Add flipped fmap

Isaac Dupree ml at
Wed Apr 7 13:31:23 EDT 2010

On 04/07/10 09:58, Felipe Lessa wrote:
>     <$$>  will be as concise and useful as<$>, maybe even more,
>     because `flip fmap` isn't valid.

This is the annoying part.  Aw, Haskell!  I suppose we could name the 
combinator `flipfmap`... and it would be clear what it meant...

I disagree with the consistency argument.  I don't think <*> -> <**> is 
a very good precedent.  It has confused me.  Far more people are 
familiar with functor than applicative (so this precedent would be 
applied mostly to an audience who's unfamiliar with it).  >>= -> =<< , 
although perhaps silly, at least looks enough like a mirrored version 
that you can guess...

I'd say "let's fix the fixity of fmap!" except we can't because Prelude 
exports it.

Besides, IMHO fixity isn't too big a deal, because one can use 
parentheses and they're rather less confusing to read than things being 
in the "wrong" order or expressions using symbol-operators that the 
reader doesn't know.  I've used `fmap`, infix like that, sometimes... 
although more recently I've come to think that it's not necessary to use 
infix operators at all for clarity. A thought: for
>  do m1
>     m2
>     m3 <$$> bigPieceOfCodeThatVisuallyBreaks
>               theSequentialNatureOfADoExpression
how about using
   do m1
      flip fmap m3 $ bigPieceOfCodeThatVisuallyBreaks
(not sure though.. I like using "forM_" more than "flip mapM_"..)


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