[Caml-list] extending user-defined polymorphic variant types

[Caml-list] extending user-defined polymorphic variant types

[Dan Bensen]

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I'm trying to write a functor that extends a user-supplied polymorphic 
variant type (PVT).  How do you declare the user's type in the signature 
for the argument to the functor without locking in the individual variant 
definitions?  
The code below (in revised syntax) generates an error message that says
the type isn't a PVT.

code:

> module type Reader = sig type ast; end;
>
> module Make (Read: Reader) = struct
>   type ast = [= Read.ast | `Lid of string];
> end;

message:

> Error: The type Read.ast is not a polymorphic variant type

How do you make ast a PVT while allowing the user to 
specify the variants?

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Re: [Caml-list] extending user-defined polymorphic variant types

[Philippe Veber]

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Hi Dan,

I'm afraid you can't. If you want to write unions of polymorphic variant
types, they have to be concrete and not abstract. See Romain Bardou's work
on this:

http://romain.bardou.fr/papers/stage2006p.pdf

Or, if you can read french:

www.math.nagoya-u.ac.jp/~garrigue/papers/index.html

(see Unions de variants polymorphes abstraits).

Cheers,
  Philippe.



2012/5/7 Dan Bensen <danbensen@att.net>

>
> I'm trying to write a functor that extends a user-supplied polymorphic
> variant type (PVT).  How do you declare the user's type in the signature
> for the argument to the functor without locking in the individual variant
> definitions?
> The code below (in revised syntax) generates an error message that says
> the type isn't a PVT.
>
> code:
>
> > module type Reader = sig type ast; end;
> >
> > module Make (Read: Reader) = struct
> >   type ast = [= Read.ast | `Lid of string];
> > end;
>
> message:
>
> > Error: The type Read.ast is not a polymorphic variant type
>
> How do you make ast a PVT while allowing the user to
> specify the variants?
>

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Re: [Caml-list] extending user-defined polymorphic variant types

[Goswin von Brederlow]

Dan Bensen <danbensen@att.net> writes:

> I'm trying to write a functor that extends a user-supplied polymorphic
> variant type (PVT).  How do you declare the user's type in the signature
> for the argument to the functor without locking in the individual variant
> definitions? 
> The code below (in revised syntax) generates an error message that says
> the type isn't a PVT.
>
> code:
>
>> module type Reader = sig type ast; end;
>>
>> module Make (Read: Reader) = struct
>>   type ast = [= Read.ast | `Lid of string];
>> end;
>
> message:
>
>> Error: The type Read.ast is not a polymorphic variant type
>
> How do you make ast a PVT while allowing the user to
> specify the variants?

Even if that did work how usefull would that be? You couldn't write

let foo = function
  | `Lid s -> ()
  | x -> Read.foo x

unless Read.foo allows [> Read.ast]. In which case you don't need a
functor but can just implement foo straight up.

What is your use case? What do you want to do?

MfG
        Goswin

Re: [Caml-list] extending user-defined polymorphic variant types

[Dan Bensen]

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> What is your use case? What do you want to do?

I've been trying to combine camlp4 with OMeta.
(http://tinlizzie.org/ometa/)
The idea is to allow the user to extend the concrete 
syntax of the parsing system itself for parsing 
different kinds of data (terminals), then layer the 
standard combinator syntax on top of it.  

Maybe the user can define a regular variant type for
terminals and the combinator ast type can wrap it in 
a single variant (e.g. `Reader of Read.ast).

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Re: [Caml-list] extending user-defined polymorphic variant types

[Dan Bensen]

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> extend the concrete syntax of the parsing system
> itself for parsing different kinds of data (terminals),

> then layer the standard combinator syntax on top of it.

That last part should read "layer the standard 
combinator ast on top of the reader ast."

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