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

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