TypeCompose

From HaskellWiki

Contents 1 Abstract 2 Type composition 3 Other features 3.1 Composable bijections 3.2 Pair- & function-like types 3.3 References 3.4 Titling 3.5 Partial values 3.6 Context-dependent monoids

1 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://code.haskell.org/TypeCompose.
• See the version history.

2 Type composition

The 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
  .

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 module is inspired by There and Back Again: Arrows for Invertible Programming, though done here in a less general setting.

3.2 Pair- & function-like types

The and patterns emerged while working on DeepArrow and Eros. generalizes and from to other functors. It also provides variants of type and . is similar with classes for lambda-like constructions. For example uses of and , see TV and Eros.

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.

3.6 Context-dependent monoids

Bit of an oddball also. defines a sort of meta-monoid, that can be supplied dynamically with choices of and . Used in Phooey (starting with version 1.3) so that layout could be a monoid but still vary in style.

Categories: Composition | Applicative | Libraries | Packages