[commit: base] master: Update comment on fmapDefault to note that it only works for Traversable instances with a given definition of traversable, and NOT for instances with only sequenceA given. (2c764e4)

[Ian Lynagh]

Repository : ssh://darcs.haskell.org//srv/darcs/packages/base

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/2c764e4266aedeea872abad159db0af62b0f26fb

>---------------------------------------------------------------

commit 2c764e4266aedeea872abad159db0af62b0f26fb
Author: Brent Yorgey <byorgey at LVN513-12.cis.upenn.edu>
Date:   Fri Apr 15 15:22:54 2011 -0400

    Update comment on fmapDefault to note that it only works for Traversable instances with a given definition of traversable, and NOT for instances with only sequenceA given.

>---------------------------------------------------------------

 Data/Traversable.hs |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/Data/Traversable.hs b/Data/Traversable.hs
index 2bdc1bc..062d1a0 100644
--- a/Data/Traversable.hs
+++ b/Data/Traversable.hs
@@ -170,7 +170,10 @@ instance Applicative (StateR s) where
 mapAccumR :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c)
 mapAccumR f s t = runStateR (traverse (StateR . flip f) t) s
 
--- | This function may be used as a value for `fmap` in a `Functor` instance.
+-- | This function may be used as a value for `fmap` in a `Functor`
+--   instance, provided that 'traverse' is defined. (Using
+--   `fmapDefault` with a `Traversable` instance defined only by
+--   'sequenceA' will result in infinite recursion.)
 fmapDefault :: Traversable t => (a -> b) -> t a -> t b
 {-# INLINE fmapDefault #-}
 fmapDefault f = getId . traverse (Id . f)