X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Data%2FEither.hs;h=1c12897eaad3230d83b9111be1e0d4a3d6b82f9e;hb=6685444335fe57d5d86b61965989e45f34fddf0e;hp=406e7e7e3bb162dbf8be697f4b8bad69ef3391fa;hpb=2b09d1369ead1a675a916a3c928e0b72c169c3fe;p=ghc-base.git

diff --git a/Data/Either.hs b/Data/Either.hs
index 406e7e7..1c12897 100644
--- a/Data/Either.hs
+++ b/Data/Either.hs
@@ -1,4 +1,5 @@
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# LANGUAGE CPP, NoImplicitPrelude #-}
+
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Either
@@ -15,14 +16,28 @@
 
 module Data.Either (
    Either(..),
-   either	-- :: (a -> c) -> (b -> c) -> Either a b -> c
+   either,           -- :: (a -> c) -> (b -> c) -> Either a b -> c
+   lefts,            -- :: [Either a b] -> [a]
+   rights,           -- :: [Either a b] -> [b]
+   partitionEithers, -- :: [Either a b] -> ([a],[b])
  ) where
 
+#include "Typeable.h"
+
 #ifdef __GLASGOW_HASKELL__
 import GHC.Base
+import GHC.Show
+import GHC.Read
 #endif
 
-#ifndef __HUGS__
+import Data.Typeable
+
+#ifdef __GLASGOW_HASKELL__
+{-
+-- just for testing
+import Test.QuickCheck
+-}
+
 {-|
 
 The 'Either' type represents values with two possibilities: a value of
@@ -33,9 +48,47 @@ either correct or an error; by convention, the 'Left' constructor is
 used to hold an error value and the 'Right' constructor is used to
 hold a correct value (mnemonic: \"right\" also means \"correct\").
 -}
-data  Either a b  =  Left a | Right b	deriving (Eq, Ord )
+data  Either a b  =  Left a | Right b   deriving (Eq, Ord, Read, Show)
 
+-- | 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
-#endif  /* __HUGS__ */
+#endif  /* __GLASGOW_HASKELL__ */
+
+INSTANCE_TYPEABLE2(Either,eitherTc,"Either")
+
+-- | Extracts from a list of 'Either' all the 'Left' elements
+-- All the 'Left' elements are extracted in order.
+
+lefts   :: [Either a b] -> [a]
+lefts x = [a | Left a <- x]
+
+-- | Extracts from a list of 'Either' all the 'Right' elements
+-- All the 'Right' elements are extracted in order.
+
+rights   :: [Either a b] -> [b]
+rights x = [a | Right a <- x]
+
+-- | 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)
+
+{-
+{--------------------------------------------------------------------
+  Testing
+--------------------------------------------------------------------}
+prop_partitionEithers :: [Either Int Int] -> Bool
+prop_partitionEithers x =
+  partitionEithers x == (lefts x, rights x)
+-}
+