You can do a lot with the format concatenation operator (^^):

  let prefixprintf prefix o fmt = Printf.fprintf o (prefix ^^ fmt)
  let suffixprintf suffix o fmt = Printf.fprintf o (fmt ^^ suffix)

# prefixprintf "foo: " stdout "bar %d" 5;;
foo: bar 5- : unit = ()
# suffixprintf ": foo" stdout "bar %d" 5;;
bar 5: foo- : unit = ()

The type is very general, you can prefix or suffix with more than a
litteral string:

# suffixprintf ": foo %d" stdout "bar %d" 5 6;;
bar 5: foo 6- : unit = ()



On Fri, Aug 12, 2016 at 2:44 PM, Soegtrop, Michael <
michael.soegtrop@intel.com> wrote:

> Dear Ocaml Users,
>
>
>
> it is quite common to have wrappers for printf to e.g. print errors or
> warnings with some context. Prefixing is easy to do in OCaml (honestly it
> took me a short while to figure it out):
>
>
>
> let prefixprintf (oc : out_channel) (fmt : ('a, out_channel, unit) format)
> : 'a =
>
>   Printf.fprintf oc "Prefix: ";
>
>   Printf.fprintf oc fmt
>
> ;;
>
>
>
> I wonder how I would postfix something. Even converting the formatted
> output first to a string and then output it together with the postfix
> doesn’t seem to be easy, because the formatting function must be the last
> call in the function, since it has to produce the returned function. Is
> there some way to make a variadic lambda expression or some other magic to
> handle such cases? A similar question would be how I can print the same
> formatted string twice.
>
>
>
> I don’t have an application for this, I ask this out of pure OCaml
> curiosity. So I am not so much interested in workarounds, more in an answer
> to the question how I can return a function of type ‘a without more or less
> directly calling some variant of printf.
>
>
>
> Best regards,
>
>
>
> Michael
>
>
>
> Intel Deutschland GmbH
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> Tel: +49 89 99 8853-0, www.intel.de
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>