let rec and polymorphic functions

["David Allsopp" <dra-news@metastack.com>]

Why is let rec apparently unable to infer polymorphic function types? In
both the expressions below, I'd expect [out] to have type [('a, unit,
string, unit) format4 -> 'a]. Why when used in a [let rec] construct is it
clearly inferred as [('_a, unit, string, unit) format4 -> '_a] and then
instantiated as [(unit, unit, string, unit) format4 -> unit] by the first
call to [out] in [f]? It seems to contradict the end of Section 6.7.1 of the
manual.

I know that [out] and [f] are not mutually recursive so there's no need to
use [let rec] but I tend to use [let rec] in situations where I'm defining
two functions where one uses the other at the [let ... in] level as it saves
writing the extra [in]! This appears potentially to be a mistake, though...

As ever, a technical explanation of why the type system behaves this way
much appreciated! I won't make judgement on the hours of time wasted by the
cryptic type errors in this case ;o)

Just in case it matters, I'm using O'Caml 3.09.3...


David

(*
 * This first example works.
 *)
let out line =
  Printf.printf line
in
  let f () =
    (*
     * [out] is clearly polymorphic
     *)
    out "TEST";
    out "%d" 0;
    out "%b" false;
  in
    f ();;

(*
 * This second example does not. Why?
 *)
let rec out line =
  Printf.printf line
and f () =
  (*
   * [out] gets inferred as string -> unit here...
   *)
  out "TEST";
  (*
   * ... and so we get a "too many parameters" error here.
   *)
  out "%d" 0;
in
  f ();;