Module Make.Equality_witness

This module provides support for type equalities and runtime type identifiers.

For two types a and b, (a, b) eq is a witness that a = b. This is a standard generalized algebraic datatype on top of which type-level programming techniques can be implemented.

Given a type a, an inhabitant of a t is a dynamic identifier for a. Identifiers can be compared for equality. They are also equipped with a hash function.

WARNING: the hash function changes at every run. Therefore, the result of the hash function should never be stored.

Notice that dynamic identifiers are not unique: two identifiers for a can have distinct hash and can be physically distinct. Hence, only eq can decide if two type identifiers correspond to the same type.

type (_, _) eq =
  1. | Refl : ('a, 'a) eq

A proof witness that two types are equal.

type 'a t

A dynamic representation for 'a.

val make : unit -> 'a t

make () is a dynamic representation for 'a. A fresh identifier is returned each time make () is evaluated.

val eq : 'a t -> 'b t -> ('a, 'b) eq option

eq ida idb returns a proof that a = b if ida and idb identify the same type.

val hash : 'a t -> int

hash id returns a hash for id.