From: Philippe Veber <philippe.veber@gmail.com>
To: Tiphaine Turpin <Tiphaine.Turpin@free.fr>
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] [GADT noob question] Building a GADT from an untyped representation
Date: Fri, 4 Jan 2013 18:25:07 +0100 [thread overview]
Message-ID: <CAOOOohRJhF2Fxy0XhNbzSS+aFXpK04zRDjqeSJtKXaB0MYwsuw@mail.gmail.com> (raw)
In-Reply-To: <50E701D3.9070008@free.fr>
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This is pretty much what I asked for, thanks Tiphaine!
ph.
2013/1/4 Tiphaine Turpin <Tiphaine.Turpin@free.fr>
> Hi Philippe,
>
> I don't think that you can achieve what you are you are asking exactly,
> unless ressorting to an existential type. You can do it using GADT too, as
> described in the "existential types" section of
>
> https://sites.google.com/site/ocamlgadt/
>
> For your example, you can write:
>
> type any_expr =
> | Expr : _ expr -> any_expr
> ;;
>
> let parse_expr x =
> try Expr (Int (int_of_string x))
> with _ ->
> Expr (Float (float_of_string x))
> ;;
>
> Cheers,
>
> Tiphaine
>
>
> On 01/04/13 16:05, Philippe Veber wrote:
>
> 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.
>>>
>>
>>
>
>
>
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next prev parent reply other threads:[~2013-01-04 17:25 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-01-04 13:32 Philippe Veber
2013-01-04 14:54 ` Yury Sulsky
2013-01-04 15:05 ` Philippe Veber
2013-01-04 16:22 ` Tiphaine Turpin
2013-01-04 17:25 ` Philippe Veber [this message]
2013-01-04 22:00 ` Jeff Meister
2013-01-05 0:27 ` Jeremy Yallop
2013-01-05 13:40 ` Philippe Veber
2013-01-05 16:26 ` Jeremy Yallop
2013-01-08 11:00 ` Philippe Veber
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