[...]
> I had the same problem a month ago,
> I ended with the following ugly function:
> 
> let log b fmt = 
>   if b then Printf.printf fmt
>   else 
>     let rec f x = Obj.magic f in 
>       f fmt
> (* val f: bool -> ('a, out_channel, unit) format -> 'a *)
> 
> (I think this is safe since f just eats its arguments, 
>  and gets typed with "-rectypes" : ('b->'a) as 'a)

No, this is not safe: you break the type system and create buggy
values like that.

# let log b fmt = 
    if b then Printf.printf fmt else 
    let rec f x = Obj.magic f in f fmt;;
val log : bool -> ('a, out_channel, unit) format -> 'a = <fun>
# log false "";;
- : unit = <unknown constructor>
# let x = log false "";;
val x : unit = <unknown constructor>
# Marshal.to_string;;
- : 'a -> Marshal.extern_flags list -> string = <fun>
# Marshal.to_string x [];;
Exception: Invalid_argument "output_value: abstract value (outside
heap)".

There is clearly something wrong!

> maybe a function "zprintf" could be added to Printf and Format to do
> this job ?

It is overkill to introduce a new printf function just for that case.

I suggest we try a bit harder to find an acceptable solution without
introducing zprintf :)

Best regards,

Pierre Weis

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


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