Re: [Caml-list] pattern matching on mapped lists

[Octachron <octa@polychoron.fr>]

On 06/03/15 19:42, Mikhail Mandrykin wrote:
> It seems GADT encoding mostly works here, but the absence of local 
> opens in patterns and redefinition of [] constructor and [_; _] 
> special syntax for lists currently makes the use of such encoding 
> inconvenient e.g.:
>
> module Flist =
> struct
>
>   type z = Z
>
>   type 'a s = S of 'a
>
>   type ('a, _) t =
>     | Nil : ('a, z) t
>     | :: : 'a * ('a, 'b) t ->  ('a, 'b s) t
>
>     let rec map : type a. _ -> (_, a) t -> (_, a) t = fun f ->
>        function
>        | Nil ->   Nil
>        | x :: xs -> f x :: map f xs
> end
>
> open Flist
>
> let a :: b :: c :: Nil = map ((+) 2) @@ 1 :: 2 ::  3 :: Nil;;  (* 
> Can't use Flist.[a; b; c] on both sides *)
>
> let d :: e :: Nil = map ((^) "2 + ") @@ "1" :: "2" :: Nil;;
>
> let g :: Nil = map ((+) 2) @@ 1 :: Nil;;
>
> Printf.printf "%d = %s = %d\n%!" a d g;;
>
> BTW, both features are already proposed as feature requests:
> https://github.com/ocaml/ocaml/pull/187
> https://github.com/ocaml/ocaml/pull/76
>
> Regards, Mikhail
>

A possibility to lighten these constructions is to use a ppx extension 
to rewrite
the special syntax [a;...] into a standard Cons(a,... Nil)...) 
construction. For instance with my experimental ppx_listlike extension 
(https://github.com/Octachron/ppx_listlike), these examples can be 
rewritten as

     let [%ll? [a; b; c] ] = map ((+) 2) [%ll 1; 2; 3 ];;

or with more syntactic sugar

     let%with_ll [ d; e ] = map ((^) "2 + ") ["1"; "2" ];;
     let%with_ll [g] = map ((+) 2) [1];;

Regards,
octachron.