diff --git a/lexer.c b/lexer.c
new file mode 100644
index 0000000..708f726
--- /dev/null
+++ b/lexer.c
@@ -0,0 +1,145 @@
+#include "lexer.h"
+#include <stdbool.h>
+#include <stdio.h>
+
+const char* token_type_to_string(TokenType type) { }
+
+void token_to_string(char* out, size_t out_size, Token token)
+{
+    snprintf(out, out_size, "Token { type }");
+}
+
+void lexer_construct(Lexer* self, const char* text, size_t length)
+{
+    *self = (Lexer) {
+        .text = text,
+        .length = length,
+        .position = {
+            .index = 0,
+            .line = 1,
+            .column = 1,
+        },
+    };
+}
+
+Token lexer_next(Lexer* self) { return lexer_make_token(self); }
+
+bool lexer_done(const Lexer* self)
+{
+    return self->position.index >= self->length;
+}
+
+void lexer_step(Lexer* self)
+{
+    if (self->text[self->position.index] == '\n') {
+        self->position.line += 1;
+        self->position.column = 1;
+    } else {
+        self->position.column += 1;
+    }
+    self->position.index += 1;
+}
+
+char lexer_current(const Lexer* self)
+{
+    return self->text[self->position.index];
+}
+
+LexerPosition lexer_position(const Lexer* self) { return self->position; }
+
+Token lexer_token_from(const Lexer* self, TokenType type, LexerPosition start)
+{
+    return (Token) {
+        .type = type,
+        .index = start.index,
+        .length = self->position.index,
+        .line = start.line,
+        .column = start.column,
+    };
+}
+
+void lexer_skip_whitespace(Lexer* self)
+{
+    lexer_step(self);
+    while (!lexer_done(self) && is_whitespace_char(lexer_current(self)))
+        lexer_step(self);
+}
+
+Token lexer_make_int_token(Lexer* self)
+{
+    LexerPosition start = self->position;
+    lexer_step(self);
+    while (!lexer_done(self) && is_int_char(lexer_current(self)))
+        lexer_step(self);
+    return lexer_token_from(self, TokenTypeInt, start);
+}
+
+Token lexer_make_id_token(Lexer* self)
+{
+    LexerPosition start = self->position;
+    lexer_step(self);
+    while (!lexer_done(self) && is_id_char(lexer_current(self)))
+        lexer_step(self);
+    return lexer_token_from(self, TokenTypeId, start);
+}
+
+Token lexer_make_single_char_token(Lexer* self, TokenType type)
+{
+    LexerPosition start = lexer_position(self);
+    lexer_step(self);
+    return lexer_token_from(self, type, start);
+}
+
+Token lexer_make_static_token(Lexer* self)
+{
+    switch (lexer_current(self)) {
+        case '+':
+            return lexer_make_single_char_token(self, TokenTypePlus);
+        case '(':
+            return lexer_make_single_char_token(self, TokenTypeLParen);
+        case ')':
+            return lexer_make_single_char_token(self, TokenTypeRParen);
+        default:
+            return lexer_token_from(
+                self, TokenTypeInvalid, lexer_position(self));
+    }
+}
+
+Token lexer_make_token(Lexer* self)
+{
+    if (lexer_done(self)) {
+        return lexer_token_from(self, TokenTypeEof, lexer_position(self));
+    } else if (is_whitespace_char(lexer_current(self))) {
+        lexer_skip_whitespace(self);
+        return lexer_make_token(self);
+    } else if (is_int_char(lexer_current(self))) {
+        return lexer_make_int_token(self);
+    } else if (is_id_char(is_int_char(lexer_current(self)))) {
+        return lexer_make_id_token(self);
+    } else {
+        return lexer_make_static_token(self);
+    };
+}
+
+void token_iterator(TokenIterator* self, Lexer* lexer)
+{
+    *self = (TokenIterator) {
+        .lexer = lexer,
+        .current = lexer_next(lexer),
+    };
+}
+
+Token token_step(TokenIterator* self)
+{
+    return self->current = lexer_next(self->lexer);
+}
+
+Token token_current(TokenIterator* self) { return self->current; }
+
+bool is_whitespace_char(char c) { return c == ' ' || c == '\t' || c == '\n'; }
+bool is_int_char(char c) { return c >= '0' && c <= '9'; }
+bool is_id_char(char c)
+{
+    return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_'
+        || is_int_char(c);
+}
diff --git a/lexer.h b/lexer.h
new file mode 100644
index 0000000..c60e84b
--- /dev/null
+++ b/lexer.h
@@ -0,0 +1,66 @@
+#ifndef LEXER_H
+#define LEXER_H
+
+#include <stdbool.h>
+#include <stdlib.h>
+
+typedef enum {
+    TokenTypeInvalid,
+    TokenTypeEof,
+    TokenTypePlus,
+    TokenTypeLParen,
+    TokenTypeRParen,
+    TokenTypeInt,
+    TokenTypeId,
+} TokenType;
+
+const char* token_type_to_string(TokenType type);
+
+typedef struct {
+    TokenType type;
+    size_t index, length;
+    int line, column;
+} Token;
+
+void token_to_string(char* out, size_t out_size, Token token);
+
+typedef struct {
+    size_t index;
+    int line, column;
+} LexerPosition;
+
+typedef struct {
+    const char* text;
+    size_t length;
+    LexerPosition position;
+} Lexer;
+
+void lexer_construct(Lexer* self, const char* text, size_t length);
+Token lexer_next(Lexer* self);
+
+bool lexer_done(const Lexer* self);
+void lexer_step(Lexer* self);
+char lexer_current(const Lexer* self);
+LexerPosition lexer_position(const Lexer* self);
+Token lexer_token_from(const Lexer* self, TokenType type, LexerPosition start);
+void lexer_skip_whitespace(Lexer* self);
+Token lexer_make_int_token(Lexer* self);
+Token lexer_make_id_token(Lexer* self);
+Token lexer_make_single_char_token(Lexer* self, TokenType type);
+Token lexer_make_static_token(Lexer* self);
+Token lexer_make_token(Lexer* self);
+
+typedef struct {
+    Lexer* lexer;
+    Token current;
+} TokenIterator;
+
+void token_iterator(TokenIterator* self, Lexer* lexer);
+Token token_step(TokenIterator* self);
+Token token_current(TokenIterator* self);
+
+bool is_whitespace_char(char c);
+bool is_int_char(char c);
+bool is_id_char(char c);
+
+#endif
diff --git a/main.c b/main.c
index 13f5f5d..649bd09 100644
--- a/main.c
+++ b/main.c
@@ -1,140 +1,6 @@
+#include "lexer.h"
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 
-typedef enum {
-    TokenTypeEof,
-    TokenTypeNewline,
-    TokenTypeSemicolon,
-    TokenTypePlus,
-    TokenTypeInt,
-    TokenTypeId,
-} TokenType;
-
-typedef struct {
-    TokenType type;
-    size_t index, length;
-    int line, column;
-} Token;
-
-typedef struct {
-    size_t index;
-    int line, column;
-} LexerPosition;
-
-typedef struct {
-    const char* text;
-    size_t length;
-    LexerPosition position;
-} Lexer;
-
-void construct_lexer(Lexer* self, const char* text, size_t length)
-{
-    *self = (Lexer) {
-        .text = text,
-        .length = length,
-        .position = {
-            .index = 0,
-            .line = 1,
-            .column = 1,
-        },
-    };
-}
-
-bool is_whitespace_char(char c) { return c == ' ' || c == '\t'; }
-bool is_int_char(char c) { return c >= '0' && c <= '9'; }
-bool is_id_char(char c)
-{
-    return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_'
-        || is_int_char(c);
-}
-
-bool lexer_done(const Lexer* self)
-{
-    return self->position.index >= self->length;
-}
-
-void lexer_step(Lexer* self)
-{
-    if (self->text[self->position.index] == '\n') {
-        self->position.line += 1;
-        self->position.column = 1;
-    } else {
-        self->position.column += 1;
-    }
-    self->position.index += 1;
-}
-
-char lexer_current(const Lexer* self)
-{
-    return self->text[self->position.index];
-}
-
-LexerPosition lexer_position(const Lexer* self) { return self->position; }
-
-Token lexer_token_from(const Lexer* self, TokenType type, LexerPosition start)
-{
-    return (Token) {
-        .type = type,
-        .index = start.index,
-        .length = self->position.index,
-        .line = start.line,
-        .column = start.column,
-    };
-}
-
-void lexer_skip_whitespace(Lexer* self)
-{
-    lexer_step(self);
-    while (!lexer_done(self) && is_whitespace_char(lexer_current(self)))
-        lexer_step(self);
-}
-
-Token lexer_make_int_token(Lexer* self)
-{
-    LexerPosition start = self->position;
-    lexer_step(self);
-    while (!lexer_done(self) && is_int_char(lexer_current(self)))
-        lexer_step(self);
-    return lexer_token_from(self, TokenTypeInt, start);
-}
-
-Token lexer_make_id_token(Lexer* self)
-{
-    LexerPosition start = self->position;
-    lexer_step(self);
-    while (!lexer_done(self) && is_id_char(lexer_current(self)))
-        lexer_step(self);
-    return lexer_token_from(self, TokenTypeId, start);
-}
-
-Token lexer_make_static_token(Lexer* self)
-{
-    switch (lexer_current(self)) {
-        case '\n':
-
-        default:
-            printf("unrecognized char '%c'\n", lexer_current(self));
-            exit(EXIT_FAILURE);
-    }
-}
-
-Token lexer_make_token(Lexer* self)
-{
-    if (lexer_done(self)) {
-        return lexer_token_from(self, TokenTypeEof, lexer_position(self));
-    } else if (is_whitespace_char(lexer_current(self))) {
-        lexer_skip_whitespace(self);
-        return lexer_make_token(self);
-    } else if (is_int_char(lexer_current(self))) {
-        return lexer_make_int_token(self);
-    } else if (is_id_char(is_int_char(lexer_current(self)))) {
-        return lexer_make_id_token(self);
-    } else {
-        return lexer_make_static_token(self);
-    };
-}
-
-Token lexer_next(Lexer* self) { return lexer_make_token(self); }
-
 int main() { printf("hello world\n"); }