diff --git a/ast.go b/ast.go
index 2e6d431..937f159 100644
--- a/ast.go
+++ b/ast.go
@@ -1,20 +1,20 @@
 package main
 
-import (
-	"fmt"
-	"strings"
-)
-
 type Visitor interface {
-	visitBinary(b *Binary)
-	visitLiteral(l *Literal)
-	visitGrouping(g *Grouping)
-	visitUnary(u *Unary)
+	visitUnary(u *Unary) any
+	visitBinary(b *Binary) any
+	visitLiteral(l *Literal) any
+	visitGrouping(g *Grouping) any
 }
 
 type Expr interface {
 	expr()
-	accept(v Visitor)
+	accept(v Visitor) any
+}
+
+type Unary struct {
+	op    Token
+	right Expr
 }
 
 type Binary struct {
@@ -23,114 +23,31 @@ type Binary struct {
 	right Expr
 }
 
-type Unary struct {
-	op    Token
-	right Expr
+type Literal struct {
+	value any
 }
 
 type Grouping struct {
 	expression Expr
 }
 
-type Literal struct {
-	value any
-}
-
 func (u *Unary) expr()    {}
-func (g *Grouping) expr() {}
-func (l *Literal) expr()  {}
 func (b *Binary) expr()   {}
+func (l *Literal) expr()  {}
+func (g *Grouping) expr() {}
 
-func (u *Unary) accept(v Visitor) {
-	v.visitUnary(u)
+func (u *Unary) accept(v Visitor) any {
+	return v.visitUnary(u)
 }
 
-func (g *Grouping) accept(v Visitor) {
-	v.visitGrouping(g)
+func (b *Binary) accept(v Visitor) any {
+	return v.visitBinary(b)
 }
 
-func (l *Literal) accept(v Visitor) {
-	v.visitLiteral(l)
+func (l *Literal) accept(v Visitor) any {
+	return v.visitLiteral(l)
 }
 
-func (b *Binary) accept(v Visitor) {
-	v.visitBinary(b)
-}
-
-type AstStringer struct {
-	str strings.Builder
-}
-
-func (as AstStringer) String(expr Expr) string {
-
-	if expr == nil {
-		return ""
-	}
-
-	expr.accept(&as)
-	return as.str.String()
-}
-
-func (as *AstStringer) visitBinary(b *Binary) {
-	as.str.WriteString("(")
-	as.str.WriteString(b.op.lexeme)
-	as.str.WriteString(" ")
-	b.left.accept(as)
-	as.str.WriteString(" ")
-	b.right.accept(as)
-	as.str.WriteString(")")
-
-}
-
-func (as *AstStringer) visitLiteral(l *Literal) {
-	as.str.WriteString(fmt.Sprintf("%v", l.value))
-}
-
-func (as *AstStringer) visitGrouping(g *Grouping) {
-	as.str.WriteString("(group ")
-	g.expression.accept(as)
-	as.str.WriteString(")")
-}
-
-func (as *AstStringer) visitUnary(u *Unary) {
-	as.str.WriteString(fmt.Sprintf("(%s ", u.op.lexeme))
-	u.right.accept(as)
-	as.str.WriteString(")")
-}
-
-type AstToRPN struct {
-	str strings.Builder
-}
-
-func (as AstToRPN) String(expr Expr) string {
-
-	if expr == nil {
-		return ""
-	}
-
-	expr.accept(&as)
-	return as.str.String()
-}
-
-func (as *AstToRPN) visitBinary(b *Binary) {
-	b.left.accept(as)
-	as.str.WriteString(" ")
-	b.right.accept(as)
-	as.str.WriteString(" ")
-	as.str.WriteString(b.op.lexeme)
-
-}
-
-func (as *AstToRPN) visitLiteral(l *Literal) {
-	as.str.WriteString(fmt.Sprintf("%v", l.value))
-}
-
-func (as *AstToRPN) visitGrouping(g *Grouping) {
-	g.expression.accept(as)
-	as.str.WriteString(" group")
-}
-
-func (as *AstToRPN) visitUnary(u *Unary) {
-	u.right.accept(as)
-	as.str.WriteString(fmt.Sprintf(" %s", u.op.lexeme))
+func (g *Grouping) accept(v Visitor) any {
+	return v.visitGrouping(g)
 }
diff --git a/ast_rpn.go b/ast_rpn.go
new file mode 100644
index 0000000..96b2e2f
--- /dev/null
+++ b/ast_rpn.go
@@ -0,0 +1,46 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+type AstToRPN struct {
+	str strings.Builder
+}
+
+func (as AstToRPN) String(expr Expr) string {
+
+	if expr == nil {
+		return ""
+	}
+
+	expr.accept(&as)
+	return as.str.String()
+}
+
+func (as *AstToRPN) visitBinary(b *Binary) any {
+	b.left.accept(as)
+	as.str.WriteString(" ")
+	b.right.accept(as)
+	as.str.WriteString(" ")
+	as.str.WriteString(b.op.lexeme)
+	return nil
+}
+
+func (as *AstToRPN) visitLiteral(l *Literal) any {
+	as.str.WriteString(fmt.Sprintf("%v", l.value))
+	return nil
+}
+
+func (as *AstToRPN) visitGrouping(g *Grouping) any {
+	g.expression.accept(as)
+	as.str.WriteString(" group")
+	return nil
+}
+
+func (as *AstToRPN) visitUnary(u *Unary) any {
+	u.right.accept(as)
+	as.str.WriteString(fmt.Sprintf(" %s", u.op.lexeme))
+	return nil
+}
diff --git a/ast_string.go b/ast_string.go
new file mode 100644
index 0000000..f54f2f9
--- /dev/null
+++ b/ast_string.go
@@ -0,0 +1,51 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+type AstStringer struct {
+	str strings.Builder
+}
+
+func (as AstStringer) String(expr Expr) string {
+
+	if expr == nil {
+		return ""
+	}
+
+	expr.accept(&as)
+	return as.str.String()
+}
+
+func (as *AstStringer) visitBinary(b *Binary) any {
+	as.str.WriteString("(")
+	as.str.WriteString(b.op.lexeme)
+	as.str.WriteString(" ")
+	b.left.accept(as)
+	as.str.WriteString(" ")
+	b.right.accept(as)
+	as.str.WriteString(")")
+	return nil
+
+}
+
+func (as *AstStringer) visitLiteral(l *Literal) any {
+	as.str.WriteString(fmt.Sprintf("%v", l.value))
+	return nil
+}
+
+func (as *AstStringer) visitGrouping(g *Grouping) any {
+	as.str.WriteString("(group ")
+	g.expression.accept(as)
+	as.str.WriteString(")")
+	return nil
+}
+
+func (as *AstStringer) visitUnary(u *Unary) any {
+	as.str.WriteString(fmt.Sprintf("(%s ", u.op.lexeme))
+	u.right.accept(as)
+	as.str.WriteString(")")
+	return nil
+}
diff --git a/error.go b/error.go
index 58940cb..8f1544f 100644
--- a/error.go
+++ b/error.go
@@ -6,6 +6,7 @@ import (
 )
 
 var hadError = false
+var hadRuntimeError = false
 
 func printError(token Token, message string) {
 	if token.typ == EOF {
@@ -19,3 +20,8 @@ func report(line int, where string, message string) {
 	log.Printf("[%d] Error %s: %s", line, where, message)
 	hadError = true
 }
+
+func reportRuntimeError(token Token, message string) {
+	log.Printf("[%d] Error: %s", token.line, message)
+	hadRuntimeError = true
+}
diff --git a/glox.go b/glox.go
index 7a5bb2b..4feeb79 100644
--- a/glox.go
+++ b/glox.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"bufio"
+	"fmt"
 	"log"
 	"os"
 )
@@ -31,6 +32,7 @@ func runPrompt() {
 		}
 		run([]byte(scanner.Text()))
 		hadError = false
+		hadRuntimeError = false
 	}
 }
 
@@ -40,15 +42,39 @@ func runFile(path string) {
 	try(err)
 
 	run(file)
+
+	switch {
+	case hadError:
+		os.Exit(65)
+	case hadRuntimeError:
+		os.Exit(70)
+	default:
+		os.Exit(0)
+	}
 }
 
 func run(source []byte) {
 	tokens := newScanner(source).scan()
 
+	if hadError {
+		return
+	}
+
 	ast := newParser(tokens).parse()
 
+	if hadError {
+		return
+	}
+
 	println(AstStringer{}.String(ast))
-	println(AstToRPN{}.String(ast))
+
+	res := newInterpreter().evaluate(ast)
+
+	if hadRuntimeError {
+		return
+	}
+
+	fmt.Printf("%v\n", res)
 }
 
 func try(err error) {
diff --git a/interpreter.go b/interpreter.go
new file mode 100644
index 0000000..d621fc4
--- /dev/null
+++ b/interpreter.go
@@ -0,0 +1,148 @@
+package main
+
+import (
+	"fmt"
+	"reflect"
+)
+
+type Interpreter struct{}
+
+type RuntimeError struct {
+	token Token
+	msg   string
+}
+
+func (re RuntimeError) Error() string {
+	return re.msg
+}
+
+func newInterpreter() *Interpreter {
+	return &Interpreter{}
+}
+
+func (i *Interpreter) evaluate(e Expr) any {
+	defer i.recover()
+	return e.accept(i)
+}
+
+func (i *Interpreter) recover() {
+	if err := recover(); err != nil {
+		pe, ok := err.(RuntimeError)
+
+		if !ok {
+			panic(err)
+		}
+
+		reportRuntimeError(pe.token, pe.msg)
+		hadRuntimeError = true
+	}
+}
+
+func (i *Interpreter) visitBinary(b *Binary) any {
+	left := i.evaluate(b.left)
+	right := i.evaluate(b.right)
+
+	switch b.op.typ {
+	case MINUS:
+		checkIfFloats(b.op, left, right)
+		return left.(float64) - right.(float64)
+	case SLASH:
+		checkIfFloats(b.op, left, right)
+		return left.(float64) / right.(float64)
+	case STAR:
+		checkIfFloats(b.op, left, right)
+		return left.(float64) * right.(float64)
+	case GREATER:
+		checkIfFloats(b.op, left, right)
+		return left.(float64) > right.(float64)
+	case LESS:
+		checkIfFloats(b.op, left, right)
+		return left.(float64) < right.(float64)
+	case GREATER_EQUAL:
+		checkIfFloats(b.op, left, right)
+		return left.(float64) >= right.(float64)
+	case LESS_EQUAL:
+		checkIfFloats(b.op, left, right)
+		return left.(float64) <= right.(float64)
+	case BANG_EQUAL:
+		return !reflect.DeepEqual(left, right)
+	case EQUAL_EQUAL:
+		return reflect.DeepEqual(left, right)
+	case PLUS:
+		if isFloats(left, right) {
+			return left.(float64) + right.(float64)
+		}
+
+		if isStrings(left, right) {
+			return left.(string) + right.(string)
+		}
+	}
+
+	panic(RuntimeError{b.op, fmt.Sprintf("Operands must be numbers or strings: %v %s %v", left, b.op.lexeme, right)})
+
+	return nil
+}
+
+func (i *Interpreter) visitLiteral(l *Literal) any {
+	return l.value
+}
+
+func (i *Interpreter) visitGrouping(g *Grouping) any {
+	return i.evaluate(g.expression)
+}
+
+func (i *Interpreter) visitUnary(u *Unary) any {
+	val := i.evaluate(u.right)
+
+	switch u.op.typ {
+	case MINUS:
+		checkIfFloat(u.op, val)
+		return -val.(float64)
+	case BANG:
+		return !isTruthy(val)
+	}
+
+	return nil
+}
+
+func checkIfFloat(op Token, val any) {
+	if _, ok := val.(float64); ok {
+		return
+	}
+
+	panic(RuntimeError{op, "value must ne a number."})
+}
+
+func checkIfFloats(op Token, a any, b any) {
+	if isFloats(a, b) {
+		return
+	}
+
+	panic(RuntimeError{op, fmt.Sprintf("Operands must be numbers: %v %s %v", a, op.lexeme, b)})
+}
+
+func isFloats(a any, b any) bool {
+	ltype := reflect.TypeOf(a)
+	rtype := reflect.TypeOf(b)
+
+	return ltype.Kind() == rtype.Kind() && ltype.Kind() == reflect.Float64
+}
+
+func isStrings(a any, b any) bool {
+	ltype := reflect.TypeOf(a)
+	rtype := reflect.TypeOf(b)
+
+	return ltype.Kind() == rtype.Kind() && ltype.Kind() == reflect.String
+}
+
+func isTruthy(val any) bool {
+	if val == nil {
+		return false
+	}
+
+	if boolean, ok := val.(bool); ok {
+		return boolean
+	}
+
+	return true
+}
diff --git a/parser.go b/parser.go
index 8dd1604..11cf9bb 100644
--- a/parser.go
+++ b/parser.go
@@ -88,7 +88,7 @@ func (p *Parser) factor() Expr {
 	for p.match(SLASH, STAR) {
 		op := p.previous()
 		right := p.unary()
-		exp = &Unary{op, right}
+		exp = &Binary{exp, op, right}
 	}
 
 	return exp
@@ -190,14 +190,7 @@ func (p *Parser) synchronize() {
 		}
 
 		switch p.peek().typ {
-		case CLASS:
-		case FOR:
-		case FUN:
-		case IF:
-		case PRINT:
-		case RETURN:
-		case VAR:
-		case WHILE:
+		case CLASS, FOR, FUN, IF, PRINT, RETURN, VAR, WHILE:
 			return
 		}