Implement quoted values

lib/chervil/ast.rb

     require "chervil/ast/identifier"
     require "chervil/ast/list"
     require "chervil/ast/number"
+    require "chervil/ast/quotation"
     require "chervil/ast/string"
   end
 end

lib/chervil/ast/quotation.rb

+module Chervil::AST
+  class Quotation
+    attr_reader :value
+
+    def initialize(value)
+      @value = value
+    end
+
+    def evaluate(env)
+      if @value.is_a?(List)
+        @value.elements
+      else
+        @value.evaluate(env)
+      end
+    end
+
+    def ==(other)
+      @value == other.value
+    end
+  end
+end

lib/chervil/lexer.rb

         else
           raise "Unexpected character #{current_char}"
         end
-      when '"', '\''
+      when '\''
+        advance
+        Token.new(:quote, '\'')
+      when '"'
         delimiter = current_char
         advance
         string = String.new

lib/chervil/parser.rb

         identifier
       elsif @current_token.type == :lparen
         list
+      elsif @current_token.type == :quote
+        quotation
       else
         @error = Error.new("Unexpected token #{@current_token.type}: #{@current_token.value}")
       end
     end
 
+    def quotation
+      eat(:quote)
+      value = expr
+      AST::Quotation.new(value)
+    end
+
     def list
       eat(:lparen)
 

spec/lexer_spec.rb

       expect(Lexer.new('"hello"').get_next_token).to eq(
         Token.new(:string, 'hello')
       )
-      expect(Lexer.new("'world'").get_next_token).to eq(
-        Token.new(:string, 'world')
-      )
     end
 
     it 'lexes booleans' do
       lexer = Lexer.new('"hello world')
       expect(lexer.get_next_token).to eq(Error.new("Unterminated string"))
     end
+
+    it 'lexes quotes' do
+      lexer = Lexer.new('\'')
+      expect(lexer.get_next_token).to eq(Token.new(:quote, '\''))
+    end
   end
 end

spec/parser_spec.rb

         )
       )
     end
+
+    it 'parses a quoted value' do
+      expect(parse('\'(1 2 3)').first).to eq(
+        AST::Quotation.new(
+          AST::List.new(
+            [
+              AST::Number.new(1.0),
+              AST::Number.new(2.0),
+              AST::Number.new(3.0),
+            ]
+          )
+        )
+      )
+    end
   end
 end