[Caml-list] printf format strings

[Caml-list] printf format strings

[Jonathan Roewen]

Hi,

What is the following for?

{ fmt %}: convert a format string argument. The argument must have the
same type as the internal format string fmt.

And how to use it? I get Bad format `%{' in the toplevel.

Jonathan

Re: [Caml-list] printf format strings

[Hendrik Tews]

Jonathan Roewen <jonathan.roewen@gmail.com> writes:

   { fmt %}: convert a format string argument. The argument must have the
   same type as the internal format string fmt.
   
   And how to use it? I get Bad format `%{' in the toplevel.
   
My toplevel accepts it, but does nothing on it:

    # Printf.printf "%{%d%s%}" "kkkk\n" ;;                   
    This expression has type
      (int -> string -> 'a, 'b, 'c, int -> string -> 'a) format4
    but is here used with type (int -> string -> 'a, 'b, 'c, 'a) format4

This is expected, because kkkk has the wrong type.

    # Printf.printf "%{%d%s%}" "kk%dkk%s\n" ;;
    - : unit = <unknown constructor>

This should print something I guess. The "unknown constructor"
looks very suspicious. Maybe the %{ format is meant to generate
random strings???

    # String.escaped(Printf.sprintf "%{kkk%}" "kkk\n") ;;
    - : string =
    "\\136LÛ\\149*\\000\\000\\000 óY\\149*\\000\\000\\0000\\tî\\149*\\000\\000"

;-)


Note that there is a typo in the docs: There is no format %\(...%\):

    # Printf.printf "%\\(aaa%\\)";;
    Bad conversion %\, at char number 0 in format string ``%\(aaa%\)''

It is %(...%):

    # Printf.printf "%(aaa%)";;    
    - : ('_a, '_b, '_c, '_a) format4 -> unit = <fun>

However, it does not seem to work either:

    # Printf.printf "%(aaa%)"  "bbbb";;
    - : unit = <unknown constructor>


Bye,

Hendrik

PS. These two conversions are new, but not marked as such in the
enhanced reference manual, because they do not appear in the txt
version (which I used for the diffs).

[Caml-list] printf format strings

[Remi Vanicat]

005/12/1, Hendrik Tews <tews@tcs.inf.tu-dresden.de>:
> Jonathan Roewen <jonathan.roewen@gmail.com> writes:
>
>    { fmt %}: convert a format string argument. The argument must have the
>    same type as the internal format string fmt.
>
>    And how to use it? I get Bad format `%{' in the toplevel.
>
> My toplevel accepts it, but does nothing on it:
>
>     # Printf.printf "%{%d%s%}" "kkkk\n" ;;
>     This expression has type
>       (int -> string -> 'a, 'b, 'c, int -> string -> 'a) format4
>     but is here used with type (int -> string -> 'a, 'b, 'c, 'a) format4
>
> This is expected, because kkkk has the wrong type.
>
>     # Printf.printf "%{%d%s%}" "kk%dkk%s\n" ;;
>     - : unit = <unknown constructor>
>
> This should print something I guess. The "unknown constructor"
> looks very suspicious. Maybe the %{ format is meant to generate
> random strings???

Well, I've guessed a bug in the type system, and it seem I'm correct:

let t = Obj.repr (Printf.printf "%{%d%s%}" "kk%dkk%s\n");;
# Obj.is_block t;;
- : bool = true
# Obj.is_block (Obj.repr ());;
- : bool = false
# Obj.tag t;;
- : int = 247
# Obj.closure_tag;;
- : int = 247

So this is a closure, so a function that should have a functional type.


I've tried to play with the devil, and here my result :
# (Obj.magic( Printf.printf "%{%d%s%}" "kk%dkk%s\n") 5 "rae" () : unit );;
%i%s- : unit = ()

Very strange indeed.

Well, this should be reported in the bug tracking system.