diff --git a/Makefile b/Makefile new file mode 100644 --- /dev/null +++ b/Makefile @@ -0,0 +1,1 @@ +include ../conal-cabal-make.inc diff --git a/README b/README new file mode 100644 --- /dev/null +++ b/README @@ -0,0 +1,17 @@ +ZipFold is a small package zipping folds, as described in a collection of +blog posts, , inspired by the post "Beautiful Folds" by Max Rabkin +. + +Please share any comments & suggestions on the discussion (talk) page at +[1]. + +You can configure, build, and install all in the usual way with Cabal +commands. + + runhaskell Setup.lhs configure + runhaskell Setup.lhs build + runhaskell Setup.lhs install + +References: + +[1] http://haskell.org/haskellwiki/ZipFold diff --git a/Setup.lhs b/Setup.lhs new file mode 100644 --- /dev/null +++ b/Setup.lhs @@ -0,0 +1,3 @@ +#!/usr/bin/env runhaskell +> import Distribution.Simple +> main = defaultMain diff --git a/ZipFold.cabal b/ZipFold.cabal new file mode 100644 --- /dev/null +++ b/ZipFold.cabal @@ -0,0 +1,35 @@ +Name: ZipFold +Version: 0.0 +Cabal-Version: >= 1.2 +Synopsis: Zipping folds +Category: Data +Description: + ZipFold is a small package zipping folds, as described in a collection of + blog posts, , inspired by a post by Max Rabkin + . + . + Project wiki page: + . + The module documentation pages have links to colorized source code and + to wiki pages where you can read and contribute user comments. Enjoy! + . + © 2008 by Conal Elliott; BSD3 license. +Author: Conal Elliott +Maintainer: conal@conal.net +Homepage: http://haskell.org/haskellwiki/ZipFold +Package-Url: http://code.haskell.org/ZipFold +Copyright: (c) 2008 by Conal Elliott +License: BSD3 +Stability: experimental +build-type: Simple + +Library + hs-Source-Dirs: src + Extensions: + Build-Depends: base, TypeCompose + Exposed-Modules: + Data.Zip.FoldL + + ghc-options: -Wall + +-- ghc-prof-options: -prof -auto-all diff --git a/src/Data/Zip/FoldL.hs b/src/Data/Zip/FoldL.hs new file mode 100644 --- /dev/null +++ b/src/Data/Zip/FoldL.hs @@ -0,0 +1,114 @@ +{-# LANGUAGE ExistentialQuantification, FlexibleContexts #-} +{-# OPTIONS_GHC -Wall #-} +---------------------------------------------------------------------- +-- | +-- Module : Data.Zip.FoldL +-- Copyright : (c) Conal Elliott 2008 +-- License : BSD3 +-- +-- Maintainer : conal@conal.net +-- Stability : experimental +-- +-- Zipping of non-strict left folds. +-- +-- Based on "Beautiful Folds" by Max Rabkin +-- +-- +-- See also +---------------------------------------------------------------------- + +module Data.Zip.FoldL + ( Fold(..), cfoldl, cfoldl' + , WithCont , FoldC , cfoldlc + , WithCont', FoldC', cfoldlc' + , Pair'(..), P + ) where + +import Data.Monoid +import Control.Applicative +import Data.List (foldl') + +import Data.Pair + +-- | Data representation of a left fold +data Fold b a = F (a -> b -> a) a + +-- | Interpretation of a 'Fold' as non-strict +cfoldl :: Fold b a -> [b] -> a +cfoldl (F op e) = foldl op e + +-- Non-strict left-fold zipping +zipF :: Fold b a -> Fold b a' -> Fold b (a,a') +F op e `zipF` F op' e' = F op'' (e,e') + where + (a,a') `op''` b = (a `op` b, a' `op'` b) + +instance Pair (Fold b) where pair = zipF + +-- | Interpretation of a 'Fold' as non-strict +cfoldl' :: Fold b a -> [b] -> a +cfoldl' (F op e) = foldl' op e + +-- Strict left-fold zipping +zipF' :: Fold b a -> Fold b a' -> Fold b (P a a') +F op e `zipF'` F op' e' = F op'' (P e e') + where + P a a' `op''` b = P (a `op` b) (a' `op'` b) + + +instance Pair' (Fold b) where pair' = zipF' + + +-- | Add a continuation. +data WithCont h b c = forall a. WC (h b a) (a -> c) + +instance Functor (WithCont h b) where + fmap g (WC f k) = WC f (fmap g k) + +instance Pair (h b) => Applicative (WithCont h b) where + pure a = WC (error "unneeded pre-cont") (pure a) + WC f k <*> WC f' k' = + WC (f `pair` f') (\ (a,a') -> (k a) (k' a')) + + +-- | Non-strict left fold with continuation. +type FoldC = WithCont Fold + +-- | Interpretation of a 'FoldC' +cfoldlc :: FoldC b a -> [b] -> a +cfoldlc (WC f k) = fmap k (cfoldl f) + + + +-- | Like 'WithCont' but with pair-strict '(<*>)' +data WithCont' h b c = forall a. WC' (h b a) (a -> c) + +instance Functor (WithCont' h b) where + fmap g (WC' f k) = WC' f (fmap g k) + +instance Pair' (h b) => Applicative (WithCont' h b) where + pure a = WC' (error "unneeded pre-cont") (pure a) + WC' f k <*> WC' f' k' = + WC' (f `pair'` f') (\ (P a a') -> (k a) (k' a')) + +-- | Strict left fold with continuation. +type FoldC' = WithCont' Fold + +-- | Interpretation of a 'FoldC' +cfoldlc' :: FoldC' b a -> [b] -> a +cfoldlc' (WC' f k) = fmap k (cfoldl' f) + + +---- + +-- | Strict pairs +data P c c' = P !c !c' + +-- | Strict generalized pair +class Pair' f where + pair' :: f a -> f b -> f (P a b) + +instance Pair' [] where pair' = liftA2 P +instance Monoid u => Pair' ((,) u) where pair' = liftA2 P +instance Pair' ((->) u) where pair' = liftA2 P +instance Pair' IO where pair' = liftA2 P diff --git a/wikipage.tw b/wikipage.tw new file mode 100644 --- /dev/null +++ b/wikipage.tw @@ -0,0 +1,13 @@ +[[Category:Packages]] + +== Abstract == + +'''ZipFold''' is a small package zipping folds, as described in a [http://conal.net/blog/tag/zip collection of blog posts' and inspired by the post [http://squing.blogspot.com/2008/11/beautiful-folding.html "Beautiful Folds"] by Max Rabkin. + +Besides this wiki page, here are more ways to find out about ZipFold: +* Visit the [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ZipFold Hackage page] for library documentation and to download & install. +* Or install with cabal install ZipFold. +* Get the code repository: darcs get http://code.haskell.org/ZipFold. + + +Please leave comments at the [[Talk:ZipFold|Talk page]].