[Haskell-cafe] What's the difference?

[Paul Johnson]

PR Stanley wrote:
> Hi
> What is the difference between
>
>     data T0 f a = MkT0 a
>     instance Eq (T0 f a) where ...
>
>         and
>
>     data T0 f a = MkT0 a
>     instance Eq a => Eq (T0 f a) where ...
>
The second one says that "TO f a" is only an instance of "Eq" if "a" is, 
while the first says that "TO f a" is an instance regardless of the type 
of its arguments.


You can regard an "instance" declaration as an inference rule for the 
type checker, with "=>" meaning "implies" (though I don't think its the 
answer to your other question about names).