append_if.ex

defmodule LangUtil do
  @doc ~S"""
  For use in a pipeline like so:

    s
    |> append_if(c.section, "Section: #{c.section}\n")
    |> append_if(true, "Description: #{c.short_description}\n")
    |> append_if(c.long_description, prefix_every_line(c.long_description, " ") <> "\n")

  `expression` is not evaluated unless evaluation of `clause` is truthy.  This avoids
  blowing up on nils and other unexpected values.
  """
  defmacro append_if(acc, clause, expression) do
    quote do
      if unquote(clause) do
        unquote(acc) <> unquote(expression)
      else
        unquote(acc)
      end
    end
  end
end