let rec and polymorphic functions

let rec and polymorphic functions

[David Allsopp]

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 ();;

Re: [Caml-list] let rec and polymorphic functions

[Jon Harrop]

There are many problems with this. Google for ad-hoc polymorphism, polymorphic 
recursion and generic printing.

On Wednesday 27 June 2007 09:40:31 David Allsopp wrote:
>   out "TEST";

val out : string -> unit

>   out "%d" 0;

val out : format -> int -> unit

As printf is ad-hoc polymorphic, you must supply the format specifier 
immediately and OCaml will generate a custom printer for you. OCaml does not 
use run-time types so you cannot have a generic print function: you must 
specific print functions for each of your (possibly higher-order) types.

Also, recursive calls ossify the function to a monomorphic type, so you cannot 
do polymorphic recursion in OCaml. There are workaround using recursive 
modules or objects but I don't think this is what you want here.

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
The OCaml Journal
http://www.ffconsultancy.com/products/ocaml_journal/?e

Re: [Caml-list] let rec and polymorphic functions

[Arnaud Spiwack]

Jon Harrop a écrit :
> There are many problems with this. Google for ad-hoc polymorphism, polymorphic 
> recursion and generic printing.
>
> On Wednesday 27 June 2007 09:40:31 David Allsopp wrote:
>   
>>   out "TEST";
>>     
>
> val out : string -> unit
>   
Actually it seems to infer properly "out : (unit, out_channel, unit) 
format -> unit". So the magic is pulled here (which surprises me a lot, 
but well). The problem seems more related to the fact that mutual 
recursive function are monomorphic.
>   
>>   out "%d" 0;
>>     
>
> val out : format -> int -> unit
>
> As printf is ad-hoc polymorphic, you must supply the format specifier 
> immediately and OCaml will generate a custom printer for you. OCaml does not 
> use run-time types so you cannot have a generic print function: you must 
> specific print functions for each of your (possibly higher-order) types.
>
> Also, recursive calls ossify the function to a monomorphic type, so you cannot 
> do polymorphic recursion in OCaml. There are workaround using recursive 
> modules or objects but I don't think this is what you want here.
>
>