Edsko de Vries wrote:
>While all of that is true, it still does not explain why all occurrences of a
>type variable need the same attribute. For instance, there is absolutely no
>problem with
>
>second :: *a .a -> *a
>second x y = x
I think you are right, but I am not expert in type systems. I also
don't like this restriction.
However, I doubt that removing the same attribute restriction will
improve the type system much. For example, if someone would
define a function 'f' with type
f :: a -> *a | Eq a
he would probably expect that the typechecker will accept:
g x = f (f x)
but because '*a' cannot be coerce to 'a' this is not allowed.
This might be more confusing for a programmer than a simple attribute
restriction.
The main problem with this restriction seems to occur in combination
with overloading. For example, it prevents class definitions like:
class T a where
g :: a -> *a
Maybe this can be fixed by allowing only instances of types that
can be coerced (something similar is implemented for the Array class).
Kind regards,
John van Groningen
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