---
name: "Type"
section: "3o"
platform: "linux"
package: "ocaml"
version: "5.5.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/Type"
---

# Type(3o)

Type introspection.

## NAME

Type - Type introspection.

## Module

Module   Type

## Documentation

Module **Type**\
  :  **sig end**

Type introspection.

**Since** 5.1

### 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.