String.valid?/1

I was recently working on a project that involved printing the contents of a file in an Elixir app. This is generally simple using File.read, but an issue arose when trying to print the contents of a binary file. The unusual bytes caused errors while trying to print.

While there is no way to be 100% sure a file is binary (mime or even file extension can help), one way to at least ensure we could print the file contents in Elixir was to use String.valid?.

This is the entire implementation from the Elixir 1.11 source:

  def valid?(<<_::utf8, t::binary>>), do: valid?(t)
  def valid?(<<>>), do: true
  def valid?(_), do: false

You can see it tries to cast each character as UTF-8. If it reaches the end of the string, it is valid - super simple, right?

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