From 4b67c7d6e1fe55463981f5e777da68c6471077eb Mon Sep 17 00:00:00 2001
From: tijani <tijani@tijanilawal.com>
Date: Mon, 8 Aug 2022 10:14:14 +0000
Subject: [PATCH] Implement the "!" evaluation operator

git-svn-id: https://svn.tlawal.org/svn/monkey@46 f6afcba9-9ef1-4bdd-9b72-7484f5705bac
---
 evaluator/evaluator.go      | 48 +++++++++++++++++++++++++++++++++++--
 evaluator/evaluator_test.go | 33 +++++++++++++++++++------
 repl/repl.go                |  6 ++---
 3 files changed, 75 insertions(+), 12 deletions(-)

diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
index 5ab22bf..632ad2b 100644
--- a/evaluator/evaluator.go
+++ b/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
+	}
+}
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index ade6bd7..cb6e505 100644
--- a/evaluator/evaluator_test.go
+++ b/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)
diff --git a/repl/repl.go b/repl/repl.go
index d2f8efe..e4a3916 100644
--- a/repl/repl.go
+++ b/repl/repl.go
@@ -4,9 +4,9 @@ import (
 	"bufio"
 	"fmt"
 	"io"
+	"monkey/evaluator"
 	"monkey/lexer"
 	"monkey/parser"
-	"monkey/evaluator"
 )
 
 const MONKEY_FACE = `            __,__
@@ -43,9 +43,9 @@ func Start(in io.Reader, out io.Writer) {
 			print_parser_errors(out, l_parser.Errors())
 			continue
 		}
-		
+
 		evaluated := evaluator.Eval(program)
-		if evaluated != nil{
+		if evaluated != nil {
 			io.WriteString(out, evaluated.Inspect())
 			io.WriteString(out, "\n")
 		}