The documentation for Format.sprintf says […] returns a string containing the result of formatting the arguments. But how can that function know how to format the arguments if it is not given a formatter argument? Indeed, it needs to know the margins (and current state) of the formatter where it…

Does sprintf not respect the margins set on str_formatter? At any rate, you can use fprintf with a custom buffer formatter that you flush to get the results, and set any properties you need on that formatter

Format.asprintf (the non-deprecated version of Format.sprintf) uses the str_formatter as a formatter. Thus, what you wanted to write was let () = Format.pp_set_geometry ~margin:3 ~max_indent:2 Format.str_formatter; let s = Format.asprintf "a@ a@ a" in Format.printf "%s" s Note however that usi…

Thanks for both the answer and the advice. [image] octachron: (the non-deprecated version of Format.sprintf) Format.sprintf appears not to be deprecated as of 5.4, see OCaml library : Format (although it looks indeed completely subsumed by Format.asprintf; same with Format.k(a)sprintf). […

Well, I should have checked the solution before replying: at least on OCaml 4.14.2 toplevel, it does not seem to work: $ ocaml OCaml version 4.14.2 [...] # let () = Format.pp_set_geometry ~margin:3 ~max_indent:2 Format.str_formatter; let s = Format.asprintf "a@ a@ a@ a@ a" in Format.printf "%s@…

You are right, the str_formatter is not connected to the sprintf function, I should have checked. It is thus probably better to create your own asprintf function: let asprintf' ~margin ~max_indent fmt = let b = Buffer.create 17 in let ppf = Format.formatter_of_buffer b in Format.pp_set_ge…

Using Format.sprintf

Learning

JohnJ October 19, 2025, 12:11pm 7

For the sake of completeness, it is also possible to get ahold of the formatter used by asprintf: https://discuss.ocaml.org/t/disabling-format-wrapping/15180/3?u=johnj

Topic		Replies	Views
Disabling Format wrapping Learning	3	210	August 23, 2024
Formatter string provided by a function. How can I do that? Learning	1	247	June 7, 2024
Obtaining the necessary type for format strings + arguments Learning pretty-printing , printf	7	1524	January 11, 2021
Generic format function for a sequence of floats Learning	5	1705	August 15, 2017
Using Format to define styled strings Learning format	8	250	October 28, 2025

Using Format.sprintf

Related topics