Module Stdlib.Type

Type introspection.

since 5.1

Type equality witness

type (_, _) eq =

type eq(_, _) =

| Equal : ('a, 'a) eq

| Equal : eq('a, 'a)

;

The purpose of eq is to represent type equalities that may not otherwise be known by the type checker (e.g. because they may depend on dynamic data).

A value of type (a, b) eq represents the fact that types a and b are equal.

If one has a value eq : (a, b) eq that proves types a and b are equal, one can use it to convert a value of type a to a value of type b by pattern matching on Equal:

  let cast (type a) (type b) (Equal : (a, b) Type.eq) (a : a) : b = a

let cast = (type a, type b, Equal: Type.eq(a, b), a: a): b => a;

At runtime, this function simply returns its second argument unchanged.

Type identifiers

module Id : sig ... end

module Id: { ... };

Type identifiers.