Type equality witness

type (_, _) eq =
| Equal : ('a, 'a) eq

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

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

Type identifiers

module Id : sig end

Type identifiers.

A type identifier is a value that denotes a type. Given two type identifiers, they can be tested for Type.Id.provably_equal to prove they denote the same type. Note that:

-Unequal identifiers do not imply unequal types: a given type can be denoted by more than one identifier.

-Type identifiers can be marshalled, but they get a new, distinct, identity on unmarshalling, so the equalities are lost.

See an Type.Id.example of use.