Am Freitag, 10. Juli 2009 23:41 schrieben Sie: > On Jul 10, 2009, at 4:35 AM, Wolfgang Jeltsch wrote: > > I fear that this instance doesn’t satisfy required laws. As far as > > I know, the following equalities should hold: > > > > (*>) = (>>) > > > > f *> empty = empty > > > > empty <|> g = g > > > > This implies the following: > > > > (f >> empty) <|> g = g > > > > But this wouldn’t hold with your instance. (f >> empty) <|> g would > > cause the side effects of f and of g, while g would (obviously) only cause > > the side effects of g. > > I think the third equality you provide is too strong (which isn't to > say that it might not be the law that people have documented and > expect). Lots of useful alternative instances fail it, not least any > parser combinator library (such as Parsec) without automatic > backtracking.
Really? The third equality is required since Alternative instances have to be monoids with empty as the neutral element and (<|>) as composition. > […] > Additionally, the second equality you provide is just wrong. > > f *> empty = empty is no more true than f *> g = g, I don’t understand this. The equation f *> g = g is much more general than f *> empty = empty. (<|>) usually denotes non-determinism and empty should be the neutral element of non-determinism, which is failing. This leads me to f *> empty = empty. > […] Best wishes, Wolfgang _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
