diff --git a/lex.go b/lex.go
index b81c9b1..012e009 100644
--- a/lex.go
+++ b/lex.go
@@ -49,6 +49,7 @@ var (
 	Newline     = &Token{value: "\n"}
 	SingleQuote = &Token{value: "'"}
 	Let         = &Token{value: "let"}
+	LambdaTok   = &Token{value: "lambda"}
 )
 
 func (t Token) String() string {
@@ -71,6 +72,7 @@ var knownTokens = []*Token{
 	Newline,
 	SingleQuote,
 	Let,
+	LambdaTok,
 }
 
 func (t *Token) Equals(a Lexeme) bool {
diff --git a/main.go b/main.go
index 712d034..adb1fe3 100644
--- a/main.go
+++ b/main.go
@@ -13,14 +13,35 @@ func (v *Value[T]) Step() (Expression, error) {
 	return v, nil
 }
 
+func (v *Value[T]) Replace(Id, Expression) {
+}
+
 func (v *Quoted) Step() (Expression, error) {
 	return v, nil
 }
 
+func (v *Quoted) Replace(Id, Expression) {
+}
+
 func (d *Declaration) Step() (Expression, error) {
 	return &Value[int]{0}, nil
 }
 
+func (v *Declaration) Replace(id Id, expr Expression) {
+	if id == v.id.value {
+		return
+	} else {
+		switch x := v.expr.(type) {
+		case *Identifier:
+			if x.value == id {
+				v.expr = expr
+			}
+		default:
+			x.Replace(id, expr)
+		}
+	}
+}
+
 func (v *Identifier) Step() (Expression, error) {
 	switch v.value {
 	case "+":
@@ -37,7 +58,6 @@ func (v *Identifier) Step() (Expression, error) {
 					sum += x.value.value
 				default:
 					return nil, fmt.Errorf("invalid type")
-
 				}
 			}
 			return &Value[int]{sum}, nil
@@ -48,6 +68,50 @@ func (v *Identifier) Step() (Expression, error) {
 	}
 }
 
+func (v *Identifier) Replace(id Id, expr Expression) {
+	panic("!")
+}
+
+func (l *Lambda) Step() (Expression, error) {
+	f := func(args []Expression) (Expression, error) {
+		if len(args) != len(l.args) {
+			return nil, fmt.Errorf("unexpected number of arguments")
+		}
+		for i, arg := range l.args {
+			argExpr := args[i]
+			for j, e := range l.body {
+				switch x := e.(type) {
+				case *Identifier:
+					if x.value == arg.value {
+						l.body[j] = argExpr
+					}
+				default:
+					e.Replace(arg.value, argExpr)
+				}
+			}
+		}
+		if len(l.body) == 0 {
+			return &Value[int]{0}, nil
+		}
+		for i, expr := range l.body {
+			if isValue(expr) {
+				continue
+			}
+			r, err := expr.Step()
+			if err != nil {
+				return nil, err
+			}
+			l.body[i] = r
+		}
+		return l.body[len(l.body)-1], nil
+	}
+	return &Value[Function]{f}, nil
+}
+
+func (l *Lambda) Replace(id Id, expr Expression) {
+	panic("!")
+}
+
 func (v *List) Step() (Expression, error) {
 	for i, expr := range v.els {
 		if !isValue(expr) {
@@ -73,6 +137,19 @@ func (v *List) Step() (Expression, error) {
 	return r, nil
 }
 
+func (v *List) Replace(id Id, expr Expression) {
+	for i, e := range v.els {
+		switch x := e.(type) {
+		case *Identifier:
+			if x.value == id {
+				v.els[i] = expr
+			}
+		default:
+			e.Replace(id, expr)
+		}
+	}
+}
+
 func isValue(e Expression) bool {
 	switch e.(type) {
 	case *Value[int], *Value[*Int], *Value[Function], *Quoted:
@@ -86,7 +163,7 @@ func fullStep(e Expression) (Expression, error) {
 	for !isValue(e) {
 		n, err := e.Step()
 		if err != nil {
-			return n, nil
+			return nil, err
 		}
 		e = n
 	}
diff --git a/parse.go b/parse.go
index c099a59..785278d 100644
--- a/parse.go
+++ b/parse.go
@@ -12,6 +12,7 @@ type Function func([]Expression) (Expression, error)
 
 type Expression interface {
 	Step() (Expression, error)
+	Replace(Id, Expression)
 	// Visit(func(Expression) error)
 }
 
@@ -32,6 +33,11 @@ type Declaration struct {
 	expr Expression
 }
 
+type Lambda struct {
+	args []*Identifier
+	body []Expression
+}
+
 func (v Value[T]) String() string {
 	return fmt.Sprint(v.value)
 }
@@ -48,69 +54,65 @@ func (v List) String() string {
 	return fmt.Sprintf("(%s)", strings.Join(s, " "))
 }
 
+func (v *Lambda) String() string {
+	// var args = new(List)
+	// args.els = append(args.els, v.args...)
+	//
+	// var s []string
+	// for _, e := range v.args {
+	// 	s = append(s, fmt.Sprint(e))
+	// }
+	return "fuck you"
+}
+
 func parseQuotedObject(in *Input) (Expression, error) {
-	start := in.Index()
 	l := in.Pop()
 	switch {
 	case l.Equals(SingleQuote):
 		n, err := consume(in)
 		if err != nil {
-			in.Seek(start)
 			return nil, err
 		}
 		return &Quoted{n}, nil
 	default:
-		in.Seek(start)
 		return nil, noMatch
 	}
 }
 
 func parseSymbol(in *Input) (Expression, error) {
-	start := in.Index()
 	l := in.Pop()
 	switch x := l.(type) {
 	case *Identifier:
 		return x, nil
 	default:
-		in.Seek(start)
 		return nil, noMatch
 	}
 }
 
 func parseValue(in *Input) (Expression, error) {
-	start := in.Index()
 	l := in.Pop()
 	switch x := l.(type) {
 	case *Int:
 		return &Value[*Int]{x}, nil
 	default:
-		in.Seek(start)
 		return nil, noMatch
 	}
 }
 
 func parseList(in *Input) (Expression, error) {
-	start := in.Index()
 	openingParens := in.Pop()
 	if !openingParens.Equals(Lparen) {
-		in.Seek(start)
 		return nil, noMatch
 	}
 	idx, ok := in.Find(Lparen, Rparen)
 	if !ok {
-		in.Seek(start)
 		return nil, fmt.Errorf("unmatched parenthesis starting at %s", openingParens.Position())
 	}
-	if idx == 0 {
-		in.Seek(start)
-		return nil, fmt.Errorf("no function declared at %s", openingParens.Position())
-	}
 	list := in.Take(idx)
 	var args = new(List)
 	for list.Len() > 0 {
 		arg, err := consume(list)
 		if err != nil {
-			in.Seek(start)
 			return nil, err
 		}
 		args.els = append(args.els, arg)
@@ -120,38 +122,79 @@ func parseList(in *Input) (Expression, error) {
 }
 
 func parseLet(in *Input) (Expression, error) {
-	start := in.Index()
 	token := in.Pop()
 	if !token.Equals(Let) {
-		in.Seek(start)
 		return nil, noMatch
 	}
 	id, ok := in.Pop().(*Identifier)
 	if !ok {
-		in.Seek(start)
 		return nil, noMatch
 	}
 	expr, err := consume(in)
 	if err != nil {
-		in.Seek(start)
 		return nil, noMatch
 	}
 	return &Declaration{id: id, expr: expr}, nil
 }
 
+func parseLambda(in *Input) (Expression, error) {
+	openingParens := in.Pop()
+	if !openingParens.Equals(Lparen) {
+		return nil, noMatch
+	}
+	idx, ok := in.Find(Lparen, Rparen)
+	if !ok {
+		return nil, fmt.Errorf("unmatched parenthesis starting at %s", openingParens.Position())
+	}
+	decl := in.Take(idx)
+	tok := decl.Pop()
+	if !tok.Equals(LambdaTok) {
+		return nil, noMatch
+	}
+	l, err := parseList(decl)
+	if err != nil {
+		return nil, err
+	}
+	list, ok := l.(*List)
+	if !ok {
+		return nil, noMatch
+	}
+	var args []*Identifier
+	for _, e := range list.els {
+		id, ok := e.(*Identifier)
+		if !ok {
+			return nil, noMatch
+		}
+		args = append(args, id)
+	}
+	var body []Expression
+	for decl.Len() > 0 {
+		n, err := consume(decl)
+		if err != nil {
+			return nil, err
+		}
+		body = append(body, n)
+	}
+	_ = in.Pop()
+	return &Lambda{args: args, body: body}, nil
+}
+
 type ParserFunc func(*Input) (Expression, error)
 
 func consume(in *Input) (n Expression, err error) {
 	var parseFunctions = []ParserFunc{
 		parseQuotedObject,
+		parseLambda,
 		parseList,
 		parseSymbol,
 		parseValue,
 		parseLet,
 	}
 	for _, f := range parseFunctions {
+		start := in.Index()
 		n, err := f(in)
 		if err != nil {
+			in.Seek(start)
 			if errors.Is(err, noMatch) {
 				continue
 			} else {