Add partitionEithers, lefts, and rights.

Patch from Russell O'Connor, trac proposal #974.

diff --git a/Data/Either.hs b/Data/Either.hs
index 38766ad..f64c3e4 100644 (file)
--- a/Data/Either.hs
+++ b/Data/Either.hs
@@ -15,12 +15,22 @@
 
 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
 
+import Data.Tuple
+
 #ifdef __GLASGOW_HASKELL__
 import GHC.Base
 
+{-
+-- just for testing
+import Test.QuickCheck
+-}
+
 {-|
 
 The 'Either' type represents values with two possibilities: a value of
@@ -40,3 +50,36 @@ either                  :: (a -> c) -> (b -> c) -> Either a b -> c
 either f _ (Left x)     =  f x
 either _ g (Right y)    =  g y
 #endif  /* __GLASGOW_HASKELL__ */
+
+-- | 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)
+-}
+