ANN: pa_polyrec: syntax for polymorphic recursion

[Jeremy Yallop <yallop@gmail.com>]

I'm pleased to announce the initial release of pa_polyrec, a syntax extension 
for polymorphic recursion in OCaml.

    https://forge.ocamlcore.org/projects/pa-polyrec/

There are several methods for encoding polymorphic-recursive functions in OCaml; 
this extension allows them to be written directly, using a natural syntax.  For 
example, given the following type of perfectly-balanced trees we might wish to 
write a function for summing the leaves.

   type 'a perfect = Zero of 'a | Succ of ('a * 'a) perfect

In standard OCaml such a function can be written as follows:

   let sump f =
     (object (o)
         method sump : 'a. ('a -> int) -> 'a perfect -> int =
           fun f -> function
            | Zero x -> f x
            | Succ x -> o#sump (fun (a, b) -> f a + f b) x
      end)#sump f

   let sum_perfect = sump id

Using pa_polyrec one can write the function in the following less obfuscated style:

   let rec sump : 'a. ('a -> int) -> 'a perfect -> int =
     fun f -> function
      | Zero x -> f x
      | Succ x -> sump (fun (a, b) -> f a + f b) x

   let sum_perfect = sump id

Note that the type variable 'a in the type of the function is quantified: this 
is what differentiates polymorphic-recursive functions from standard OCaml 
recursive function definitions.

More complex usage is supported, including mutual recursion.  A number of 
examples are included in the distribution, including several modules from Chris 
Okasaki's thesis "Purely Functional Data Structures" and code from Richard Bird 
and Ross Paterson's paper "de Bruijn notation as a nested datatype".