Re: [Caml-list] creating GADTs

[Reed Wilson <cedilla@gmail.com>]

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I think I've gotten everything pretty much straightened out now, but there
is something I didn't expect with GADTs and pattern-matching. Using my
previous samplerate_t type:

type mpeg1_t
type mpeg2_t
type _ samplerate_t =
| S48000 : mpeg1_t samplerate_t
| S44100 : mpeg1_t samplerate_t
| S24000 : mpeg2_t samplerate_t
| S22050 : mpeg2_t samplerate_t


I can't seem to do something like this:

let samples_per_frame : type id. id samplerate_t -> int = function
| S48000 | S44100 -> 1152
| S24000 | S22050 -> 576


Instead I have to have each of the constructors use a separate branch:

let samples_per_frame : type id. id samplerate_t -> int = function
| S48000 -> 1152
| S44100 -> 1152
| S24000 -> 576
| S22050 -> 576


even though S48000 and S44100 are the exact same type! OCaml complains:
Error: This pattern matches values of type mpeg1_t samplerate_t
       but a pattern was expected which matches values of type
         id samplerate_t


I certainly don't know a lot about these new type features, but I know
there shouldn't be a limitation in what I'm doing with them here. Is there
any way to tell the typing system to let me combine the branches?

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