printf "%a" vs sprintf "%a"

printf "%a" vs sprintf "%a"

[Bertrand Jeannet]

For people that do not use OCaml batteries, a sprintf function similar 
to  (Printf|Format).printf can be defined using kfprintf:

let sprintf format =
   let buffer = Buffer.create 512 in
   let fmt = Format.formatter_of_buffer buffer in
   Format.kfprintf
     (begin fun fmt ->
       Format.pp_print_flush fmt ();
       let s = Buffer.contents buffer in
       Buffer.clear buffer;
       s
     end)
     fmt
     format

The types are different:
Format.fprintf : Format.formatter -> ('a, Format.formatter, unit, unit) 
format4 -> 'a
val sprintf : ('a, Format.formatter, unit, string) format4 -> 'a = <fun>

but that is normal, as sprintf should return a string.

Bertrand Jeannet

printf "%a" vs sprintf "%a"

[Tiphaine Turpin]

Hi,

I'am confused by the interpretation of "%a":

# Printf.printf "%a";;
- : (out_channel -> '_a -> unit) -> '_a -> unit = <fun>
# Printf.sprintf "%a";;
- : (unit -> '_a -> string) -> '_a -> string = <fun>

Usually, the typing of formatting functions is such that

printf something

has type unit if and only if

sprintf something

has type string. But %a breaks this rule. Wouldn't it be simpler to have
two separate directives which accept respectively string printers and
channel printers, regardless of the outer printing function ?

Tiphaine