Hi Yury,

Thanks for your answer! It is true your fix is accepted by the compiler,
but this is not what I was looking for: in the end I'd like to have an 'a
expr such that 'a is the type of the value obtained when evaluating the
expression. With the polymorphic variants, I cannot write an eval function
of type 'a eval -> 'a, which is the main motivation behind using GADT in
that (canonical) case. Sorry for ommiting this point!

ph.


2013/1/4 Yury Sulsky <yury.sulsky@gmail.com>

> Hi Philippe,
>
> I think you can do this by using a polymorphic variant as the type
> variable:
>
> type _ expr =
> | Int : int -> [> `int ] expr
> | Float : float -> [> `float ] expr
>
> let parse_expr : string -> [ `int | `float ] expr = fun x ->
>   try Int (int_of_string x)
>   with _ ->
>     Float (float_of_string x)
> ;;
>
> - Yury
>
>
>
> On Fri, Jan 4, 2013 at 8:32 AM, Philippe Veber <philippe.veber@gmail.com>wrote:
>
>> Dear list,
>>
>> Suppose I define a GADT for expressions:
>>
>> type _ expr =
>> | Int : int -> int expr
>> | Float : float -> float expr
>>
>> Now I want to write a parser, that will build an ['a expr] from a string.
>> Without thinking much, I tried the following:
>>
>> let parse_expr : type s. string -> s expr = fun x ->
>>   try Int (int_of_string x)
>>   with _ ->
>>     Float (float_of_string x)
>> ;;
>>
>> Which fails with the following error message:
>>
>> Error: This expression has type int expr but an expression was expected
>> of type s expr
>>
>> That makes sense, since [s] is a locally abstract type. I tried a couple
>> of variants and finally realised that I could not even write the type of
>> [parse_expr]: it should be [string -> 'a expr] for some ['a], but I'm not
>> sure that really means something.
>>
>> So to put it simple, how does one construct a GADT value from a string ?
>>
>> Cheers,
>>   ph.
>>
>
>