2009/9/2 zaxis <z_a...@163.com>: > > Isnot it clear without the 'forall' ? > data Branch tok st a = Branch (PermParser tok st (b -> a)) (GenParser tok st > b) > > thanks! >
The situation is not so simple. Consider, for example, a procedure that takes as input a *generic* sorting algorithm: sortThem :: (forall a. Ord a => [a] -> [a]) -> [Int] -> [String] -> ([Int], [String]) sortThem sortAlgo ints strings = (sortAlgo ints, sortAlgo strings) Here you can't omit the 'forall' because if you do, then inside the body of the sortThem function the 'a' type variable has a fixed value and can't be both Int and String! This is a somewhat esoteric example, but one could consider, for instance, a procedure that takes as input some trees of different types and a generic tree traversal algorithm. Well, have a look at the haskellwiki page, there's a pile of examples :) P.S. I tried to write up the difference between datatype and function declarations in this respect, but my explanations turned into a mess, so I erased them in the hope that someone will explain it better than me. > > jkff wrote: >> >> This means that for any type 'b' you can construct a value of type >> 'Branch tok st a' by passing to Branch an argument of type >> '(PermParser tok st (b -> a))' and 'GenParser tok st b'. >> This also means that when you're given a value of type Branch tok st >> a, you don't know what that 'b' type was; the only thing you know is >> that the 'b' in 'b -> a' in the first argument of Branch is the same >> as the 'b' in 'GenParser tok st b'. >> >> See also: the haskellwiki page on existential types. >> >> 2009/9/2 zaxis <z_a...@163.com>: >>> >>> data Branch tok st a = forall b. Branch (PermParser tok st (b -> a)) >>> (GenParser tok st b) >>> >>> please shed a light on me, thanks! >>> -- >>> View this message in context: >>> http://www.nabble.com/How-to-understand-the-%27forall%27---tp25250783p25250783.html >>> Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. >>> >>> _______________________________________________ >>> 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 >> >> > > -- > View this message in context: > http://www.nabble.com/How-to-understand-the-%27forall%27---tp25250783p25251783.html > Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. > > _______________________________________________ > 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