TypeCompose
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: git clone git@github.com:conal/TypeCompose.git.
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.