diff --git a/lexer/lexer.go b/lexer/lexer.go
new file mode 100644
index 0000000..8033769
--- /dev/null
+++ b/lexer/lexer.go
@@ -0,0 +1,104 @@
+package lexer
+
+import (
+	"errors"
+	"fmt"
+
+	"asciigoat.org/core/runes"
+)
+
+// state function
+type StateFn func(Lexer) StateFn
+
+type Lexer interface {
+	Run() // run state machine
+
+	Position() TokenPosition // base for the next token
+	Tokens() <-chan Token    // tokens output
+
+	NewLine()
+	Step(n int)
+
+	Emit(TokenType)
+	EmitError(error)
+	EmitErrorf(string, ...interface{})
+	EmitSyntaxError(string, ...interface{})
+}
+
+type lexer struct {
+	start StateFn // initial state
+
+	in     *runes.Feeder // runes source
+	pos    TokenPosition // base for the next token
+	cursor int           // look ahead pointer
+	tokens chan Token    // tokens output
+}
+
+func NewLexer(start StateFn, in *runes.Feeder, tokens int) Lexer {
+	return &lexer{
+		start:  start,
+		in:     in,
+		pos:    TokenPosition{1, 1},
+		tokens: make(chan Token, tokens),
+	}
+}
+
+func (lex *lexer) Run() {
+	defer close(lex.tokens)
+
+	for state := lex.start; state != nil; {
+		state = state(lex)
+	}
+}
+
+func (lex *lexer) Position() TokenPosition {
+	return lex.pos
+}
+
+func (lex *lexer) Step(n int) {
+	lex.cursor += n
+}
+
+func (lex *lexer) NewLine() {
+	lex.pos.NewLine()
+}
+
+func (lex *lexer) Tokens() <-chan Token {
+	return lex.tokens
+}
+
+func (lex *lexer) Emit(typ TokenType) {
+	var text []rune
+
+	pos := lex.pos
+
+	// extract text to emit, and update cursor for the next
+	if n := lex.cursor; n > 0 {
+		text = lex.in.Runes()[:n]
+		lex.in.Skip(n)
+		lex.pos.Step(n)
+		lex.cursor = 0
+	}
+
+	lex.tokens <- NewToken(typ, text, pos)
+}
+
+func (lex *lexer) EmitError(err error) {
+	lex.tokens <- NewErrorToken(err, lex.pos)
+}
+
+func (lex *lexer) EmitErrorf(s string, args ...interface{}) {
+	if len(args) > 0 {
+		s = fmt.Sprintf(s, args...)
+	}
+
+	lex.tokens <- NewErrorToken(errors.New(s), lex.pos)
+}
+
+func (lex *lexer) EmitSyntaxError(s string, args ...interface{}) {
+	if len(args) > 0 {
+		s = fmt.Sprintf(s, args...)
+	}
+
+	lex.tokens <- NewSyntaxErrorToken(s, lex.pos, lex.cursor, lex.in.Runes())
+}