Re: [Caml-list] strange typechecking result

[Leo White <lpw25@cam.ac.uk>]

On Feb 22 2013, Matej Kosik wrote:

>Hello,
>
>For one of my modules, the typechecker started to raise strange 
>complaints. I was not able to figure out exactly why, but at least I 
>wanted to narrow down the problem.
>
>This small program:
>
>   type r1 = {l1 : unit list}
>
>   and r2 = {l2 : int64 list}
>
>   let rec f1 _ =
>     ()
>
>   and _ r1 =
>     f1 r1.l1
>
>   and _ r2 =
>     f1 r2.l2
>
>is rejected by the typechecker with a following error message:
>
>   File "test.ml", line 12, characters 5-10:
>   Error: This expression has type int64 list
>          but an expression was expected of type unit list
>
>I do not understand why the given program was rejected.
>

I think that by default recursive uses of a function are monomorphic. You 
can fix this with an explicit polymorphic annotation:

   let rec f1: 'a. 'a -> unit = fun _ -> ()
   
   and f2 r1 = 
     f1 r1.l1
   
   and f3 r2 = 
     f1 r2.l2;;

Regards,

Leo