5]:
<blockquote>''[...] the laugh is on us: the program prints only a single <code>"ha"</code>, at the time variable <br><code>x</code> is bound. In the presence of side effects, equational reasoning in its simplest form <br>becomes invalid.''</blockquote>
''Equational reasoning'' is the basis for that simple optimisation and many others in implementations like GHC - so far they've been serving us quite well.
What - just treat I/O-centric definitions as some special case by modifying GHC? Haskell implementations like GHC are complicated enough as is!
The problem is being caused by the code being treated as though it's pure, so let's modify the code instead. In this case, the easiest solution is to make all calls to I/O-centric definitions unique:
main u = let !(u1, u2) = part u in
putchars "ha" u1 `seq` putchars "ha" u2
But what about:
oops g h u = g u `seq` h u
main' = oops (putchars "ha") (putchars "ha")
Will the laugh be on us, again?
This is Haskell, not Clean[6] - there are no uniqueness types to help fend off such potentially-troublesome expressions. For now, the simplest way to make sure <code>OI</code> values are only used once is to have the implementation treat their reuse as being invalid e.g. by throwing an exception or raising an error to stop the offending program.
In the prototype implementation, the maintenance of this all-important ''single-use'' property is performed by <code>expire#</code>.
Now for the much-maligned[7] <code>seq</code>...you could be tempted into avoiding its use by using a new data type:
newtype Result a = Is a
getchar :: OI -> Result Char
putchar :: Char -> OI -> Result ()
and case-expressions:
respond' :: Request -> OI -> Response
respond' Getq = \u -> case getchar u of Is c -> Getp c
respond' (Putq c) = \u -> case putchar c u of Is _ -> Putp
But before you succumb:
unit_Result :: a -> Result a
unit_Result = Is
bind_Result :: Result a -> (a -> Result b) -> Result b
bind_Result (Is x) k = k x
Oh look - <code>Result</code> is one of '''those''' types...
The bang-pattern extension? So you can instead write:
respond'' :: Request -> OI -> Response
respond'' Getq = \u -> let !c = getchar u in Getp c
respond'' (Putq c) = \u -> let !z = putchar c u in Putp
As you can see, <code>z</code> isn't used anywhere - there is no need for it. This being Haskell, if it isn't needed, it isn't evaluated - that allows implementations like GHC to rewrite a definition like <code>respond''</code> as:
respond'' :: Request -> OI -> Response
respond'' Getq = \u -> let !c = getchar u in Getp c
respond'' (Putq c) = \u -> Putp
You could try all manner of ways to avoid using <code>seq</code> - you might even find one that you like; all well and good...but others might not. For me, the simplest way I've found to make this approach to I/O work is with <code>seq</code> - one that's actually sequential.
But maybe - after all that - you still want <code>seq</code> banished from Haskell. Perhaps you still don't understand I/O in Haskell. It could be that you're dismayed by what you've read here. Alternately, you may have seen or tried this all before, and know it doesn't work - darn...
If that's you, the corresponding language proposal[8] has a list of other articles and research papers I've found which describe or refer to other approaches - perhaps one (or more) of them will be more acceptable.
As noted by Owen Stephens[9]:
<blockquote>''I/O is not a particularly active area of research, but new approaches are still being discovered, <br>iteratees being a case in point.''</blockquote>
Who knows - the Haskell language could return to having a pure, fully-defined approach to I/O...and it could be you that finds it :-D
P.S: Why the name <code>OI</code>? Many years ago I was tinkering with arrows for performing I/O, labelling them <code>OI a b</code> out of expediency. More recently, I discovered a set of slides[10] describing another approach to I/O which used values of type <code>OI a</code> in a similar fashion to what I've been describing here. I've reused the name because of that similarity.
[1] [[Sequential ordering of evaluation]]; Haskell Wiki.<br>
[2] [https://gitlab.haskell.org/ghc/ghc/-/issues/5129 Ticket# 5129: "evaluate" optimized away]; GHC bug tracker.<br>
[3] [https://www.cs.nott.ac.uk/~pszgmh/appsem-slides/peytonjones.ppt Wearing the hair shirt: a retrospective on Haskell]; Simon Peyton Jones.<br>
[4] [https://citeseerx.ist.psu.edu/viewdoc/download?doi= A History of Haskell: being lazy with class]; Paul Hudak, John Hughes, Simon Peyton Jones and Philip Wadler.<br>
[5] [https://citeseerx.ist.psu.edu/viewdoc/download?doi= How to Declare an Imperative]; Philip Wadler.<br>
[6] [https://clean.cs.ru.nl/Clean The Clean homepage]; Radboud University, Nijmegen, The Netherlands.<br>
[7] [https://mail.haskell.org/pipermail/haskell/2002-May/009622.html Thread: State monads don't respect the monad laws in Haskell]; Haskell mail archive.<br>
[8] [[Partibles for composing monads]]; Haskell Wiki.<br>
[9] [https://www.owenstephens.co.uk/assets/static/research/masters_report.pdf Approaches to Functional I/O]; Owen Stephens.<br>
[10] <span style="color:#ba0000">Non-Imperative Functional Programming</span>; Nobuo Yamashita.<br>
[[User:Atravers|Atravers]] 03:05, 20 August 2020 (UTC)

Latest revision as of 22:54, 16 April 2021

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