Re: [Caml-list] Specifying abstract type in a record

[Richard Jones <rich@annexia.org>]

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On Fri, Nov 12, 2004 at 10:11:51AM -0600, josh wrote:
> OK,  Here's what I'm trying to do:
> 
> # type doer = { file_name:string ; actor: ('a -> unit) };;
> 
> But when I do this, it tells me that I've got "Unbound type parameter 'a ".

You need to do this:

# type 'a doer = { file_name:string ; actor: ('a -> unit) };;
type 'a doer = { file_name : string; actor : 'a -> unit; }

because you're creating a polymorphic type ('a doer).  Note that this
won't let you store an actor function of type, say, int -> unit and
then replace it with another function of type, say, string -> unit.
For the same reason it won't let you pass an ``int doer'' to a
function expecting a ``string doer''.

> # type t
> # type doer = { file_name:string; actor (t -> unit) };;
> 
> It works until I try to use a created record:
> 
> # let b = {file_name = "one"; actor = (fun x -> () ) };;
> # b.actor 10;;
> The expression has type int but is used with type t
> 
> even if I try to do this
> 
> # type t = int;;
> 
> it doesn't work.

OK, this is another problem.  Here what you're doing is defining a
_new_ type ``t'', which is unrelated to your old type ``t''.

Here's another example:

# let f () = "hello";;
val f : unit -> string = <fun>
# f ();;
- : string = "hello"
# let g = f;;    
val g : unit -> string = <fun>
# let f () = "goodbye";;
val f : unit -> string = <fun>
# f ();;
- : string = "goodbye"
# g ();;
- : string = "hello"

Notice that ``g'' still calls the old ``f'', even after ``f'' has been
redefined.

There's a beginner's list:

> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners

Rich.

-- 
Richard Jones.  http://www.annexia.org/  http://www.j-london.com/
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