From: Brent Yorgey <byorgey@LVN513-12.cis.upenn.edu>
Date: Fri, 15 Apr 2011 19:22:54 +0000 (-0400)
Subject: Update comment on fmapDefault to note that it only works for Traversable instances... 
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=2c764e4266aedeea872abad159db0af62b0f26fb;p=ghc-base.git

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.
---

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)