Re: [Haskell-cafe] Optimization with Strings ?

Am Donnerstag, den 03.12.2009, 16:23 +0100 schrieb Emmanuel CHANTREAU:

Le Thu, 3 Dec 2009 13:20:31 +0100, David Virebayre a écrit :

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It doesn't work this way : Strings are just lists of Chars. Comparison is made recursively, Char by Char. You can have a look at the source to make sure :

instance (Eq a) => Eq [a] where [] == [] = True (x:xs) == (y:ys) = x == y && xs == ys _xs == _ys = False

Hello

Thank you David and Bulat for your answers.

I don't see the proof you see. Because GHC could store two sames objects juste once and by the definition of == on lists it could deduce that "forall x; List x => x==x". GHC have all informations to do this optimization job, because haskell functions definitions are mathematics definitions.

This does not always hold, because the equivalence known from mathematics differs from Haskell's strict equality when it comes to infinite or undefined values. Consider let x = repeat () in x == x and let x = error "oops" in x == x In both cases your optimized program would return True, although the value is undefined (bottom).