Ruby Koans: Why convert list of symbols to strings

Question

I'm referring to this test in about_symbols.rb in Ruby Koans https://github.com/edgecase/ruby_koans/blob/master/src/about_symbols.rb#L26

def test_method_names_become_symbols   symbols_as_strings = Symbol.all_symbols.map { |x| x.to_s }   assert_equal true, symbols_as_strings.include?("test_method_names_become_symbols") end     # THINK ABOUT IT:   #   # Why do we convert the list of symbols to strings and then compare   # against the string value rather than against symbols? 

Why exactly do we have to convert that list into strings first?

Answer 1

This has to do with how symbols work. For each symbol, only one of it actually exists. Behind the scenes, a symbol is just a number referred to by a name (starting with a colon). Thus, when comparing the equality of two symbols, you're comparing object identity and not the content of the identifier that refers to this symbol.

If you were to do the simple test :test == "test", it will be false. So, if you were to gather all of the symbols defined thus far into an array, you would need to convert them to strings first before comparing them. You can't do this the opposite way (convert the string you want to compare into a symbol first) because doing that would create the single instance of that symbol and "pollute" your list with the symbol you're testing for existence.

Hope that helps. This is a bit of an odd one, because you have to test for the presence of a symbol without accidentally creating that symbol during the test. You usually don't see code like that.

Answer 2

Because if you do:

assert_equal true, all_symbols.include?(:test_method_names_become_symbols) 

It may (depending on your ruby implementation) automatically be true, because :test_method_names_become_symbols creates the symbol. See this bug report.