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From: Lukasz Stafiniak <lukstafi@gmail.com>
Date: Sun, 5 Dec 2010 09:10:01 +0100
Message-ID: <AANLkTinVSDB-XWXKvJPxDzhzxox-Vmxnw9Lw2ucLy0De@mail.gmail.com>
Subject: Re: [Caml-list] GADT constructor syntax
To: Jacques Le Normand <rathereasy@gmail.com>
Cc: caml-list caml-list <caml-list@yquem.inria.fr>
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X-Spam: no; 0.00; lukasz:01 syntax:01 restricting:01 quantified:01 notation:01 existential:01 ocaml:01 conciseness:01 wrote:01 typing:01 caml-list:01 constructor:01 exceptions:01 constraint:01 constraint:01 

On Sat, Dec 4, 2010 at 10:06 PM, Jacques Le Normand
<rathereasy@gmail.com> wrote:
> in this case, yes, but if you have constraints then it is different. cons=
ider:
>
> type 'a term =3D
> =A0 =A0 =A0| Ignore of 'a : int term
> =A0 =A0 =A0| Embed of 'a
> =A0 constraint 'a =3D int
>
> this is different from:
>
> type 'a term =3D
> =A0 =A0 | Ignore of 'a : int term
> =A0 =A0 | Embed of 'a : 'a term
> =A0 constraint 'a =3D int
>
> (in fact, an error is flagged on the second one)
>
> Also, it can save the user some typing.

I don't think that it is very different. Standard-language constraints
are outside of implications, so they apply to each branch. It already
behaves this way in the standard language, basically restricting the
type family to "int term". So I don't see why

> | Embed of 'a : 'a term
> =A0 constraint 'a =3D int

shouldn't mean that Embed is "basically" Embed of int : int term. My
position is that if a type variable should be treated as local to a
branch, it should be explicitly quantified (using the dot notation,
for example "Ignore of 'a. 'a : int term", without any exceptions to
"existential" variables). And that the patterns

type X =3D
  | Branch of Y

and

type X =3D
  | Branch of Y : X

should be equivalent (where X could for example be 'a term).

I would accept concessions to my general outlook on the grounds of
being conservative over standard OCaml programmer intuitions and
conciseness of code... when they apply...