newtype (empty) → 0.1

raw patch · 4 files changed

+159/−0 lines, 4 filesdep +basesetup-changed

Dependencies added: base

Files

• Control/Newtype.hs +107/−0
• LICENSE +30/−0
• Setup.hs +2/−0
• newtype.cabal +20/−0

@@ -0,0 +1,107 @@+{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, TypeFamilies #-}+{- |+The 'Newtype' typeclass and related functions: 'op', 'ala', 'ala'', 'under'. Primarly pulled from Conor McBride's Epigram work. Some examples:++@ala Sum foldMap [1,2,3,4] -- foldMaps the list ala the Sum newtype. This results in 10.@++@ala Product foldMap [1,2,3,4] -- foldMaps the list ala the Product newtype. This results in 24.@++@ala Endo foldMap [(+1), (+2), (subtract 1), (*2)] 3 -- foldMaps the list ala the Endo newtype. This results in 8.@++NB: 'Data.Foldable.foldMap' is a generalized @mconcatMap@ which is a generalized @concatMap@.++This package includes 'Newtype' instances for all the (non-GHC\/foreign) newtypes in base (as seen in the examples). However, there are neat things you can do with this with /any/ newtype and you should definitely define your own 'Newtype' instances for the power of this library. For example, see @ala Cont traverse@, with the proper 'Newtype' instance for Cont.+-}+module Control.Newtype ( Newtype(..), op, ala, ala', under ) where++import Data.Monoid+import Control.Applicative+import Control.Arrow++-- | Given a newtype @n@, we will always have the same unwrapped type @o@, meaning we can represent this with a fundep @n -> o@.+--+-- Any instance of this class just needs to let @pack@ equal to the newtype's constructor, and let @unpack@ destruct the newtype with pattern matching.+class Newtype n o | n -> o where+  pack :: o -> n+  unpack :: n -> o++{-+This would be nice, but it breaks in odd ways with GHC < 7.+Oh, and it also makes the instances an extra line longer. :(+class Newtype n where+  type Orig n+  pack :: Orig n -> n+  unpack :: n -> Orig n+-}++-- |+-- This function serves two purposes:+--+-- 1. Giving you the unpack of a newtype without you needing to remember the name.+--+-- 2. Showing that the first parameter is /completely ignored/ on the value level, meaning the only reason you pass in the constructor is to provide type information. Typeclasses sure are neat.+op :: Newtype n o => (o -> n) -> n -> o+op _ = unpack++-- | The workhorse of the package. Given a pack and a \"higher order function\", it handles the packing and unpacking, and just sends you back a regular old function, with the type varying based on the hof you passed.+--+-- The reason for the signature of the hof is due to 'ala' not caring about structure. To illustrate why this is important, another function in this package is 'under'. It is not extremely useful; @under2@ might be more useful (with e.g., @mappend@), but then we already digging the trench of \"What about @under3@? @under4@?\". The solution utilized here is to just hand off the \"packer\" to the hof. That way your structure can be imposed in the hof, whatever you may want it to be (e.g., List, Traversable). +ala :: (Newtype n o, Newtype n' o') => (o -> n) -> ((o -> n) -> b -> n') -> (b -> o')+ala pa hof = ala' pa hof id++-- | This is the original function seen in Conor McBride's work. The way it differs from the 'ala' function in this package, is that it provides an extra hook into the \"packer\" passed to the hof. However, this normally ends up being @id@, so 'ala' wraps this function and passes @id@ as the final parameter by default. If you want the convenience of being able to hook right into the hof, you may use this function.+ala' :: (Newtype n o, Newtype n' o') => (o -> n) -> ((a -> n) -> b -> n') -> (a -> o) -> (b -> o')+ala' _ hof f = unpack . hof (pack . f)++-- | A very simple operation involving running the function \'under\' the newtype. Suffers from the problems mentioned in the 'ala' function's documentation.+under :: (Newtype n o, Newtype n' o') => (o -> n) -> (n -> n') -> (o -> o')+under _ f = unpack . f . pack+++instance Newtype All Bool where+  pack = All+  unpack (All a) = a++instance Newtype Any Bool where+  pack = Any+  unpack (Any a) = a++instance Newtype (Sum a) a where+  pack = Sum+  unpack (Sum a) = a++instance Newtype (Product a) a where+  pack = Product+  unpack (Product a) = a++instance Newtype (WrappedMonad m a) (m a) where+  pack = WrapMonad+  unpack (WrapMonad a) = a++instance Newtype (WrappedArrow a b c) (a b c) where+  pack = WrapArrow+  unpack (WrapArrow a) = a++instance Newtype (ZipList a) [a] where+  pack = ZipList+  unpack (ZipList a) = a++instance Newtype (Const a x) a where+  pack = Const+  unpack (Const a) = a++instance Newtype (Endo a) (a -> a) where+  pack = Endo+  unpack (Endo a) = a++instance Newtype (First a) (Maybe a) where+  pack = First+  unpack (First a) = a++instance Newtype (Last a) (Maybe a) where+  pack = Last+  unpack (Last a) = a++instance ArrowApply a => Newtype (ArrowMonad a b) (a () b) where+  pack = ArrowMonad+  unpack (ArrowMonad a) = a

@@ -0,0 +1,30 @@+Copyright (c)2011, Darius Jahandarie++All rights reserved.++Redistribution and use in source and binary forms, with or without+modification, are permitted provided that the following conditions are met:++    * Redistributions of source code must retain the above copyright+      notice, this list of conditions and the following disclaimer.++    * Redistributions in binary form must reproduce the above+      copyright notice, this list of conditions and the following+      disclaimer in the documentation and/or other materials provided+      with the distribution.++    * Neither the name of Darius Jahandarie nor the names of other+      contributors may be used to endorse or promote products derived+      from this software without specific prior written permission.++THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

@@ -0,0 +1,2 @@+import Distribution.Simple+main = defaultMain

@@ -0,0 +1,20 @@+Name:                newtype+Version:             0.1+Synopsis:            A typeclass and set of functions for working with newtypes.+Description:         Per Conor McBride, the Newtype typeclass represents the packing and unpacking of a newtype, and allows you to operatate under that newtype with functions such as ala.+License:             BSD3+License-file:        LICENSE+Author:              Darius Jahandarie, Conor McBride+Maintainer:          Darius Jahandarie <djahandarie at gmail>+-- Copyright:           +Category:            Control+Build-type:          Simple+-- Extra-source-files:  +Cabal-version:       >=1.2++Library+  Exposed-modules:     Control.Newtype+  Build-depends:       base >= 3.0 && < 6+  -- Other-modules:       +  -- Build-tools:         +  Ghc-options: -Wall