Polymorphism and Let-Generalization

The most visible “HM feature” in Rex is that let bindings can be reused at multiple types.

If you’re new to HM languages, this is the big shift from “everything has one type” to “some definitions are generic”.

A polymorphic helper

let id = \x -> x in
  (id 1, id true, id "hi")

Why lambdas aren’t generalized

Inside a lambda body, parameters are monomorphic unless you explicitly abstract:

\f ->
  let x = f 1 in
    f x

If f were required to work at multiple unrelated types, that would be rejected.

Practical implication

If you want something reusable, let-bind it at the outer level:

let
  id = \x -> x,
  use = \x -> (id x, id x)
in
  use 1

Practical tip

If a definition should be reusable, prefer let-binding it and giving it a clear name (and often a type annotation).

In practice, you’ll use:

• let for reusable helpers,
• lambdas for “inline glue” (callbacks passed to map, foldl, bind, …),
• fn for API-like top-level functions with stable signatures.