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Constructors are not functions
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Date: 2009-10-06 (12:19)
From: Philippe Wang <philippe.wang.lists@g...>
Subject: Re: [Caml-list] Constructors are not functions

I don't know the actual reason, but I guess it's simply a choice of semantics.

It woud be weird to be able to deconstruct (Bar 42) while not be able
to deconstruct (Bar) as it's not constructed.
I mean we can write (match x with Bar y -> y).
If partial construction were accepted we may like to write
(match x with Bar -> x) but we couldn't because Bar is like a function then.

With type t = A | B of int
what would be the error warning for
(match x with A -> 42 | B -> 43) ?

Well, then I guess the reason is that it would be complicated to
choose some sound enough semantics for partial application of
constructors, since the solution of having to write (fun x -> Bar x)
is much simpler.

If someone has the actual historical explanation, I am also interested.


Philippe Wang

On Tue, Oct 6, 2009 at 2:01 PM, Chantal KELLER
<chantal.keller@wanadoo.fr> wrote:
>    Dear Ocaml users,
>  Is there a reason for constructors not to behave like functions? For
> instance, one cannot make a partial application from a constructor:
> # type foo = | Bar of int;;
> type foo = Bar of int
> # let foobar = Bar in foobar 17;;
> Error: The constructor Bar expects 1 argument(s),
>       but is applied here to 0 argument(s)
>  Thanks,
> --
> Chantal KELLER