Introduced the evaluator (and integrated it with the REPL)

It supports integer and boolean literals, prefix expressions and infix
expressions with integer operands.

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

pkg/evaluator/evaluator.go

@@ -0,0 +1,104 @@
+package evaluator
+
+import (
+	"code.jmug.me/jmug/interpreter-in-go/pkg/ast"
+	"code.jmug.me/jmug/interpreter-in-go/pkg/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.
+	case *ast.Program:
+		return evalStatements(node.Statements)
+	case *ast.ExpressionStatement:
+		return Eval(node.Expression)
+		// 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)
+	case *ast.InfixExpression:
+		left := Eval(node.Left)
+		right := Eval(node.Right)
+		return evalInfixExpression(node.Operator, left, right)
+	}
+	return nil
+}
+
+func evalStatements(stmts []ast.Statement) object.Object {
+	var res object.Object
+	for _, stmt := range stmts {
+		res = Eval(stmt)
+	}
+	return res
+}
+
+func evalPrefixExpression(op string, right object.Object) object.Object {
+	switch op {
+	case "!":
+		return evalBangOperatorExpression(right)
+	case "-":
+		return evalMinusPrefixOperatorExpression(right)
+	}
+	return _NULL
+}
+
+func evalBangOperatorExpression(obj object.Object) object.Object {
+	switch obj {
+	case _TRUE:
+		return _FALSE
+	case _FALSE:
+		return _TRUE
+	case _NULL:
+		return _TRUE
+	default:
+		return _FALSE
+	}
+}
+
+func evalMinusPrefixOperatorExpression(obj object.Object) object.Object {
+	if obj.Type() != object.INTEGER_OBJ {
+		return _NULL
+	}
+	val := obj.(*object.Integer).Value
+	return &object.Integer{Value: -val}
+}
+
+func evalInfixExpression(op string, left, right object.Object) object.Object {
+	if left.Type() == object.INTEGER_OBJ && right.Type() == object.INTEGER_OBJ {
+		return evalIntegerInfixExpression(op, left, right)
+	}
+	return _NULL
+}
+
+func evalIntegerInfixExpression(op string, left, right object.Object) object.Object {
+	l := left.(*object.Integer).Value
+	r := right.(*object.Integer).Value
+	switch op {
+	case "+":
+		return &object.Integer{Value: l + r}
+	case "-":
+		return &object.Integer{Value: l - r}
+	case "*":
+		return &object.Integer{Value: l * r}
+	case "/":
+		return &object.Integer{Value: l / r}
+	}
+	return _NULL
+}
+
+func nativeBoolToBooleanObject(b bool) object.Object {
+	if b {
+		return _TRUE
+	}
+	return _FALSE
+}

pkg/evaluator/evaluator_test.go

@@ -0,0 +1,108 @@
+package evaluator
+
+import (
+	"testing"
+
+	"code.jmug.me/jmug/interpreter-in-go/pkg/lexer"
+	"code.jmug.me/jmug/interpreter-in-go/pkg/object"
+	"code.jmug.me/jmug/interpreter-in-go/pkg/parser"
+)
+
+func TestEvalIntegerExpression(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected int64
+	}{
+		{"5", 5},
+		{"10", 10},
+		{"-5", -5},
+		{"-10", -10},
+		{"5 + 5 + 5 + 5 - 10", 10},
+		{"2 * 2 * 2 * 2 * 2", 32},
+		{"-50 + 100 + -50", 0},
+		{"5 * 2 + 10", 20},
+		{"5 + 2 * 10", 25},
+		{"20 + 2 * -10", 0},
+		{"50 / 2 * 2 + 10", 60},
+		{"2 * (5 + 10)", 30},
+		{"3 * 3 * 3 + 10", 37},
+		{"3 * (3 * 3) + 10", 37},
+		{"(5 + 10 * 2 + 15 / 3) * 2 + -10", 50},
+	}
+
+	for _, tt := range tests {
+		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)
+	}
+}
+
+func testEval(input string) object.Object {
+	l := lexer.New(input)
+	p := parser.New(l)
+	program := p.ParseProgram()
+
+	return Eval(program)
+}
+
+func testIntegerObject(t *testing.T, obj object.Object, expected int64) bool {
+	result, ok := obj.(*object.Integer)
+	if !ok {
+		t.Errorf("object is not Integer. got=%T (%+v)", obj, obj)
+		return false
+	}
+	if result.Value != expected {
+		t.Errorf("object has wrong value. got=%d, want=%d",
+			result.Value, expected)
+		return false
+	}
+
+	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
+}

pkg/object/object.go

@@ -0,0 +1,47 @@
+package object
+
+import "fmt"
+
+type ObjectType string
+
+const (
+	INTEGER_OBJ = "INTEGER"
+	BOOLEAN_OBJ = "BOOLEAN"
+	NULL_OBJ    = "NULL"
+)
+
+type Object interface {
+	Type() ObjectType
+	Inspect() string
+}
+
+type Integer struct {
+	Value int64
+}
+
+func (i *Integer) Type() ObjectType {
+	return INTEGER_OBJ
+}
+func (i *Integer) Inspect() string {
+	return fmt.Sprintf("%d", i.Value)
+}
+
+type Boolean struct {
+	Value bool
+}
+
+func (b *Boolean) Type() ObjectType {
+	return BOOLEAN_OBJ
+}
+func (b *Boolean) Inspect() string {
+	return fmt.Sprintf("%t", b.Value)
+}
+
+type Null struct{}
+
+func (n *Null) Type() ObjectType {
+	return NULL_OBJ
+}
+func (n *Null) Inspect() string {
+	return "null"
+}

pkg/repl/repl.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 
+	"code.jmug.me/jmug/interpreter-in-go/pkg/evaluator"
 	"code.jmug.me/jmug/interpreter-in-go/pkg/lexer"
 	"code.jmug.me/jmug/interpreter-in-go/pkg/parser"
 )
@@ -25,9 +26,12 @@ func Start(in io.Reader, out io.Writer) {
 			printParserErrors(out, p.Errors())
 			continue
 		}
-		io.WriteString(out, program.String())
+		res := evaluator.Eval(program)
+		if res != nil {
+			io.WriteString(out, res.Inspect())
 			io.WriteString(out, "\n")
 		}
+	}
 }
 
 const MONKEY_FACE = `            __,__