Implement the "!" evaluation operator

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

evaluator/evaluator.go

@@ -5,6 +5,12 @@ 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
@@ -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

@@ -37,6 +37,25 @@ func TestEvalBooleanExpression(l_test *testing.T){
 	}
 }
 
+// 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 {

repl/repl.go

@@ -4,9 +4,9 @@ import (
 	"bufio"
 	"fmt"
 	"io"
+	"monkey/evaluator"
 	"monkey/lexer"
 	"monkey/parser"
-	"monkey/evaluator"
 )
 
 const MONKEY_FACE = `            __,__