How to make these dynamically typed functions type-safe? [closed]

Answer 1

1# This is not typeable with a finite type. This means that very few (if any) programming languages will be able to type this.

However, as you have demonstrated, there is a specific type for x that allows the function to be typed:

x :: t -> B

Where B is some concrete type. This results in apply being typed as:

apply :: (t -> B) -> B

Note that Hindley-Milner will not derive this type.

2# This is easy to represent in Haskell (left as an exercise to the reader...)