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[Caml-list] Narrowing coercions for functions with optional parameters
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Date: 2002-02-24 (19:50)
From: Tomasz Zielonka <zielony@c...>
Subject: Re: [Caml-list] Narrowing coercions for functions with optional parameters
On Sun, Feb 24, 2002 at 07:33:23PM +0100, Stefano Zacchiroli wrote:
> On Sun, Feb 24, 2002 at 03:01:25PM +0100, Tomasz Zielonka wrote:
> > # let coerce f x = f x
> > val coerce : ('a -> 'b) -> 'a -> 'b = <fun>
> > 
> > # coerce g ;;
> > - : int -> int = <fun>
> With such a call you haven't change the original "g" function so the
> coersion of "g" to "int -> int" doesn't work because it was applied to
> the old "g" function.

Yes, I want it that way.
Maybe I wasn't clear enough. Let's try again.

My original question was: Is is possible to implement narrowing coercions
for functions with optional parameters. 

Such coercion would erase optional parameters not found in target type. 
It is similar to coercing one class to another, when methods not found
in the target class are hidden/"erased".

Lets say you have several functions of the same type - some request
handlers which take request as argument and return reply.

  type request and reply
  type handler = request -> reply

You could place them in a list, hash table or some other structure, from
which they are taken to handle typical requests.

Imagine that in some situation you would like to call one handler from
another passing some additional parameters. Of course the former handler
will have different type (optional labeled parameters). Now you can't
put it in the list with others (unless you wrap it in other function
with appropriate type).

Then it would be nice if you could just do (g :> handler), and get the
optional parameters erased.

When I was preparing an example, it came to me that such coercions are
sometimes performed automatically:

We have:
  val coerce : ('a -> 'b) -> 'a -> 'b = <fun>
  val g : ?add:int -> int -> int = <fun>

When 'coerce' is applied to 'g' the type of 'g' must be unified with
type of 'coerce's argument, that is:

  ?add:int -> int -> int

is unified with

  'a -> 'b

The only way to do it is to _erase_ optional argument ?add and then
substitute int for both 'a and 'b.

So here 'g' was used with type int -> int, optional argument ?add being
automatically erased. 
Here the coercion was implicit.

It DOES WORK in Ocaml 3.04

I would like that it was also possible to explicitly coerce 'g' to
type int -> int like in:

  (g : int -> int)

or rather

  (g :> int -> int)

But this DOESN'T WORK.

So now the question is:
  Why implicit coercions are performed and explicit coercions are not

> Try this (after executing your examples of course):
>      # let g = coerce g;;
>      val g : int -> int = <fun>
>      # (g : int -> int) ;;  
>      - : int -> int = <fun>

Then the second phrase is only type assertion (or something) not
coercion. Not an interesting coercion.

> Hope this helps.
> Cheers.

Hope I was clear enough this time.

   .-.   Tomasz Zielonka                           CYBER SERVICE
   oo|   programista                        http://www.cs.net.pl
  /`'\   zielony@cs.net.pl
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