diff --git a/parser.c b/parser.c
new file mode 100644
index 0000000..9b7d8bc
--- /dev/null
+++ b/parser.c
@@ -0,0 +1,158 @@
+#include "parser.h"
+#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
+
+void lexer_construct(Lexer* lexer, const char* text, size_t length)
+{
+    *lexer = (Lexer) {
+        .text = text,
+        .text_length = length,
+        .index = 0,
+        .line = 1,
+        .col = 1,
+    };
+}
+
+static inline bool is_id_start_char(char c)
+{
+    return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_';
+}
+
+static inline bool is_id_char(char c)
+{
+    return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')
+        || (c >= '0' && c <= '9') || c == '_';
+}
+
+struct MatchIdToTokenTypeCase {
+    const char* keyword;
+    TokenType token_type;
+};
+
+static inline TokenType match_id_to_token_type(
+    const char* source, size_t length, struct MatchIdToTokenTypeCase cases[])
+{
+    for (size_t i = 0; cases[i].keyword != NULL; ++i) {
+        if (strncmp(source, cases[i].keyword, length) == 0) {
+            return cases[i].token_type;
+        }
+    }
+    return TokenType_Id;
+}
+
+Token lexer_next(Lexer* lexer)
+{
+    Pos pos = lexer_pos(lexer);
+    if (lexer_done(lexer)) {
+        return lexer_token(lexer, TokenType_EOF, pos);
+    }
+    char c = lexer_current(lexer);
+    if (c == ' ' || c == '\t' || c == '\n') {
+        lexer_step(lexer);
+        return lexer_next(lexer);
+    }
+    if (is_id_start_char(c)) {
+        lexer_step(lexer);
+        while (is_id_char(c)) {
+            lexer_step(lexer);
+        }
+        size_t length = lexer->index - pos.index;
+        TokenType token_type
+            = match_id_to_token_type(&lexer->text[pos.index], length,
+                (struct MatchIdToTokenTypeCase[]) {
+                    { "not", TokenType_Not },
+                    { "and", TokenType_And },
+                    { "or", TokenType_Or },
+                    { "loop", TokenType_Loop },
+                    { "fn", TokenType_Fn },
+                    { "return", TokenType_Return },
+                    { "break", TokenType_Break },
+                    { NULL, TokenType_Id },
+                });
+        return lexer_token(lexer, token_type, pos);
+    }
+    if (c >= '1' && c <= '9') {
+        lexer_step(lexer);
+        while (c >= '1' && c <= '9') {
+            lexer_step(lexer);
+        }
+        return lexer_token(lexer, TokenType_Int, pos);
+    }
+    return lexer_token(lexer, TokenType_Error, pos);
+}
+
+Token lexer_token(Lexer* lexer, TokenType token_type, Pos pos)
+{
+    return (Token) {
+        .token_type = token_type,
+        .pos = pos,
+        .length = lexer->index - pos.index,
+    };
+}
+
+void lexer_step(Lexer* lexer)
+{
+    if (lexer_done(lexer)) {
+        return;
+    }
+    lexer->index += 1;
+    if (lexer_current(lexer) == '\n') {
+        lexer->line += 1;
+        lexer->col = 1;
+    } else {
+        lexer->col += 1;
+    }
+}
+
+bool lexer_done(const Lexer* lexer)
+{
+    return lexer->index >= lexer->text_length;
+}
+
+char lexer_current(const Lexer* lexer) { return lexer->text[lexer->index]; }
+
+Pos lexer_pos(const Lexer* lexer)
+{
+    return (Pos) {
+        .index = lexer->index,
+        .line = lexer->line,
+        .col = lexer->col,
+    };
+}
+
+int ast_node_vec_construct(ASTNodeVec* vec)
+{
+    const size_t capacity_start = 4;
+    *vec = (ASTNodeVec) {
+        .data = malloc(capacity_start),
+        .length = 0,
+        .capacity = capacity_start,
+    };
+    if (vec->data == NULL) {
+        return -1;
+    }
+    return 0;
+}
+
+void ast_node_vec_destroy(ASTNodeVec* vec)
+{
+    if (vec->data != NULL) {
+        free(vec->data);
+    }
+}
+
+int ast_node_vec_push(ASTNodeVec* vec, ASTNode* item)
+{
+    if (vec->length + 1 > vec->capacity) {
+        vec->capacity *= 2;
+        ASTNode** data = realloc(vec->data, vec->capacity);
+        if (data == NULL) {
+            return -1;
+        }
+        vec->data = data;
+    }
+    vec->data[vec->length] = item;
+    vec->length += 1;
+    return 0;
+}
diff --git a/parser.h b/parser.h
index 417c1b5..8d5594b 100644
--- a/parser.h
+++ b/parser.h
@@ -1,10 +1,18 @@
 #ifndef PARSER_H
 #define PARSER_H
 
+#include <stdbool.h>
 #include <stddef.h>
 
+typedef struct {
+    size_t index;
+    int line;
+    int col;
+} Pos;
+
 typedef enum {
     TokenType_Error,
+    TokenType_EOF,
     TokenType_Id,
     TokenType_Int,
     TokenType_Not,
@@ -12,6 +20,7 @@ typedef enum {
     TokenType_Or,
     TokenType_Loop,
     TokenType_Fn,
+    TokenType_Return,
     TokenType_Break,
     TokenType_LParen,
     TokenType_RParen,
@@ -21,25 +30,45 @@ typedef enum {
     TokenType_RBracket,
     TokenType_Comma,
     TokenType_Semicolon,
-    TokenType_Exclamation,
     TokenType_Plus,
+    TokenType_PlusEqual,
     TokenType_Minus,
+    TokenType_MinusEqual,
     TokenType_Asterisk,
-    TokenType_EE,
-    TokenType_NE,
+    TokenType_AsteriskEqual,
+    TokenType_EqualEqual,
+    TokenType_Exclamation,
+    TokenType_ExclamationEqual,
     TokenType_LT,
-    TokenType_GT,
     TokenType_LTE,
+    TokenType_GT,
     TokenType_GTE,
+    TokenType_Pipe,
+    TokenType_PipeGT,
 } TokenType;
 
 typedef struct {
-    TokenType token;
-    size_t index;
+    TokenType token_type;
+    Pos pos;
     size_t length;
-    int line;
 } Token;
 
+typedef struct {
+    const char* text;
+    size_t text_length;
+    size_t index;
+    int line;
+    int col;
+} Lexer;
+
+void lexer_construct(Lexer* lexer, const char* text, size_t text_length);
+Token lexer_next(Lexer* lexer);
+Token lexer_token(Lexer* lexer, TokenType token_type, Pos pos);
+void lexer_step(Lexer* lexer);
+bool lexer_done(const Lexer* lexer);
+char lexer_current(const Lexer* lexer);
+Pos lexer_pos(const Lexer* lexer);
+
 typedef enum {
     ASTNodeType_Error,
     ASTNodeType_Id,
@@ -65,9 +94,9 @@ typedef struct {
     size_t capacity;
 } ASTNodeVec;
 
-void ast_node_vec_construct(ASTNodeVec* vec);
+int ast_node_vec_construct(ASTNodeVec* vec);
 void ast_node_vec_destroy(ASTNodeVec* vec);
-void ast_node_vec_push(ASTNodeVec* vec, ASTNode* item);
+int ast_node_vec_push(ASTNodeVec* vec, ASTNode* item);
 
 typedef struct {
     ASTNode* condition;