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Re: [Caml-list] Generalized Algebraic Datatypes
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Date: -- (:)
From: Jacques Le Normand <rathereasy@g...>
Subject: Re: [Caml-list] Re: Generalized Algebraic Datatypes
On Fri, Oct 29, 2010 at 5:37 PM, bluestorm <bluestorm.dylc@gmail.com> wrote:

> On Fri, Oct 29, 2010 at 11:10 PM, Stefan Monnier <monnier@iro.umontreal.ca
> > wrote:
>
>> > type _ t =
>> >   | IntLit : int -> int t
>> >   | BoolLit : bool -> bool t
>> >   | Pair : 'a t * 'b t -> ('a * 'b) t
>> >   | App : ('a -> 'b) t * 'a t -> 'b t
>> >   | Abs : ('a -> 'b) -> ('a -> 'b) t
>>
>> > There's something "Haskellish" about this syntax, in the sense that type
>> > constructors are portrayed as being like functions.
>>
>> Indeed IIRC OCaml does not accept "App" as an expression (you have to
>> provide arguments to the construct).  Maybe this is a good opportunity
>> to lift this restriction.
>
>
>  It was actually the case in Caml Light : each datatype constructor
> implicitly declared a constructor function with the same name. I don't
> exactly know why this feature was dropped in Objective Caml, but I think I
> remember (from a previous discussion) that people weren't sure it was worth
> the additional complexity.
>
> Note that, as in Jacques's examples, the constructor function was not
> curryfied. (type t = A of bool * int) would generate a function (A : bool *
> int -> t). It doesn't help the already tricky confusion between (A of bool *
> int) and (A of (bool * int))...
> By the way, it is unclear if
>   | App : ('a -> 'b) t -> 'a t -> 'b t
> would be accepted in Jacques proposal. If not, I think going back to a "of
> ..." syntax would be wiser.
>
>
It is accepted. In fact, that constructor is part of an example on my
webpage. If there's any doubt, you can always download the source and try it
out.
cheers,
Jacques

>
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