# TypeCompose

## Contents

## 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://darcs.haskell.org/packages/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.