End in EOF

lib/chervil/lexer.rb

       when ')'
         advance
         Token.new(:rparen, ")")
+      when nil
+        Token.new(:eof, "eof")
       end
     end
 
     def tokenize
       tokens = Array.new
-      until @position == @source.size
-        tokens << get_next_token
+      loop do
+        token = get_next_token
+        tokens << token
+        break if token.type == :eof
       end
       tokens
     end

spec/lexer_spec.rb

   RSpec.describe Lexer do
     it 'lexes parens' do
       expect(Lexer.new('()').tokenize).to eq(
-        [Token.new(:lparen, '('), Token.new(:rparen, ')')]
+        [
+          Token.new(:lparen, '('),
+          Token.new(:rparen, ')'),
+          Token.new(:eof, 'eof')
+        ]
       )
     end
   end