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[[Category:Composition]]
[[Category:Applicative]]
[[Category:Libraries]]
[[Category:Packages]]
[[Category:Type-level programming]]
== 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 [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/TypeCompose Hackage page] for library documentation and to download & install.
* Or install with <tt>cabal install TypeCompose</tt>.
* Get the code repository: <tt>darcs get http://darcs.haskell.org/packages/TypeCompose</tt>.
<!--* See the [[TypeCompose/Versions| version history]].-->
== Type composition ==
The <hask>Control.Compose</hask> module includes
* 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.
=== Pair- & function-like types ===
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.
For example uses of <hask>Pair</hask> and <hask>Lambda</hask>, see [[TV]] and [[Eros]].
=== References ===
Monads with references. Direct rip-off from [http://citeseer.ist.psu.edu/473734.html 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 [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
posts.
=== Context-dependent monoids ===
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.