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Date: Tue, 26 Oct 2010 17:55:21 +0200
From: Emmanuel Dieul <emmanuel.dieul@free.fr>
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To: caml-list@yquem.inria.fr
Subject: type inference problem with Printf.sprintf ?
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X-Spam: no; 0.00; inference:01 printf:01 sprintf:01 inference:01 printf:01 sprintf:01 ocaml:01 compiler:01 1900:98 1900:98 unix:01 unix:01 int:01 int:01 expression:02 

Hi everyone.

I've just noticed a type inference problem (or am I wrong ?). Here's my example :

let format_date =
   let format_valeur = function valeur ->
     (if valeur < 10 then "0" else "") ^ (string_of_int valeur)
   in
   function date ->
     Printf.sprintf "%s/%s/%l %s:%s:%s"
       (format_valeur(date.Unix.tm_mday) : string)
       (format_valeur(date.Unix.tm_mon + 1) : string)
       (date.Unix.tm_year + 1900)
       (format_valeur(date.Unix.tm_hour) : string)
       (format_valeur(date.Unix.tm_min) : string)
       (format_valeur(date.Unix.tm_sec) : string)
;;

The "format_valeur" function has type "int -> string".
The "format_date" function has type "Unix.tm -> string".
The problem is about the Printf.sprintf function.

In fact, if I don't force the type of every date field (like 
"(format_valeur(date.Unix.tm_mday) : string)"),
the ocaml 3.12.0 compiler detects a type error.
This example

let format_date =
   let format_valeur = function valeur ->
     (if valeur < 10 then "0" else "") ^ (string_of_int valeur)
   in
   function date ->
     Printf.sprintf "%s/%s/%l %s:%s:%s"
       format_valeur(date.Unix.tm_mday)
       (format_valeur(date.Unix.tm_mon + 1) : string)
       (date.Unix.tm_year + 1900)
       (format_valeur(date.Unix.tm_hour) : string)
       (format_valeur(date.Unix.tm_min) : string)
       (format_valeur(date.Unix.tm_sec) : string)
;;

generates this error around the "%s/%s/%l %s:%s:%s" string :
Error: This expression has type
          (string -> string -> int -> string -> string -> string -> 'a -> 'b,
           unit, string, string, string, 'a -> 'b)
          format6
        but an expression was expected of type
          (string -> string -> int -> string -> string -> string -> 'a -> 'b,
           unit, string)
          format =
            (string -> string -> int -> string -> string -> string -> 'a -> 'b,
             unit, string, string, string, string)
            format6

Did I do something wrong or is there a real problem ?

Emmanuel