Add a "with" syntax for patterns

Git branch: with-patterns

This project is to add a "with" syntax for pattern guards to OCaml. This has previously been implemented in the ocaml-patterns preprocessor.

A with guard is similar to a when guard except that instead of matching an expression against true it can match an expression against any pattern. This also allows with guards to bind variables for use in the case's expression.

Examples

The following example calls lookup on the matched x and then checks that the result matches Some v. This v can then be used in the case expression.

match e with
  Var x with Some v = lookup x env -> v
| Var x -> failwith ("Unbound variable " ^ x)
| Const v -> v

With guards can also be used with or-patterns to bind variables:

match x with
  Foo(y, z)
| Bar y with z = 3 -> y + z

or similarly:

let f (Some x | None with x = 0) = x

Things to do

Add with guards to the parser and parsetree

Add a with <pattern> = <expression> production to the parser. This should be placed wherever a when guard is allowed. This will require adding a contructor for with guards to the parsetree.

Add with guards to or-patterns in the parser

with guards should also be allowed on or-patterns, even though when guards are not allowed on them . We will only allow exhaustive patterns for with guards on or-patterns, so the reasons for not allowing when guards on then do not apply.

Add translations for with guards to Typecore

Typecore is where the guards must be typechecked and translated into the typedtree.

Add check for exhaustivity of or-pattern with guards

The exhaustivity checker (parmatch.ml) should be used to check that with guards on or-patterns are exhaustive.

Add with guards to the pattern matching engine

The actual compilation of with guards would be done in bytecomp/matching.ml. This file is complicated, and it helps to read the paper that is referenced in the comments at the top of the file.