Update: Jake Donham has updated this package to work with OCaml 3.12: http://github.com/jaked/patterns

A framework for writing extensions to OCaml pattern matching. Examples included with the distribution:

Pattern-matching for lazy values

Conjunctive patterns

Conjunctive patterns (as found in F#) generalise "as"-patterns. In standard OCaml the syntax `patt as var' may be used to bind a value simultaneously to both a pattern and a variable; with conjunctive patterns the syntax `patt & patt' may be used to bind a value simultaneously to two patterns. For example,

          let ((`A a, b) & (c, `B d)) = (`A 3, `B 4) in (a,b,c,d)

evaluates to

         (3, `B 4, `A 3, 4)

Object patterns

Object patterns bind the results of calling an object's methods to other patterns during a pattern match. This makes it more convenient to use objects as structurally-typed records. The notation mirrors that in Jacques Garrigue's pa_oo extension. For example,

          let {| x = x; y = _ |} =
             object method x = 3 method y = 4 method z = 5 end
          in
             x + 1

evaluates to

          4

Negative patterns

Matching with negative patterns succeeds if the value does not match the pattern given. For example,

         let nonzero = function
            | ~0 -> true
            | _  -> false
         in (nonzero 4, nonzero 0)

evaluates to

         (true, false)

N+K patterns

The infamous n+k patterns (as found in Haskell) offer a "Peano-number" view of integers. Matching an integer value v against `patt+k' (where k is an integer literal) succeeds, binding patt to v-k, if v>=k. For example

         let pred = function
            | x+1 -> x
            | 0   -> 0
         in (pred 10, pred 0)

evaluates to

         (9, 0) 

Release 0.3 included

• pattern guards

which are gone for now, but should reappear in a future release.

Links:

• Download source
• Documentation
• Announcement (v 0.1)
• Announcement (v 0.2)
• Announcement (v 0.3)
• Announcement (v 0.4)