Implement builtin procedures #car and #cdr

lib/schemer/ast.rb

@@ -90,6 +90,10 @@ def to_a
         @elements.to_a
       end
 
+      def to_list
+        self
+      end
+
       def empty?
         @elements.empty?
       end

lib/schemer/interpreter.rb

@@ -64,6 +64,14 @@ def initialize(ast = nil)
           end
         end)
 
+        env.add_binding(:car, lambda do |list|
+          list.to_list.elements.first
+        end)
+
+        env.add_binding(:cdr, lambda do |list|
+          list.to_list.elements.last
+        end)
+
       end
     end
 

spec/schemer/interpreter_spec.rb

@@ -90,6 +90,30 @@ module Schemer
         end
       end
 
+      describe "#car" do
+        it 'returns the first element from a list' do
+          expression = "(car (x 3))"
+          lexer = Schemer::Lexer.new
+          parser = Schemer::Parser.new
+          ast = parser.apply(lexer.parse expression) 
+
+          interpreter = Schemer::Interpreter.new(ast)
+          interpreter.walk.should be_a(AST::Identifier)
+        end
+      end
+
+      describe "#cdr" do
+        it 'returns the last element from a list' do
+          expression = "(cdr (x 3))"
+          lexer = Schemer::Lexer.new
+          parser = Schemer::Parser.new
+          ast = parser.apply(lexer.parse expression) 
+
+          interpreter = Schemer::Interpreter.new(ast)
+          interpreter.walk.should == 3
+        end
+      end
+
     end
 
   end