# TypeCompose

### From HaskellWiki

m (→Type composition: "O" --> ":.") |
m (→Pair- & function-like types) |
||

Line 39: | Line 39: | ||

=== Pair- & function-like types === | === Pair- & function-like types === | ||

− | The <hask>Data.Pair</hask> and <hask>Data.Lambda</hask> patterns emerged while working on [[DeepArrow]] and [[Eros]]. <hask>Data.Pair</hask> generalizes <hask>zip</hask> and | + | The <hask>Data.Pair</hask> and <hask>Data.Lambda</hask> patterns emerged while working on [[DeepArrow]] and [[Eros]]. <hask>Data.Pair</hask> generalizes <hask>zip</hask> and unzip 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]]. | For example uses of <hask>Pair</hask> and <hask>Lambda</hask>, see [[TV]] and [[Eros]]. |

## Revision as of 22:30, 18 December 2007

## Contents |

## 1 Abstract

*Warning*: The Haddock docs are out of date. I'm trying to get a working haddock 2.0 running (on my windows machine).

**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 learn about TypeCompose:

- Read the Haddock docs (with source code, additional examples, and Comment/Talk links).
- Get the code repository:
, or`darcs get http://darcs.haskell.org/packages/TypeCompose` - Grab a distribution tarball.
- See the version history.
- See the use of TypeCompose in DataDriven.

Please leave comments at the Talk page.

## 2 Type composition

The- Various type compositions (unary/unary, binary/unary, etc). Most are from Applicative Programming with Effects. In particular, 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.g :. f
- 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 , forFlip (->) o.(-> o)
- Constructor in pairs: .(f a, g a)
- Constructor in arrows/functions: .f a ~> g a

## 3 Other features

### 3.1 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### 3.2 Pair- & function-like types

The### 3.3 References

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

### 3.4 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.

### 3.5 Partial values

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