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From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
In-Reply-To: <000001ce83bf$879e6520$96db2f60$@metastack.com>
Date: Fri, 19 Jul 2013 15:59:47 +0900
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To: David Allsopp <dra-news@metastack.com>
X-Mailer: Apple Mail (2.1508)
Subject: Re: [Caml-list] GADT in an optional parameter

On 2013/07/18, at 23:03, David Allsopp <dra-news@metastack.com> wrote:

> I then wanted to try to make the first parameter optional, defaulting to =
Exn
> (so you'd get the vanilla behaviour of Map.find):
>=20
> module StringMap =3D
>  struct
>    include Map.Make(String)
>=20
>    let find : type s t . ?mode:(s, t) wrap -> string -> s
> Map.Make(String).t -> t =3D fun ?(mode =3D Exn) key map ->
>      match mode with
>        Exn ->
>          find key map
>      | Option ->
>          try
>            Some (find key map)
>          with Not_found -> None
>  end
>=20
> But I get: This expression has type (s, s) wrap but an expression was
> expected of type (s, t) wrap on the Exn default value.
>=20
> Now, in my woolly way, I thought that the default value for an optional
> parameter behaved /almost/ like syntactic sugar, but clearly not. Is ther=
e a
> way to annotate this to achieve what I'm after?=20

No, clearly no.
Optional arguments have no impact on typing: the absence of an optional
argument cannot force some extra equation.
I agree this would be interesting, but then we would need a way to express
it in the type. Otherwise, why would the user expect (s=3Dt) when there is =
no
argument, whereas there is no way to see what the default is?
Something like:
val  find : ?mode:(('s,'t) wrap =3D Exn) -> string -> 's Map.Make(String).t=
 -> 't

It would be good to see whether this has many applications.
In the past, there were some request to be able to default to the identity =
function.

	Jacques=