For future readers, here’s the format needed:

:io.fwrite("#~2.16.0B~2.16.0B~2.16.0B", [r, g, b])

# prints the string "#F27B04"

Values smaller than zero or larger than 255 will be replaced with **:

:io.fwrite("#~2.16.0B~2.16.0B~2.16.0B", [-r, g, b])

# prints "#**7B04"

:io.fwrite("#~2.16.0B~2.16.0B~2.16.0B", [10*r, g, b])

# prints "#**7B04"

Breakdown in detail:

~2.16.0B

• 2 is the field width. Without it, values less than 16 will only output a single character
• 16 is called “precision” in the API, but for the B control sequence it’s the base to use
• 0 is the padding character, because we want 4 to print as 04 not <space>4

That topic gave me an idea for a library. I will try to draft something this weekend.

IMO that’s too big of a change - one of the nice things about Elixir is the regularity of the syntax, and that would make #{} magical in a way that’s not available elsewhere. A more “Elixir-y” solution would be to make the conversion explicit:

"##{to_hex(r)}#{to_hex(g)}#{to_hex(b)}"
...
defp to_hex(v), do: :io_lib.format("~2.16.0B", [v])

Some other gotchas that come to mind:

• making runtime-determined values work (* in the various positions) means more tricky syntax
• what would "#{some_calculation():i}" do? “Ignore one thing from the input” makes a lot more sense when the format and the data are fully separated (as in :io)
• the term-handling codes don’t output Elixir-shaped strings:

:io.fwrite("~w", [DateTime.utc_now()])

prints

#{'__struct__' => 'Elixir.DateTime',calendar => 'Elixir.Calendar.ISO',day => 1,hour => 22,microsecond => {937508,6},minute => 47,month => 5,second => 18,std_offset => 0,time_zone => <<69,116,99,47,85,84,67>>,utc_offset => 0,year => 2022,zone_abbr => <<85,84,67>>}