Why does a partial application have value restriction?

Question

I can understand that allowing mutable is the reason for value restriction and weakly polymorphism. Basically a mutable ref inside a function may change the type in

Answer 1

The value restriction is pretty simple: only let-bound expressions that are syntactically values are generalized. Applications, including partial applications, are not values and thus are not generalized.

Note that in general it is impossible to tell whether an application is partial, and thus whether the application could have an effect on the value of a reference cell. Of course in this particular case it is obvious that no such thing occurs, but the inference rules are designed to be sound in the event that it does.