A tag struct.

Use Owl.Data.tag/2 to build a tag.

Tag is a container for data and ANSI sequences associated with it. It allows having local binding for styles in console, similar to tags in HTML.

Let's say you have a string that should be written to stdout in red color. This can be easily done by using a naive approach:

substring = "world"
IO.puts([IO.ANSI.red(), "Hello #{substring}!!"])

It works well for easy cases. If you want to make substring in another color you can try this:

substring = [IO.ANSI.green(), "world"]
IO.puts([IO.ANSI.red(), "Hello #{substring}!!"])

but you'll notice, that the text after substring is green too. In order make "!!" part red again, you have to write color explicitly:

IO.puts([IO.ANSI.red(), "Hello #{substring}#{IO.ANSI.red()}!!"])

if substring changes background color, you have to return to the previous one too:

substring = [IO.ANSI.green(), IO.ANSI.red_background() "world"]
IO.puts([IO.ANSI.red(), "Hello #{substring}#{[IO.ANSI.red(), IO.ANSI.default_background()]}!!"])

Such code is very hard to maintain. This is how the issue can be addressed with Owl.Data.tag/2:

substring = Owl.Data.tag("world", :green)
Owl.IO.puts(Owl.Data.tag(["Hello ", substring, "!!"], :red))

substring = Owl.Data.tag("world", [:green, :red_background])
Owl.IO.puts(Owl.Data.tag(["Hello ", substring, "!!"], :red))