Hi all,
I have made a small extension to OCaml to allow open datatypes (like exn)
to be created by the programmer. A patch and more details are available at
http://sites.google.com/site/ocamlopen/.
The basic syntax is:
open type foo
open type 'a bar
extend foo with Foo1 of int
extend foo with Foo2 of string
extend 'a bar with Bar of int * 'a
Open GADTs can also be created (with two possible syntaxes):
open type 'a gadt
extend 'a gadt with A : int -> int gadt
extend string gadt with B of string
Although open types are in some ways similar to polymorphic variants, they
are more similar to ordinary variants:
* Constructors for open types can be used anywhere that an ordinary
variant
constructor can be used.
* Constructors for open types have a single type, which can make them
less
prone to errors than polymorphic variants
* Constructors for open types are associated with modules, so they can
be
hidden by signatures.
* Open types can be Generalized Abstract Data Types
My original motivation for this was wanting to allow arbitrary data to be
attached onto various points of a very large data structure (an AST). This
can be done using local exceptions, but I wanted these values to have their
own type. I think it might also be useful, when using GADTs to represent
types, to allow these representations to be extended for user defined
types.
Regards,
Leo
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