Re: [Caml-list] [ANN] ppx_monadic.1.0.2, ppx for monadic do, pattern

[Jun Furuse <jun.furuse@gmail.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.)
>