#cond and other builtin procedures

lib/schemer/ast.rb

@@ -1,7 +1,16 @@
 module Schemer
   module AST
 
-    class CharacterLiteral
+    class Node
+      def true?
+        true
+      end
+      def false?
+        !true?
+      end
+    end
+
+    class CharacterLiteral < Node
       attr_reader :value
 
       def initialize(character)
@@ -13,7 +22,7 @@ def inspect
       end
     end
 
-    class Identifier
+    class Identifier < Node
       attr_reader :value
 
       def initialize(identifier)
@@ -29,7 +38,7 @@ def inspect
       end
     end
 
-    class QuotedIdentifier
+    class QuotedIdentifier < Node
       attr_reader :value
 
       def initialize(identifier)
@@ -41,7 +50,7 @@ def inspect
       end
     end
 
-    class Expression
+    class Expression < Node
       attr_reader :proc, :args
 
       def initialize(expression)
@@ -56,7 +65,11 @@ def eval(context)
           if arg.is_a?(Identifier)
             arg = context.get_binding(arg.value) || arg
           else
-            arg ||= arg.eval(context)
+            begin
+              arg = arg.eval(context)
+            rescue
+              arg
+            end
           end
         end if args
 
@@ -68,6 +81,12 @@ def eval(context)
         end
 
         block.call(*arguments)
+      rescue NoMethodError=>e
+        if e.message =~ /#{@proc.value}/
+          raise "#{@proc.value} is not a defined procedure."
+        else
+          raise e
+        end
       end
 
       def to_list
@@ -79,7 +98,7 @@ def inspect
       end
     end
 
-    class List
+    class List < Node
       attr_reader :elements
 
       def initialize(elements)
@@ -103,7 +122,7 @@ def inspect
       end
     end
 
-    class QuotedList
+    class QuotedList < Node
       attr_reader :elements
 
       def initialize(elements)
@@ -115,7 +134,7 @@ def inspect
       end
     end
 
-    class Vector
+    class Vector < Node
       attr_reader :elements
 
       def initialize(elements)
@@ -127,51 +146,5 @@ def inspect
       end
     end
 
-    class AddOperator
-      def inspect
-        "#<Operator::Add>"
-      end
-    end
-    class SubtractOperator
-      def inspect
-        "#<Operator::Subtract>"
-      end
-    end
-    class MultiplyOperator
-      def inspect
-        "#<Operator::Multiply>"
-      end
-    end
-    class DivideOperator
-      def inspect
-        "#<Operator::Divide>"
-      end
-    end
-    class GteOperator
-      def inspect
-        "#<Operator::GreaterThanOrEqual>"
-      end
-    end
-    class GtOperator
-      def inspect
-        "#<Operator::GreaterThan>"
-      end
-    end
-    class LteOperator
-      def inspect
-        "#<Operator::LowerThanOrEqual>"
-      end
-    end
-    class LtOperator
-      def inspect
-        "#<Operator::LowerThan>"
-      end
-    end
-    class EqualOperator
-      def inspect
-        "#<Operator::Equal>"
-      end
-    end
-
   end
 end

lib/schemer/environment.rb

@@ -8,11 +8,11 @@ def initialize(parent = nil)
     end
 
     def add_binding(name, value)
-      @bindings.update({ name => value })
+      @bindings.update({ name.to_s => value })
     end
 
     def get_binding(name)
-      if got = @bindings[name]
+      if got = @bindings[name.to_s]
         return got
       elsif ! @parent.nil?
         @parent.get_binding(name)

lib/schemer/interpreter.rb

@@ -72,6 +72,35 @@ def initialize(ast = nil)
           list.to_list.elements.last
         end)
 
+        env.add_binding(:list, lambda do |*args|
+          AST::List.new(args)
+        end)
+
+        env.add_binding(:null?, lambda do |object|
+          (object.respond_to?(:empty?) && object.empty?) || object.nil?
+        end)
+
+        env.add_binding("=", lambda do |one, another|
+          one == another
+        end)
+
+        env.add_binding(">", lambda do |one, another|
+          one > another
+        end)
+
+        env.add_binding("<", lambda do |one, another|
+          one < another
+        end)
+
+        env.add_binding(:cond, lambda do |*conditions|
+          conditions.map!(&:to_list).map!(&:to_a)
+          conditions.each do |condition, result|
+            return result.eval(env) if condition.eval(env)
+          end
+          nil
+        end)
+
+
       end
     end
 

spec/schemer/interpreter_spec.rb

@@ -114,6 +114,102 @@ module Schemer
         end
       end
 
+      describe "#list" do
+        it 'converts elements to a list' do
+          expression = "(list 3 4 x y)"
+          lexer = Schemer::Lexer.new
+          parser = Schemer::Parser.new
+          ast = parser.apply(lexer.parse expression) 
+
+          interpreter = Schemer::Interpreter.new(ast)
+          output = interpreter.walk
+          output.should be_a(AST::List)
+          output.should have(4).elements
+        end
+      end
+
+      describe "#null?" do
+        context 'if the object is nil' do
+          it 'returns true' do
+            expression = "(null? 3)"
+            lexer = Schemer::Lexer.new
+            parser = Schemer::Parser.new
+            ast = parser.apply(lexer.parse expression) 
+
+            interpreter = Schemer::Interpreter.new(ast)
+            interpreter.walk.should be_false
+          end
+        end
+        context 'if the object is empty or nil' do
+          it 'returns true' do
+            expression = "(null? ())"
+            lexer = Schemer::Lexer.new
+            parser = Schemer::Parser.new
+            ast = parser.apply(lexer.parse expression) 
+
+            interpreter = Schemer::Interpreter.new(ast)
+            interpreter.walk.should be_true
+          end
+        end
+      end
+
+      describe "#=" do
+        it 'returns the equality of two arguments' do
+          expression = "(= 3 (+ 1 2))"
+          lexer = Schemer::Lexer.new
+          parser = Schemer::Parser.new
+          ast = parser.apply(lexer.parse expression) 
+
+          interpreter = Schemer::Interpreter.new(ast)
+          interpreter.walk.should be_true
+        end
+      end
+
+      describe "#<" do
+        it 'returns true if foo is less than bar' do
+          expression = "(< 3 (+ 2 2))"
+          lexer = Schemer::Lexer.new
+          parser = Schemer::Parser.new
+          ast = parser.apply(lexer.parse expression) 
+
+          interpreter = Schemer::Interpreter.new(ast)
+          interpreter.walk.should be_true
+        end
+      end
+
+      describe "#>" do
+        it 'returns true if foo is more than bar' do
+          expression = "(> 3 (+ 2 2))"
+          lexer = Schemer::Lexer.new
+          parser = Schemer::Parser.new
+          ast = parser.apply(lexer.parse expression) 
+
+          interpreter = Schemer::Interpreter.new(ast)
+          interpreter.walk.should be_false
+        end
+      end
+
+      describe "#cond" do
+        it 'returns the first condition that evaluates to true' do
+          expression = "(cond ((< 3 1) 9) ((>3 1) 7) )"
+          lexer = Schemer::Lexer.new
+          parser = Schemer::Parser.new
+          ast = parser.apply(lexer.parse expression) 
+
+          interpreter = Schemer::Interpreter.new(ast)
+          interpreter.walk.should == 7
+        end
+        it 'even if the result is an expression' do
+          expression = "(cond ((< 3 1) 9) ((>3 8) 7) ((= 3 3) (+ 3 9)) )"
+          lexer = Schemer::Lexer.new
+          parser = Schemer::Parser.new
+          ast = parser.apply(lexer.parse expression) 
+
+          interpreter = Schemer::Interpreter.new(ast)
+          interpreter.walk.should == 12
+        end
+      end
+
     end
 
   end