Re: [Caml-list] Printf question

[Pierre Weis <pierre.weis@inria.fr>]

> On Mon, Sep 29, 2003 at 09:57:19PM +0200, Pierre Weis wrote:
> > Hence, I think you are seeking for the %a conversion. For instance,
> > defining a quotation function for SQL as in
> > 
> > # let sql_quoting oc = Printf.fprintf oc "'%s'";;   
> > val sql_quoting : out_channel -> string -> unit = <fun>
> > 
> > You can define sth with a %a conversion to apply quoting on the fly:
> > 
> > # let sth x =
> >     prepare "select salary from emp where id = %d and name = %a" x;;
> > val sth : int -> (out_channel -> 'a -> unit) -> 'a -> unit = <fun>
> > 
> > # sth 1 sql_quoting "Jones";;
> > select salary from emp where id = 1 and name = 'Jones'- : unit = ()
> > 
> > Still fully type-checked! Sounds better, no ?
> 
> Hmmm ... but as I explained earlier, that's really not type safe
> at all.
> 
> Rich.

What do you mean by ``not type safe at all'' ?

The common acceptation is ``it could lead to type errors at runtime''.
If you ever exhibit such a problem with format strings, please show us
a complete example, since you indeed discovered an unpleasant bug in
the compiler :(

Remember that our claim for the printf/scanf and format strings
implementation is the same as for the rest of the language: programs
are safe and can never go wrong (i.e. they never produce bus errors or
jeopardize type safety).

(To be precise, this claim evidently does not apply to programs that
could use faulty interfaces with foreign languages, functions from the
Obj module, uncautious input_value/output_value calls, or are compiled
with the -unsafe option.)

Pierre Weis

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


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