nil.step

message <<-CONTENT
nil is a special Ruby data type that means "nothing".   It's equivalent to null or None in other programming languages.


CONTENT

goals do
goal "Know what nil is for"
end

step do
  message <<-CONTENT
    How do you set a variable to empty?

    Some of the values we could use can be legitimate values, like:

    `temperature = 0`

    `today_is_friday = false`

    `empty_phrase = ""`

    Nil to the rescue.  Nil means 'nothing'.  It's used to show that a variable hasn't been assigned anything yet, or that a function didn't return a value.
  CONTENT

  irb <<-IRB
    # Where is the letter "a" in apple?
    "apple".index("a")

    # Where is the letter "z"?
    "apple".index("z")
  IRB
end

step do
    message "You may have noticed this already:"
    irb 'puts "What does puts evaluate to?"'
    message "Ruby returns what an expression evaluated to.  `puts` puts a string on the screen, but always evaluates to nil."
end

step do
    message "Here's an error message you will see often."
    irb <<-IRB
      word = "hello"
      word.upcase
      word = nil
      word.upcase
    IRB
    result "NoMethodError: undefined method 'upcase' for nil:NilClass"
    message "What happened?"
    irb "word.class"
    message "`word` was set to nil. Let's look at the alphabetized list of methods the Nil class has."
    irb "word.methods.sort"
    message "The error message told us Nil doesn't have a method called upcase."
end

step do
  message "How can we avoid these errors?  There are a few ways to tell if something is nil. Let's try them!"

  irb <<-IRB
    word = "something"
    word.nil?
    word = nil
    word.nil?
  IRB

  message "Remember, two equals signs asks if something is equal:"
  irb "word == nil"

  message "What's going on here?"

  # Putting the marker in single quotes prevents interpolation
  source_code <<-'CONTENT'
    name = nil
    if name.nil?
      puts "The name hasn't been set yet."
    else
      puts "Your name is #{name}"
    end
  CONTENT
end


explanation do

end

further_reading do
    message "[Nil Class documentation](http://www.ruby-doc.org/core-2.1.0/NilClass.html)"
end


next_step 'symbols'