为什么`forall`需要在数据定义中具有多态类型？

Question

It seems that I need to explicitly say forall to have a parametric type in data definition. For example, this

data A = A (forall s. ST s (STUArray s Int Int))

will work while this

data A = A (ST s (STUArray s Int Int))

won't.

Maybe I'm asking something too obvious, but the reason for this is not clear to me because in most other cases you don't need an explicit forall to specify a parametric type; the compiler does it instead. So what is the difference here?

Answer 1

There are two different places the forall could go, only one of which is what you intend. The other is

data A = forall s . A (ST s (STUArray s Int Int))

Which is equivalent to the GADT syntax

data A where
  A :: ST s (STUArray s Int Int) -> A

This can be thought of as a box holding an ST state transformer with some s or other—completely useless. But the type checker isn't specially tailored to the needs of ST , and similar types are useful in other contexts.