{-# LANGUAGE TemplateHaskell, ScopedTypeVariables, TypeFamilies, GADTs, RankNTypes, UndecidableInstances, DataKinds, PolyKinds #-} ----------------------------------------------------------------------------- -- | -- Module : Data.Singletons.Prelude.Either -- Copyright : (C) 2013-2014 Richard Eisenberg, Jan Stolarek -- License : BSD-style (see LICENSE) -- Maintainer : Ryan Scott -- Stability : experimental -- Portability : non-portable -- -- Defines functions and datatypes relating to the singleton for 'Either', -- including a singletons version of all the definitions in @Data.Either@. -- -- Because many of these definitions are produced by Template Haskell, -- it is not possible to create proper Haddock documentation. Please look -- up the corresponding operation in @Data.Either@. Also, please excuse -- the apparent repeated variable names. This is due to an interaction -- between Template Haskell and Haddock. -- ---------------------------------------------------------------------------- module Data.Singletons.Prelude.Either ( -- * The 'Either' singleton Sing(SLeft, SRight), -- | Though Haddock doesn't show it, the 'Sing' instance above declares -- constructors -- -- > SLeft :: Sing a -> Sing (Left a) -- > SRight :: Sing b -> Sing (Right b) SEither, -- | 'SEither' is a kind-restricted synonym for 'Sing': -- @type SEither (a :: Either x y) = Sing a@ -- * Singletons from @Data.Either@ either_, Either_, sEither_, -- | The preceding two definitions are derived from the function 'either' in -- @Data.Either@. The extra underscore is to avoid name clashes with the type -- 'Either'. Lefts, sLefts, Rights, sRights, PartitionEithers, sPartitionEithers, IsLeft, sIsLeft, IsRight, sIsRight, -- * Defunctionalization symbols LeftSym0, LeftSym1, RightSym0, RightSym1, Either_Sym0, Either_Sym1, Either_Sym2, Either_Sym3, LeftsSym0, LeftsSym1, RightsSym0, RightsSym1, IsLeftSym0, IsLeftSym1, IsRightSym0, IsRightSym1 ) where import Data.Singletons.Prelude.Instances import Data.Singletons.Prelude.Base import Data.Singletons.Single -- NB: The haddock comments are disabled because TH can't deal with them. $(singletons [d| -- Renamed to avoid name clash -- -| Case analysis for the 'Either' type. -- If the value is @'Left' a@, apply the first function to @a@; -- if it is @'Right' b@, apply the second function to @b@. either_ :: (a -> c) -> (b -> c) -> Either a b -> c either_ f _ (Left x) = f x either_ _ g (Right y) = g y |]) $(singletonsOnly [d| -- -| Extracts from a list of 'Either' all the 'Left' elements -- All the 'Left' elements are extracted in order. -- Modified to avoid list comprehensions lefts :: [Either a b] -> [a] lefts [] = [] lefts (Left x : xs) = x : lefts xs lefts (Right _ : xs) = lefts xs -- -| Extracts from a list of 'Either' all the 'Right' elements -- All the 'Right' elements are extracted in order. -- Modified to avoid list comprehensions rights :: [Either a b] -> [b] rights [] = [] rights (Left _ : xs) = rights xs rights (Right x : xs) = x : rights xs -- -| Partitions a list of 'Either' into two lists -- All the 'Left' elements are extracted, in order, to the first -- component of the output. Similarly the 'Right' elements are extracted -- to the second component of the output. partitionEithers :: [Either a b] -> ([a],[b]) partitionEithers = foldr (either_ left right) ([],[]) where left a (l, r) = (a:l, r) right a (l, r) = (l, a:r) -- -| Return `True` if the given value is a `Left`-value, `False` otherwise. -- -- /Since: 4.7.0.0/ isLeft :: Either a b -> Bool isLeft (Left _) = True isLeft (Right _) = False -- -| Return `True` if the given value is a `Right`-value, `False` otherwise. -- -- /Since: 4.7.0.0/ isRight :: Either a b -> Bool isRight (Left _) = False isRight (Right _) = True |])