[Haskell-cafe] Re: Why is $ right
associative insteadofleftassociative?
Chris Kuklewicz
haskell at list.mightyreason.com
Sun Feb 5 15:41:38 EST 2006
Brian Hulley wrote:
> Ben Rudiak-Gould wrote:
>> Paul Hudak wrote:
>>> Minor point, perhaps, but I should mention that : is not special
>>> syntax -- it is a perfectly valid infix constructor.
>>
>> > <snip>
>> ... 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.
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