8.10  Substituting inside a signature

(Introduced in OCaml 3.12, generalized in 4.06)

A “destructive” substitution (with ... := ...) behaves essentially like normal signature constraints (with ... = ...), but it additionally removes the redefined type or module from the signature.

Prior to OCaml 4.06, there were a number of restrictions: one could only remove types and modules at the outermost level (not inside submodules), and in the case of with type the definition had to be another type constructor with the same type parameters.

A natural application of destructive substitution is merging two signatures sharing a type name.

 module type Printable = sig
   type t
   val print : Format.formatter -> t -> unit
 end
 module type Comparable = sig
   type t
   val compare : t -> t -> int
 end
 module type PrintableComparable = sig
   include Printable
   include Comparable with type t := t
 end

One can also use this to completely remove a field:

 module type S = Comparable with type t := int
module type S = sig val compare : int -> int -> int end

or to rename one:

 module type S = sig
   type u
   include Comparable with type t := u
 end
module type S = sig type u val compare : u -> u -> int end

Note that you can also remove manifest types, by substituting with the same type.

 module type ComparableInt = Comparable with type t = int ;;
module type ComparableInt = sig type t = int val compare : t -> t -> int end

 module type CompareInt = ComparableInt with type t := int
module type CompareInt = sig val compare : int -> int -> int end