[Haskell-cafe] How to understand the 'forall' ?

[zaxis]

seems a bit understanding, i still need to think it  for a while
thanks!


jkff wrote:
> 
> 2009/9/2 zaxis <z_axis at 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_axis at 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!
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>>>
>>>
>>>
>>> --
>>> Eugene Kirpichov
>>> Web IR developer, market.yandex.ru
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>>
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> 
> 
> 
> -- 
> Eugene Kirpichov
> Web IR developer, market.yandex.ru
> _______________________________________________
> Haskell-Cafe mailing list
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