String object extensions for handling proper indentation duration HEREDOC interpolation of multi-line strings.

heredoc.rb

# Created in response to Stack Overflow question: 
#   http://stackoverflow.com/questions/38504004/interpolate-multiline-string-with-correct-indent

module CoreExtensions
  module String
    module Heredoc

      # Special character to flag lines that are part of a multiline HEREDOC interpolation
      # Any character that is not part of the output string will work.  Using "\r" because
      # it's rarely used in hard-coded strings.
      ML_CHAR = "\r"

      def hd_multiline
        # mark al lines in this string as a multiline heredoc insert
        self.split("\n").join("#{ML_CHAR}\n")
      end

      # Same as Rails' #strip_heredoc without requiring ActiveSupport
      def hd_strip
        indent = scan(/^[ \t]*(?=\S)/).min.size rescue 0
        gsub(/^[ \t]{#{indent}}/, '')
      end

      # Method to properly indent a HEREDOC with multiline interpolations
      def hd_render
        lines = self.split("\n") # split the string into separate lines

        # current indentation
        indent = ""

        new_lines = [] # buffer array for result string's lines
        new_lines << lines.first # always take the first line as-is

        lines.each_with_index do |line, i|
          # get the indentation for this line
          this_indent = line.scan(/^ +/).first || ""

          # if it's greater than the previous line's, update the current indentation
          indent = this_indent unless this_indent.length < indent.length

          # Load the next line
          next_line = lines[i+1]
          if next_line
            if line.scan(/#{ML_CHAR}+/).first # we have a multiline indent
              new_lines << "#{indent}#{next_line}" # add the current indentation to the next line
            else # not a multiline indent, keep the line as-is
              new_lines << next_line
            end
          end
        end

        # clean up the added ML_CHAR characters
        new_lines = new_lines.map{|line| "#{line.gsub(/#{ML_CHAR}+/, '')}"}

        result = result = new_lines.join("\n") # re-join the lines as a contiguous string
        result
      end
    end
  end
end

class String
  self.send(:include, CoreExtensions::String::Heredoc)
end

## Example use case

def parse_string
  sub_substring = <<-STRING.hd_strip
    indented line2
      double indented line1
  STRING

  # Must call #hd_render on any HEREDOCS with interpolated multiline strings
  # Note that #hd_render must be called before #hd_strip
  substring = <<-STRING.hd_render.hd_strip
    first line
    second line
      indented line1
      #{ sub_substring.hd_multiline }
        double indented line2
    third_line
  STRING

  string = <<-STRING.hd_render.hd_strip
    Quote
      #{ substring.hd_multiline }
    from substring
  STRING

  string
end

puts parse_string

=begin

Output:

Quote
  first line
  second line
    indented line1
    indented line2
      double indented line1
      double indented line2
  third_line
from substring

=end