[Haskell-cafe] Re: Newbie question

[Brent Yorgey]

2008/1/21 Alexander Seliverstov <seliverstov.a at gmail.com>:

> So, the function type "(Num a)=>Integer->a" means that return value of
> this function can be cast to any particular instance of class Num.
>
> Ok. I have a my own class "class A a" and want to write function like
> this  "f:: (A a)=>Integer->a". Can I do it?
>
>
Only if you make the function f a member of class A.  Then it is the
responsibility of each particular type which is an instance of A to define a
way of converting an Integer into a value of that type.  For example:

class A a where
  f :: Integer -> a
  ... other functions...

instance A Integer where
  f = id

instance A String where
  f = show

and so on.

-Brent
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