How do I write higher-order functions that take polymorphic functions as arguments in Typed Racket?

Question

For example, how can I write a version of map that will work with polymorphic functions in Typed Racket? I use a simple id function defined as:

Answer 1

You have to manually instantiate the polymorphism in this case:

->  (map (inst identity Integer) '(1 2 3))
- : (Listof Integer) [more precisely: (Pairof Integer (Listof Integer))]
'(1 2 3)

The reason is explained in the Typed Racket Guide here:

Typed Racket’s local type inference algorithm is currently not able to infer types for polymorphic functions that are used on higher-order arguments that are themselves polymorphic.

(see docs for more explanation and examples)