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From: Jun Furuse <jun.furuse@gmail.com>
To: oleg@okmij.org, caml-list <caml-list@inria.fr>
Subject: Re: [Caml-list] [ANN] ppx_monadic.1.0.2, ppx for monadic do, pattern
Date: Sun, 18 Jan 2015 22:47:16 +0800	[thread overview]
Message-ID: <CAAoLEWvdRt=nVRKD7d2fVvxeLujurV4+jMaEkm6VWg+oQHrdoQ@mail.gmail.com> (raw)
In-Reply-To: <20150114084056.140F6C38A1@www1.g3.pair.com>

Thanks,

let! is not valid in the vanilla syntax therefore I guess you have
patched the compiler.

Current OCaml syntax has shortage of pattern binding expression and
only usable is let%xxx p = e in which is a bit pain for ppx writers.

The pattern part of   p <-- e  is actually an expression and you
cannot simply write real patterns like   (x, y) as z. You still can
write it using an extension like [%p? (x,y) as z] but it is lousy...

Jun


On Wed, Jan 14, 2015 at 4:40 PM,  <oleg@okmij.org> wrote:
>
> 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.)
>

  reply	other threads:[~2015-01-18 14:47 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-13 14:23 [Caml-list] [ANN] ppx_monadic.1.0.2, ppx for monadic do, pattern guards and monadic comprehension Jun Furuse
2015-01-14  8:40 ` [Caml-list] [ANN] ppx_monadic.1.0.2, ppx for monadic do, pattern oleg
2015-01-18 14:47   ` Jun Furuse [this message]
2015-01-19  7:33     ` [Caml-list] [ANN] ppx_monadic.1.0.2, ppx for monadic do, oleg
2015-01-19  8:34       ` Alain Frisch
2015-01-19  9:06         ` Gabriel Scherer
2015-01-19  9:40           ` Alain Frisch
2015-01-19 16:10             ` Jeremy Yallop
2015-01-19  8:56     ` [Caml-list] [ANN] ppx_monadic.1.0.2, ppx for monadic do, pattern Alain Frisch
2015-01-19 21:52       ` Drup
2015-01-20  3:53         ` Jun Furuse

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