name:                catamorphism
version:             0.6.1.1
synopsis:            Exposes a Template Haskell function for generating catamorphisms.
description:
  This module exposes a 'makeCata' function which can create catamorphisms for
  arbitrary Haskell types. Catamorphisms are functions which deconstruct some
  value by replacing each data constructor with a custom function yielding a
  new value. See <http://www.haskell.org/haskellwiki/Catamorphisms> for a more
  in-depth discussion of catamorphisms in Haskell.
  .
  The Haskell base package already comes with a couple of standard
  catamorphisms, such as 'maybe' (for 'Maybe' values). The 'maybe' function
  could have been generated using 'makeCata' as follows:
  .
  > -- Defines 'maybe :: b -> (a -> b) -> Maybe a -> b'
  > $(makeCata defaultOptions ''Maybe)
  .
  However, catamorphisms are especially useful for recursive data structures.
  Consider the following simple example which defines a basic data type for
  modelling sums of numbers, supporting variables:
  .
  > import Data.Morphism.Cata
  > import Data.Maybe (fromJust)
  >
  > data Expr a = Number a
  >             | Variable Char
  >             | Sum (Expr a) (Expr a)
  >
  > -- Defines 'expr :: (a -> b) -> (Char -> b) -> (b -> b -> b) -> Expr a -> b'
  > $(makeCata defaultOptions ''Expr)
  .
  The 'makeCata' invocation defines a 'expr' function which works like a fold on
  'Expr' values; it can be used to implement various useful other functions:
  .
  > -- Evaluate an Expr, given some variable bindings
  > eval :: Num a => [(Char, a)] -> Expr a -> a
  > eval vars = expr id (fromJust . (`lookup` vars)) (+)
  >
  > -- Pretty-prints an Expr
  > pprint :: Show a => Expr a -> String
  > pprint = expr show show (\a b -> a ++ " + " ++ b)
  >
  > -- Counts the number of variables used in an expr
  > numVars :: Expr a -> Int
  > numVars = expr (const 1) (const 0) (+)
homepage:            https://github.com/frerich/catamorphism
license:             BSD3
license-file:        LICENSE
author:              Frerich Raabe
maintainer:          frerich.raabe@gmail.com
bug-reports:         https://github.com/frerich/catamorphism/issues
copyright:           Copyright (c) 2014, 2015, 2016, 2017, 2018 Frerich Raabe <frerich.raabe@gmail.com>
category:            Development
build-type:          Simple
cabal-version:       >=1.10
stability:           experimental

source-repository head
  type:       git
  location:   https://github.com/frerich/catamorphism

library
  exposed-modules:     Data.Morphism.Cata
  build-depends:       base >=4.6 && <4.12, template-haskell >=2.8 && <2.14
  hs-source-dirs:      src
  default-language:    Haskell2010
  ghc-options:         -Wall

test-suite spec
  type:                exitcode-stdio-1.0
  main-is:             Tests.hs
  hs-source-dirs:      test
  ghc-options:         -Wall
  build-depends:       base, catamorphism, hspec
  other-modules:       Data.Morphism.CataSpec
  default-language:    Haskell2010