[Caml-list] printf arguments

[Caml-list] printf arguments

[Henri Dubois-Ferriere]

In trying to write a wrapper around printf for debug message logging, 
(a la syslog), I am coming across the following problem:

# let myprintf fmt = if (true) then Printf.fprintf stdout fmt;;
val myprintf : (unit, out_channel, unit) format -> unit = <fun>

# Printf.fprintf;;
- : out_channel -> ('a, out_channel, unit) format -> 'a = <fun>


so the myprintf function above does not have the same type as the original 
fprintf. Note that removing the if clause returns us to the proper type:

# let myprintf fmt = Printf.fprintf stdout fmt;;
val myprintf : ('a, out_channel, unit) format -> 'a = <fun>



Any clue why this is so, and how to get around it?

Thanks,

Henri

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Re: [Caml-list] printf arguments

[Pierre Weis]

> In trying to write a wrapper around printf for debug message logging, 
> (a la syslog), I am coming across the following problem:
> 
> # let myprintf fmt = if (true) then Printf.fprintf stdout fmt;;
> val myprintf : (unit, out_channel, unit) format -> unit = <fun>

If cond then expr == if cond then expr else ()

[...]
> so the myprintf function above does not have the same type as the original 
> fprintf. Note that removing the if clause returns us to the proper type:
> 
> # let myprintf fmt = Printf.fprintf stdout fmt;;
> val myprintf : ('a, out_channel, unit) format -> 'a = <fun>

If you want to preserve the original typing of printf, you must
imperatively use a tail call to printf (due to the typing discipline
of string format).

Regards,

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/


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