[Caml-list] recursive values of abstract types, aka Batteries ParserCo

[Caml-list] recursive values of abstract types, aka Batteries ParserCo

[Christoph Höger]

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Hi all,

I wanted to parse a small lambda-calculus in OCaml and had batteries
already as a dependency. Batteries comes with ParserCo, a small parser
combinator library. Since I did not want to complicate things further,
I decided to stick with it and avoid any parser generator. Using a
monadic combinator library for a scannerless parser should be a
relatively simple exercise, right?

Unfortunately, it seems that ParserCo only exposes an abstract type:

type ('a, 'b, 'c) t

which is, of course, a function:

type ('a, 'b, 'c) t = ('a, 'c) Source.t -> ('a, 'b, 'c) result

This prevents me from defining any recursive parsers, since OCaml
won't let me let-rec a value of an abstract function type. I cannot do
eta-expansion, because the type is abstract, and I also do not have a
type constructor at hand.

How does one handle such cases in OCaml? Does this really mean, you
cannot have recursive values of abstract function types?

thanks,

Christoph



- -- 
Christoph Höger

Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Übersetzerbau und Programmiersprachen

Sekr. TEL12-2, Ernst-Reuter-Platz 7, 10587 Berlin

Tel.: +49 (30) 314-24890
E-Mail: christoph.hoeger@tu-berlin.de
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Re: [Caml-list] recursive values of abstract types, aka Batteries ParserCo

[Leo White]

> I wanted to parse a small lambda-calculus in OCaml and had batteries
> already as a dependency. Batteries comes with ParserCo, a small parser
> combinator library. Since I did not want to complicate things further,
> I decided to stick with it and avoid any parser generator. Using a
> monadic combinator library for a scannerless parser should be a
> relatively simple exercise, right?
>
> Unfortunately, it seems that ParserCo only exposes an abstract type:
>
> type ('a, 'b, 'c) t
>
> which is, of course, a function:
>
> type ('a, 'b, 'c) t = ('a, 'c) Source.t -> ('a, 'b, 'c) result
>
> This prevents me from defining any recursive parsers, since OCaml
> won't let me let-rec a value of an abstract function type. I cannot do
> eta-expansion, because the type is abstract, and I also do not have a
> type constructor at hand.
>
> How does one handle such cases in OCaml? Does this really mean, you
> cannot have recursive values of abstract function types?

Hi Christoph,

I haven't used ParserCo (or even looked at the documentation), but the
'lazy' keyword has a relaxed notion of what can be defined recursively,
so creating a recusive `('a, 'b, 'c) t Lazy.t` and then forcing it may
well solve your problem.

Regards,

Leo