Re: [Caml-list] Y combinator and type-checking

[Brian Hurt <bhurt@spnz.org>]

On Fri, 2 Jul 2004, Martin Berger wrote:

> now i annotate the definition above with the types just
> inferred:
> 
>      let rec y  ( f : ( 'a -> 'b ) -> 'a -> 'b )
>             : ( ( 'a -> 'b ) -> 'a -> 'b ) -> 'a -> 'b
>        = f (fun x -> (y f) x );;

This is where your mistake is.  Don't duplicate the type of f.  Instead, 
you should have done:

let rec y (f : ('a -> 'b) -> 'a -> 'b) : 'a -> 'b = f (fun x -> (y f) x);;

And now it works:
$ ocaml
        Objective Caml version 3.07
 
# let rec y (f : ('a -> 'b) -> 'a -> 'b) : 'a -> 'b = f (fun x -> (y f) 
x);;
val y : (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b = <fun>
#

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Brian

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