[Haskell-cafe] Folding function composition: L or R?

[Graham Klyne]

At 14:24 07/07/04 +0200, Henning Thielemann wrote:

>On Wed, 7 Jul 2004, Graham Klyne wrote:
>
> > [[
> > sss :: [ShowS]
> > sss = map (++) ["abc","def","ghi","jkl"]
> >
> > ssr = foldr (.) id sss
> > ssl = foldl (.) id sss
> >
> > sr = ssr ""
> > sl = ssl ""
> >
> > -- sl == sr == "abcdefghijkl"
> > -- is there any efficiency difference?
> > ]]
> >
> > Conventional wisdom suggests that foldr is better, but I'm not sure if it
> > really matters in this case.
>
>foldl performs worse, e.g. for infinite lists it will need infinite
>time :-)

Of course!  And your answer makes me notice an important touchstone when 
dealing with such questions:  "what happens with an infinite list?".

Thanks!

#g


------------
Graham Klyne
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