Strange Error when Unit Testing a particular function in Ruby

Question

I'm working on this Exercise 49 in Learn Ruby the Hard Way

The exercise asks to write a unit test for each function provided. One of the unit tests I am writing is giving me an error.

Here is the code I am testing (word_list is an array of Pair structs)

Pair = Struct.new(:token, :word)

def peek(word_list)
    begin
        word_list.first.token
    rescue
        nil
    end
end

def match(word_list, expecting)
    begin
        word = word_list.shift
        if word.token == expecting
            word
        else
            nil
        end
    rescue
        nil
    end
end

def skip(word_list, token)
    while peek(word_list) == token
        match(word_list, token)
    end
end

Here is the test:

def test_skip
    small_list = [Pair.new(:verb, 'go'), Pair.new(:verb, 'do')]
    skip(small_list, :verb)
    assert_equal([],small_list) 
end

Here is the error I get when I run the unit test:

  1) Error:
test_skip(SentenceTests):
TypeError: backtrace must be Array of String
    test_sentence.rb:23:in `test_skip'

In this case, line 23 refers to "skip(small_list, :verb)". I am not sure why this error is happening, the above two functions were unit tested as well and those tests came out fine.

@Zabba, I did put them exactly as specified in the exercise above:

class ParserError < Exception

end

If anyone needs to see the exact files I am using here is the link to a gist: https://gist.github.com/1190148

Answer 1

The line

skip(small_list, :verb)

makes your test skip through the whole list (2 verbs). Finally, it is empty. So small_list.firstis nil which has no method/field word.

I get the error

undefined method `word' for nil:NilClass (NoMethodError)

though.

Probably in your code this exception is caught by some other code which itself raises the exception you are seeing.

UPDATE

It turns out in Ruby 1.9's test/unit (more specifically in the minitest/unit it is based on), there is a method skip which collides with your skip. See minitest/unit.rb, line 610. That method raised the exception because it expected its 2nd parameter to be an array.

Just rename your skip.