Difference between revisions of "TypeCompose"
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== Abstract ==
== Abstract ==
Revision as of 13:35, 8 December 2008
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/TypeCompose.
- See the version history.
Control.Compose module includes
- Various type compositions (unary/unary, binary/unary, etc). Most are from Applicative Programming with Effects. In particular,
g `O` fcomposes 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
- Constructor in pairs:
(f a, g a).
- Constructor in arrows/functions:
f a ~> g a.
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.
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
Data.Lambda patterns emerged while working on DeepArrow and Eros.
 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.
Monads with references. Direct rip-off from Global Variables in Haskell.
Bit of an oddball also.
Data.CxMonoid defines a sort of meta-monoid, that can be supplied dynamically with choices of
mappend. Used in Phooey (starting with version 1.3) so that layout could be a monoid but still vary in style.