Chapter 8  Language extensions

7  Recovering the type of a module

(Introduced in OCaml 3.12)

The construction module type of module-expr expands to the module type (signature or functor type) inferred for the module expression module-expr. To make this module type reusable in many situations, it is intentionally not strengthened: abstract types and datatypes are not explicitly related with the types of the original module. For the same reason, module aliases in the inferred type are expanded.

A typical use, in conjunction with the signature-level include construct, is to extend the signature of an existing structure. In that case, one wants to keep the types equal to types in the original module. This can done using the following idiom.

 module type MYHASH = sig
   include module type of struct include Hashtbl end
   val replace: ('a, 'b) t -> 'a -> 'b -> unit
 end

The signature MYHASH then contains all the fields of the signature of the module Hashtbl (with strengthened type definitions), plus the new field replace. An implementation of this signature can be obtained easily by using the include construct again, but this time at the structure level:

 module MyHash : MYHASH = struct
   include Hashtbl
   let replace t k v = remove t k; add t k v
 end

Another application where the absence of strengthening comes handy, is to provide an alternative implementation for an existing module.

 module MySet : module type of Set = struct
   …
 end

This idiom guarantees that Myset is compatible with Set, but allows it to represent sets internally in a different way.