2 files changed, 91 insertions, 2 deletions
diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
index 8d1b26d..0cc003f 100644
--- a/evaluator/evaluator.go
+++ b/evaluator/evaluator.go
@@ -5,6 +5,12 @@ import (
 	"mana/object"
 )
 
+var (
+	NULL  = &object.Null{}
+	TRUE = &object.Boolean{Value: true}
+	FALSE = &object.Boolean{Value: false}
+)
+
 // Eval evaluates the given ast.Node and returns an object.Object.
 func Eval(node ast.Node) object.Object {
 	switch node := node.(type) {
@@ -18,6 +24,13 @@ func Eval(node ast.Node) object.Object {
 	// Expressions
 	case *ast.IntegerLiteral:
 		return &object.Integer{Value: node.Value}
+
+	case *ast.Boolean:
+		return nativeBoolToBooleanObject(node.Value)
+
+	case *ast.PrefixExpression:
+		right := Eval(node.Right)
+		return evalPrefixExpression(node.Operator, right)
 	}
 
 	return nil
@@ -32,3 +45,32 @@ func evalStatements(stmts []ast.Statement) object.Object {
 
 	return result
 }
+
+func nativeBoolToBooleanObject(input bool) *object.Boolean {
+	if input {
+		return TRUE
+	}
+	return FALSE
+}
+
+func evalPrefixExpression(operator string, right object.Object) object.Object {
+	switch operator {
+	case "!":
+		return evalBangOperatorExpression(right)
+	default:
+		return NULL
+	}
+}
+
+func evalBangOperatorExpression(right object.Object) object.Object {
+	switch right {
+	case TRUE:
+		return FALSE
+	case FALSE:
+		return TRUE
+	case NULL:
+		return TRUE
+	default:
+		return FALSE
+	}
+}
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index cd1edb1..aa1833a 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -18,8 +18,42 @@ func TestEvalIntegerExpression(t *testing.T) {
 	}
 
 	for _, tt := range tests {
-		evalated := testEval(tt.input)
-		testIntegerObject(t, evalated, tt.expected)
+		evaluated := testEval(tt.input)
+		testIntegerObject(t, evaluated, tt.expected)
+	}
+}
+
+func TestEvalBooleanExpression(t *testing.T) {
+	tests := []struct {
+		input	string
+		expected bool
+	}{
+		{"true", true},
+		{"false", false},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		testBooleanObject(t, evaluated, tt.expected)
+	}
+}
+
+func TestBangOperator(t *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 := testEval(tt.input)
+		testBooleanObject(t, evaluated, tt.expected)
 	}
 }
 
@@ -42,3 +76,16 @@ func testIntegerObject(t *testing.T, obj object.Object, expected int64) bool {
 	}
 	return true
 }
+
+func testBooleanObject(t *testing.T, obj object.Object, expected bool) bool {
+	result, ok := obj.(*object.Boolean)
+	if !ok {
+		t.Errorf("object is not Boolean. got=%T (%+v)", obj, obj)
+		return false
+	}
+	if result.Value != expected {
+		t.Errorf("object has wrong value. got=%t, want=%t", result.Value, expected)
+		return false
+	}
+	return true
+}