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	id 140F6C38A1; Wed, 14 Jan 2015 03:40:56 -0500 (EST)
From: oleg@okmij.org
cc: jun.furuse@gmail.com
To: caml-list@inria.fr
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Message-Id: <20150114084056.140F6C38A1@www1.g3.pair.com>
Date: Wed, 14 Jan 2015 03:40:56 -0500 (EST)
Subject: Re: [Caml-list] [ANN] ppx_monadic.1.0.2, ppx for monadic do,
 pattern


Jun Furuse wrote:
> * Monadic do notation:  do_;
> * Pattern guards:  when p <-- e -> ..

This reminds me: MetaOCaml incorporates the let! notation suggested by
Nicolas Pouillard and implemented by Alain Frisch. The following is
the example of using the notation, excerpted from the file
        metalib/test/pythagorian_triples.ml
in the BER MetaOCaml distribution/source. 

(* The example uses left recursion and truly infinite streams! *)
(* Don't try this in Prolog or in Haskell's MonadPlus. *)

let rec numb () = 			(* infinite stream of integers *)
    yield (mplus (let! n = numb in ret (n+1))         (* left recursion! *)
	       (ret 0)) ()
;;

let pyth : (int * int * int) NonDet.stream =
  let! i  = numb in
  let! () = guard (i>0) in
  let! j  = numb in
  let! () = guard (j>0) in
  let! k  = numb in
  let! () = guard (k>0) in
  (* Just to illustrate the `let' form within let! *)
  let test x = x*x = j*j + k*k in
  let! () = guard (test i) in
  ret (i,j,k)
;;

let [(5, 4, 3); (5, 3, 4); (10, 8, 6); (10, 6, 8); (13, 12, 5); (13, 5, 12);
     (15, 12, 9); (15, 9, 12); (17, 15, 8); (17, 8, 15)]
 =
run 10 pyth;;


(The file also implements the non-determinism monad that is
illustrated in the example.)