Difference between revisions of "TypeCompose"

Revision as of 13:08, 29 June 2015

Abstract

TypeCompose provides some classes & instances for forms of type composition, as well as some modules that haven't found another home.

Besides this wiki page, here are more ways to find out about TypeCompose:

Visit the Hackage page for library documentation and to download & install.
Or install with cabal install TypeCompose.
Get the code repository: darcs get http://code.haskell.org/~conal/code/TypeCompose.

Type composition

The Control.Compose module includes

Various type compositions (unary/unary, binary/unary, etc). Most are from Applicative Programming with Effects. In particular, g `O` f composes functors in to functors and applicative functors (AFs) into AFs. (In contrast, monads do not in general compose.) Composition makes AF-based programming simple and elegant, partly because we don't need an AF counterpart to monad transformers.
Cofunctors (contravariant functors). Great for "consumer" types, just as functors suit "producer" (container) types. There are several composition options.
Type argument flip. Handy for cofunctors: use Flip (->) o, for (-> o).
Constructor in pairs: (f a, g a).
Constructor in arrows/functions: f a ~> g a.

Other features

Composable bijections

Given all the type constructors and compositions of them, I found myself writing some pretty awkward code to wrap & unwrap through multiple layers. Composable bijections help a lot.

The Data.Bijection module is inspired by There and Back Again: Arrows for Invertible Programming, though done here in a less general setting.

Pair- & function-like types

The Data.Zip and Data.Lambda patterns emerged while working on DeepArrow and Eros. Data.Zip generalizes zip and unzip from [] to other functors. It also provides variants of type f a -> f (a,b) and f a -> f (a,b). Data.Lambda is similar with classes for lambda-like constructions.

For example uses of Pair and Lambda, see TV and Eros.

References

Monads with references. Direct rip-off from Global Variables in Haskell.

Titling

For giving titles to things. I know it sounds kind of random. More useful than I first thought. Used in Phooey, TV, and Eros.

Partial values

A monoid of partial values. See the teaser and solution blog posts.

Context-dependent monoids

Bit of an oddball also. Data.CxMonoid defines a sort of meta-monoid, that can be supplied dynamically with choices of mempty and mappend. Used in Phooey (starting with version 1.3) so that layout could be a monoid but still vary in style.

@@ Line 1: / Line 1: @@
+[[Category:Composition]]
+[[Category:Applicative Functor]]
+[[Category:Libraries]]
+[[Category:Packages]]
+[[Category:Type-level programming]]
 == Abstract ==
-'''TypeCompose''' provides some classes & instances for forms of type composition.  It also includes a very simple implementation of data-driven computation.
+'''TypeCompose''' provides some classes & instances for forms of type composition, as well as some modules that haven't found another home.
+Besides this wiki page, here are more ways to find out about TypeCompose:
-* Read [http://darcs.haskell.org/packages/TypeCompose/doc/html the Haddock docs] (with source code, additional examples, and Comment/Talk links).
+* Visit the [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/TypeCompose Hackage page] for library documentation and to download & install.
-* Get the code repository: '''<tt>darcs get http://darcs.haskell.org/packages/TypeCompose</tt>''', or
+* Or install with <tt>cabal install TypeCompose</tt>.
-* Grab a [http://darcs.haskell.org/packages/TypeCompose/dist distribution tarball].
-* See the [http://darcs.haskell.org/packages/TypeCompose/CHANGES version changes].
+* Get the code repository: <tt>darcs get http://code.haskell.org/~conal/code/TypeCompose</tt>.
+<!--* See the [[TypeCompose/Versions| version history]].-->
-TypeCompose is used in [[Phooey]], a functional GUI library.
 == Type composition ==
+The <hask>Control.Compose</hask> module includes
-For now, see the [http://darcs.haskell.org/packages/TypeCompose/doc/html/Control-Compose.html Haddock docs] and [http://darcs.haskell.org/packages/TypeCompose/doc/html/src.Control.Compose.hs.html source code].
+* Various type compositions (unary/unary, binary/unary, etc).  Most are from [http://www.soi.city.ac.uk/~ross/papers/Applicative.html Applicative Programming with Effects].  In particular, <hask>g `O` f</hask> composes functors in to functors and applicative functors (AFs) into AFs.  (In contrast, monads do not in general compose.)  Composition makes AF-based programming simple and elegant, partly because we don't need an AF counterpart to monad transformers.
+* Cofunctors (contravariant functors).  Great for "consumer" types, just as functors suit "producer" (container) types.  There are several composition options.
+* Type argument flip.  Handy for cofunctors: use <hask>Flip (->) o</hask>, for <hask>(-> o)</hask>.
+* Constructor in pairs: <hask>(f a, g a)</hask>.
+* Constructor in arrows/functions: <hask>f a ~> g a</hask>.
+== Other features ==
+=== Composable bijections ===
+Given all the type constructors and compositions of them, I found myself writing some pretty awkward code to wrap & unwrap through multiple layers.  Composable bijections help a lot.
+The <hask>Data.Bijection</hask> module is inspired by [http://citeseer.ist.psu.edu/alimarine05there.html There and Back Again: Arrows for Invertible Programming], though done here in a less general setting.
-== Data-driven computation ==
+=== Pair- & function-like types ===
-The representation of data-driven computations is quite simple and general.   They have a ''news'' publisher (<hask>news</hask>) and a source of new values (<hask>src</hask>).  Clients interested in the value subscribe to <hask>news</hask> and extract a new value from <hask>src</hask> when notified that the value may have changed.
+The <hask>Data.Zip</hask> and <hask>Data.Lambda</hask> patterns emerged while working on [[DeepArrow]] and [[Eros]].  <hask>Data.Zip</hask> generalizes <hask>zip</hask> and <hask>unzip</hask>  from <hask>[]</hask> to other functors.  It also provides variants of type <hask>f a -> f (a,b)</hask> and <hask>f a -> f (a,b)</hask>.  <hask>Data.Lambda</hask> is similar with classes for lambda-like constructions.
-<haskell>
-type DataDrivenG news src = (,) news `O` src
-</haskell>
+For example uses of <hask>Pair</hask> and <hask>Lambda</hask>, see [[TV]] and [[Eros]].
-Thanks to properties of type composition ([http://darcs.haskell.org/packages/TypeCompose/doc/html/Control-Compose.html#t%3AO <hask>O</hask>]), when <hask>news</hask> is a [http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Monoid.html#t%3AMonoid monoid] and <hask>src</hask> is an [http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Applicative.html#t%3AApplicative applicative functor], <hask>DataDriven news src</hask> is an applicative functor also.  The applicative property is very convenient for composition.
+=== References ===
-To see how our simple definitions manage data-driven computations, expand the <hask>Applicative</hask> instances of <hask>Compose</hask> and <hask>(,) a</hask>:
-<haskell>
-instance (Applicative src) => Applicative (DataDrivenG news src) where
-  pure a = Comp (mempty, pure a)
-  Comp (newsf,srcf) <*> Comp (newsx, srcx) =
-    Comp (newsf `mappend` newsx) (srcf <*> srcx)
-</haskell>
+Monads with references.  Direct rip-off from [http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.23.145 Global Variables in Haskell].
-The idea here is that <hask>mempty</hask> is publisher that never has news to report, while <hask>mappend</hask> combines publishers into one that reports all the news of either.  Thus <hask>DataDrivenG</hask> accumulates event sources as it composes, as well as delegating to whatever composition is done by <hask>src</hask>.
-=== Specializing ===
+=== Titling ===
+For giving titles to things.  I know it sounds kind of random.  More useful than I first thought.  Used in [[Phooey]], [[TV]], and [[Eros]].
-Specializing, introduce types of "updaters" (actions), "sinks" (consumers) of values, and "news publishers" (somewhere to register updaters to be executed when events occur).
+=== Partial values ===
-<haskell>
-type Updater src = src ()
-type Sink src a  = a -> Updater src
-type News src    = Sink src (Updater src)
-</haskell>
+A monoid of partial values.  See the [http://conal.net/blog/posts/a-type-for-partial-values/ teaser] and [http://conal.net/blog/posts/implementing-a-type-for-partial-values/ solution] blog
-And specialize <hask>DataDriven</hask> for news publishers:
+posts.
-<haskell>
-type DataDriven src = DataDrivenG (News src) src
-</haskell>
+=== Context-dependent monoids ===
-For instance, the "Source" types used in [[Phooey]] are defined simply as
-<haskell>
-type Source = DataDriven IO
-</haskell>
+Bit of an oddball also.  <hask>Data.CxMonoid</hask> defines a sort of meta-monoid, that can be supplied dynamically with choices of <hask>mempty</hask> and <hask>mappend</hask>.  Used in [[Phooey]] (starting with version 1.3) so that layout could be a monoid but still vary in style.
-Note that <hask>News src</hask> is a [http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Monoid.html#t%3AMonoid monoid] when <hask>src ()</hask> is.  In particular, given any applicative functor <hask>f</hask>, we can supply the following:
-<haskell>
--- Standard instance: Applicative functor applied to monoid
-instance Monoid a => Monoid (f a) where { mempty = pure mempty; mappend = (*>) }
-</haskell>
-Note that <hask>()</hask> is a monoid.  See an example in the [http://darcs.haskell.org/packages/TypeCompose/doc/html/src.Control.Instances.hs.html <hask>Control.Instances</hask> module].