[Haskell-cafe] Re: Why is $ right associative insteadofleftassociative?

Paul Hudak paul.hudak at yale.edu
Sun Feb 5 20:16:29 EST 2006

Chris Kuklewicz wrote:
> Brian Hulley wrote:
>>Ben Rudiak-Gould wrote:
>>>... but no more confusing than the fact that [f x | x <- xs] is
>>>not the same as (map f xs).
>>Can you explain why? On page 258 of Paul Hudak's book "The Haskell
>>School of Expression" he states that do x<- xs; return (f x) is
>>equivalent to [f x | x <- xs] "which is clearly just map f xs"
>>I can't find anything wrong with the example in the book but perhaps
>>I've missed something?
> He may mean that if you *redefine* the operator Prelude.((:)) then the
> desugaring and other steps may end up binding the old or the new (:) and no
> longer be identical.  This is touched on in
> http://www.haskell.org/ghc/docs/6.4.1/html/users_guide/syntax-extns.html#rebindable-syntax
> In particular, if you redefine Monad, then [ f x | x<-xs ] and do {x<-xs; return
> x} may no longer mean the same thing.

Right, but the original question is whether or not [f x | x <- xs] is 
the same as map f xs.  My book's been out for six years and no one has 
mentioned this issue, so if it's a problem I'd like to know why so that 
I can add it to my "Errata" list!

Thanks,  -Paul

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