Why, then, not permit such definitions only for members of Eq? My thoughts: - because it will break an important invariant indicated by Martin, namely the one that says that pattern variables are not forced - it will make efficient compilation of pattern matching much harder - it will make the termination behavior of pattern matching dependent not only on the term being matched, but also on how Eq is implemented for that type - it will make the very rules of evaluation for pattern matching much more complex, fragile and hard to understand: in what order should the equated parts be evaluated? That influences termination behavior, so it is important. In what order should equality comparisons be performed in f (Node a (Leaf b a) (Node a))) = b ?
2009/5/15 Salvador Lucas <slu...@dsic.upv.es>: > Dear Martin, > > I think that the (practical) reason is avoiding equality checks during > pattern matching. > > For instance, how do you evaluate this: > > foo ((+1):(1+):[]) ? > > Both expressions in the first and second entries of the list are > semantically equivalent, > but from an operational point of view, you have to ensure the equality of > two functions > over an infinite domain (of integer numbers). > > Best regards, > > Salvador. > > > Martin Hofmann escribió: >> >> It is pretty clear, that the following is not a valid Haskell pattern: >> >> foo (x:x:xs) = x:xs >> >> My questions is _why_ this is not allowed. IMHO, the semantics should be >> clear: The pattern is expected to succeed, iff 'x' is each time bound to >> the same term. >> Isn't this allowed, because this would require a strict evaluation of >> the 'x' variables? >> >> Thanks, >> >> Martin >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe@haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Eugene Kirpichov Web IR developer, market.yandex.ru _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
