Re: [Haskell-beginners] Type classes are not like interfaces, after all

2009/1/23 Francesco Bochicchio :

2009/1/23 Jan Jakubuv

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hi,

2009/1/23 Francesco Bochicchio :

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The problem whith your implementation of 'a'

a = 3 :: Integer

is that it provides too specific result. Its type signature says that its result has to be of the type n for *any* instance of the class Num. But your result is simply Integer that is just *one* specific instance of Num. In other words it has to be possible to specialize ("retype") 'a' to any other instance of Num, which is no longer possible because (3 :: Integer) is already specialized.

Uhm. Now I think I start to get it ... You are saying that if a value is a Num, it shall be possible to convert it in _any_ of the num instances?

Well, in this case of the above constant yes. The main point here is that when you want to implement a function of a type say (Num a => a -> a) then the implementation has to work for *all* instances of the class Num. Usually you can use only "abstract" functions defined in a class declaration to write such functions. Try to start with some function that mentions the quantified type in one of its arguments. They are easier to understand. Constants like (Num a => a) and functions like (Num a => Bool -> a) are rare (also they have a special name I can not recall ;-). Also note that you can take: a :: (Num t) => t a = 3 and then specialize it: spec = (a :: Integer) Sincerely, jan.