On 27-Mar-2000, Jan Brosius <[EMAIL PROTECTED]> wrote:
> 1. In Haskell there are 2 sorts of variables : variables that range over values of
>a specific type and "type variables".
>
> e.g.in fact n = n * fact (n - 1) (the factorial function n ranges over the values
>of type Int.
>
> and in the type ST s a where s and a are variables that only range over
>types
>
> Am I correct?
Yes.
> 2. I would first like to come back to the type signature
>
> f :: a -> b
>
> I can say the type of f is a -> b , isn't it?
Well, people often do say that, but it is a little sloppy; if you want
to be precise, it is more correct to say that the type of `f' is
`forall a,b . a -> b'.
> But a and b are both variables. Question
>
> can I replace the General type b by the type c -> d ?
In general that transformation does not preserve type-correctness.
Changing `b' to `c -> d' in the type signature might
change a type-correct program into an ill-typed one.
For example, if the program contains
foo :: Int -> Int
foo x = f x
then changing the type signature of `f' to `f :: a -> c -> d'
would mean that the previously type-correct call to `f' here
would now become a type error.
--
Fergus Henderson <[EMAIL PROTECTED]> | "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh> | of excellence is a lethal habit"
PGP: finger [EMAIL PROTECTED] | -- the last words of T. S. Garp.
