So, I was in the process of writing this code today, and went on a wild goose chase due to this weird error message. I'd expected to see the nil come back from String#index, and give a different error, but no NilClass to be found in this error. String isn't even a type that String#index can return. It should only return a Fixnum or nil. Instead:…

reorder.rb

#!/usr/bin/env ruby

require 'minitest/autorun'

class Reorder
  SINGLE_CHAR = /./

  def initialize(word, order)
    @word, @order = word, order
  end

  def order
    @word.scan(SINGLE_CHAR).sort { |a, b| @order.index(a) <=> @order.index(b) }.
      join
  end
end

describe Reorder do
  it 'order banana with abc' do
    Reorder.new('banana', 'abc').order.must_equal 'aaabnn'
  end
end

#   1) Error:
# Reorder#test_0001_order banana with abc:
# ArgumentError: comparison of String with String failed
#     ./reorder.rb:13:in `sort'
#     ./reorder.rb:13:in `order'
#     ./reorder.rb:20:in `block (2 levels) in <main>'