Re: [Caml-list] GADTs and parsers

[Dario Teixeira <darioteixeira@yahoo.com>]

Hi,

> Here is a more complete version of his code.

> I also changed a bit your types, because there is another problem
> using polymorphic variants with GADTs: when you get a GADT
> equation involving a refinable polymorphic variant, you are going
> to get get a local private type. If your type is unbounded, you
> cannot even use subtyping on it. Also [< `Nonlink] has exactly
> the same elements as [ `Nonlink], so it is better to use the latter.
> 
>    Jacques
> 
> type inkind = [ `Link | `Nonlink ]
> 
> type _ inline_t =
>     | Text: string -> [< inkind > `Nonlink ] inline_t
>     | Bold: 'a inline_t list -> 'a inline_t
>     | Link: string -> [< inkind > `Link ] inline_t
>     | Mref: string * [ `Nonlink ] inline_t list -> [< inkind > `Link ] 
> inline_t

Thanks for the input!  You are right, it is easy to forget that a declaration
such as [> `Nonlink ] is way more open than intended, particularly in this
case where only `Nonlink and `Link are in play.

Anyway, this sort of "closed variant", though mentioned in the language reference,
is a subtlety of polymorphic variants which is not mentioned in section 4.2 (tutorial)
of the manual.  I know the manual is not intended as a full-blown tutorial on the
language, but I reckon it could benefit from mentioning this aspect...

Best regards,
Dario Teixeira