add Control.Monad.Instances to nhc98 build

[haskell-directory.git] / Data / Traversable.hs

diff --git a/Data/Traversable.hs b/Data/Traversable.hs
index 6754094..c2c6033 100644 (file)
--- a/Data/Traversable.hs
+++ b/Data/Traversable.hs
@@ -8,7 +8,8 @@
 -- Stability   :  experimental
 -- Portability :  portable
 --
--- Class of data structures that can be traversed from left to right.
+-- Class of data structures that can be traversed from left to right,
+-- performing an action on each element.
 --
 -- See also
 --
@@ -20,9 +21,16 @@
 --    by Jeremy Gibbons and Bruno Oliveira,
 --    in /Mathematically-Structured Functional Programming/, 2006, and online at
 --    <http://web.comlab.ox.ac.uk/oucl/work/jeremy.gibbons/publications/#iterator>.
+--
+-- Note that the functions 'mapM' and 'sequence' generalize "Prelude"
+-- functions of the same names from lists to any 'Traversable' functor.
+-- To avoid ambiguity, either import the "Prelude" hiding these names
+-- or qualify uses of these function names with an alias for this module.
 
 module Data.Traversable (
        Traversable(..),
+       for,
+       forM,
        fmapDefault,
        foldMapDefault,
        ) where
@@ -100,6 +108,16 @@ instance Ix i => Traversable (Array i) where
 
 -- general functions
 
+-- | 'for' is 'traverse' with its arguments flipped.
+for :: (Traversable t, Applicative f) => t a -> (a -> f b) -> f (t b)
+{-# INLINE for #-}
+for = flip traverse
+
+-- | 'forM' is 'mapM' with its arguments flipped.
+forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b)
+{-# INLINE forM #-}
+forM = flip mapM
+
 -- | This function may be used as a value for `fmap` in a `Functor` instance.
 fmapDefault :: Traversable t => (a -> b) -> t a -> t b
 fmapDefault f = getId . traverse (Id . f)