Re: [Caml-list] type error

[Martin Jambon <martin.jambon@ens-lyon.org>]

On Sun, 23 Mar 2008, Jacques Le Normand wrote:

> Hello caml-list,
> I don't quite understand what the error is here:
>
> type foo = <bar:int;..>
>
> error:
>
> A type variable is unbound in this type declaration.
> In definition < bar : int; .. > the variable 'a is unbound

Basically ".." means "any number of methods of any kind", which needs to 
be represented by a type variable.

You can do something like this, which is probably not very useful:

# type 'a foo = < bar: int; .. > as 'a;;
type 'a foo = 'a constraint 'a = < bar : int; .. >


A more idiomatic way is to declare the following type:

type foo = < bar: int >

And when you need an object to behave as having type foo, you use ":>" to 
hide the extra methods.

# type foo = < bar: int >;;
type foo = < bar : int >
# let print_bar (x : foo) = print_int x # bar;;
val print_bar : foo -> unit = <fun>
# let x =
     (object
      method bar = 123
        method hello () = print_endline "hello"
        end);;
val x : < bar : int; hello : unit -> unit > = <obj>
# print_bar x;;
This expression has type < bar : int; hello : unit -> unit >
but is here used with type foo
Only the first object type has a method hello
# print_bar (x :> foo);;
123- : unit = ()



Martin

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