Implement the "!" evaluation operator

git-svn-id: https://svn.tlawal.org/svn/monkey@46 f6afcba9-9ef1-4bdd-9b72-7484f5705bac

evaluator/evaluator.go

@@ -5,9 +5,15 @@ import (
 	"monkey/object"
 )
 
+var (
+	NULL  = &object.Null{}
+	TRUE  = &object.Boolean{Value: true}
+	FALSE = &object.Boolean{Value: false}
+)
+
 func Eval(node ast.Node) object.Object {
 	switch node := node.(type) {
-  	// Statements
+	// Statements
 	case *ast.Program:
 		return eval_statements(node.Statements)
 
@@ -19,7 +25,12 @@ func Eval(node ast.Node) object.Object {
 		return &object.Integer{Value: node.Value}
 
 	case *ast.Boolean:
-		return &object.Boolean{Value: node.Value}
+		return native_bool_to_boolean_object(node.Value)
+
+	case *ast.PrefixExpression:
+		right := Eval(node.Right)
+		return eval_prefix_expression(node.Operator, right)
+
 	}
 
 	return nil
@@ -33,3 +44,36 @@ func eval_statements(statements []ast.Statement) object.Object {
 	}
 	return result
 }
+
+func native_bool_to_boolean_object(input bool) *object.Boolean {
+	if input {
+		return TRUE
+	}
+	return FALSE
+}
+
+func eval_prefix_expression(operator string, right object.Object) object.Object {
+	switch operator {
+	case "!":
+		return eval_bang_operator_expression(right)
+
+	default:
+		return NULL
+	}
+}
+
+func eval_bang_operator_expression(right object.Object) object.Object {
+	switch right {
+	case TRUE:
+		return FALSE
+
+	case FALSE:
+		return TRUE
+
+	case NULL:
+		return TRUE
+
+	default:
+		return FALSE
+	}
+}

evaluator/evaluator_test.go

@@ -22,21 +22,40 @@ func TestEvalIntegerExpression(l_test *testing.T) {
 	}
 }
 
-func TestEvalBooleanExpression(l_test *testing.T){
+func TestEvalBooleanExpression(l_test *testing.T) {
 	tests := []struct {
-		input string
+		input    string
 		expected bool
 	}{
 		{"true", true},
 		{"false", false},
 	}
 
-	for _, tt := range tests{
-			evaluated := test_eval(tt.input)
-			test_boolean_object(l_test, evaluated, tt.expected)
-		}
+	for _, tt := range tests {
+		evaluated := test_eval(tt.input)
+		test_boolean_object(l_test, evaluated, tt.expected)
+	}
+}
+
+// Test to convert the '!' operator to boolean value and negate it
+func TestBangOperator(l_test *testing.T) {
+	tests := []struct {
+		input    string
+		expected bool
+	}{
+		{"!true", false},
+		{"!false", true},
+		{"!5", false},
+		{"!!true", true},
+		{"!!false", false},
+		{"!!5", true},
 	}
 
+	for _, tt := range tests {
+		evaluated := test_eval(tt.input)
+		test_boolean_object(l_test, evaluated, tt.expected)
+	}
+}
 
 // Helpers
 func test_eval(input string) object.Object {
@@ -60,7 +79,7 @@ func test_integer_object(l_test *testing.T, l_object object.Object, expected int
 	return true
 }
 
-func test_boolean_object(l_test *testing.T, l_object object.Object, expected bool)bool {
+func test_boolean_object(l_test *testing.T, l_object object.Object, expected bool) bool {
 	result, ok := l_object.(*object.Boolean)
 	if !ok {
 		l_test.Errorf("object is not Boolean, got=%T (%+v)", l_object, l_object)

repl/repl.go

@@ -4,9 +4,9 @@ import (
 	"bufio"
 	"fmt"
 	"io"
+	"monkey/evaluator"
 	"monkey/lexer"
 	"monkey/parser"
-	"monkey/evaluator"
 )
 
 const MONKEY_FACE = `            __,__
@@ -45,7 +45,7 @@ func Start(in io.Reader, out io.Writer) {
 		}
 
 		evaluated := evaluator.Eval(program)
-		if evaluated != nil{
+		if evaluated != nil {
 			io.WriteString(out, evaluated.Inspect())
 			io.WriteString(out, "\n")
 		}