Console output (io)

Plan 11 step 40 (2026-10-07): io:format/1,2 and io:put_chars/1 write to standard output. They are runtime builtins of module io in the bridge catalog (builtins); the runtime implements OTP's io_lib_format and io_lib_pretty rules in C++ (io_format.cpp, io_pretty.cpp).

Calls

• io:format(Format) is io:format(Format, []). Both return ok.
• io:put_chars(Chardata) writes chardata: Unicode characters, UTF-8 binaries and nested lists ending in [] or a binary. Anything else (an atom, a surrogate, invalid UTF-8, a bitstring) raises badarg.
• Text is built whole, then written as UTF-8, as OTP's standard output device does in -noshell mode (encoding unicode). An error writes nothing. A rejected write is the runtime failure output_failure (exit 70).
• Qualified calls, fun io:format/2, apply(io, format, Args) and M:format(...) all reach the builtins. Other io functions keep the unknown module io diagnostic. A batch module named io does not replace the builtins (OTP's io is sticky).

Formats

• Format is an atom (its characters), a binary (its bytes as Latin-1 characters) or a list. A list element that is not a character is chardata written as it is, without control sequences, and moves the column by one (OTP's quirk).
• Control sequences ~F.P.PadModC: field width F (negative or -: left adjusted), precision P, pad character, * taking an integer (or the pad) from the arguments, modifiers t (Unicode), l (no string detection) and k (ordered maps; Clause always prints maps in key order).
• Supported C: ~w, ~p, ~s, ~c, ~b, ~B, ~i, ~n, ~~, with OTP's field rules: ~s truncates or pads to P within F; ~w and ~b print * characters when the text does not fit F; ~c repeats the character; ~b/~B take the base from P (2..36); ~n with F writes F newlines.
• Without t, ~s takes Latin-1 characters only and ~c writes the low byte of its integer; ~w/~p escape atom characters beyond Latin-1 as \x{...}. With t, ~s reads binaries as UTF-8 (falling back to Latin-1) and ~p writes <<"..."/utf8>> for printable UTF-8 binaries.
• badarg: an unknown control character, missing or extra arguments, an argument of the wrong type, F smaller than P for ~s/~c, a left adjusted ~p or ~n, a base outside 2..36, output that is not Unicode.
• Not implemented (badarg, see differences): ~e, ~f, ~g, ~x, ~X, ~+, ~#, ~W, ~P and the K modifier.

Pretty printing (~p)

• Line length is F (default 80, 0 writes one line); the first column is P, else the column the output has reached on its line (tabs to multiples of 8).
• The term becomes OTP's intermediate form: each value with its one-line length; strings (flat lists of printable Latin-1 characters, the default io:printable_range()), printable binaries, atoms and numbers are text.
• What fits on the rest of the line is written whole. Lists and tuples break between elements, aligned after the bracket; tagged tuples ({atom, ...}) align after the tag, or indent by 4 (or 1) when that would pass half the line; map pairs and native record fields that do not fit put the value on the next line; long binaries wrap between bytes.
• Containers nested more than 256 deep raise system_limit (the layout recurses on the native stack).
• Formatting runs to completion, however large the term (portions).