seems a bit understanding, i still need to think it for a while thanks!
jkff wrote: > > 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 > > -- View this message in context: http://www.nabble.com/How-to-understand-the-%27forall%27---tp25250783p25254016.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