diff --git a/.vscode/settings.json b/.vscode/settings.json
new file mode 100644
index 0000000..a5a2d31
--- /dev/null
+++ b/.vscode/settings.json
@@ -0,0 +1,5 @@
+{
+    "files.associations": {
+        "print": "cpp"
+    }
+}
\ No newline at end of file
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7c8690a..9d0e055 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -6,4 +6,5 @@ set(SOURCES
     src/main.cpp
 )
 
+include_directories(pl ${CMAKE_CURRENT_SOURCE_DIR}/include)
 add_executable(pl ${SOURCES})
diff --git a/example.rx b/example.rx
new file mode 100644
index 0000000..47deb63
--- /dev/null
+++ b/example.rx
@@ -0,0 +1,7 @@
+extern putchar;
+
+fn main() {
+    local a = 34;
+    local b = 35;
+    putchar(a + b);
+}
\ No newline at end of file
diff --git a/include/ast.hpp b/include/ast.hpp
new file mode 100644
index 0000000..44de678
--- /dev/null
+++ b/include/ast.hpp
@@ -0,0 +1,154 @@
+#pragma once
+#include <cstring>
+#include "lexer.hpp"
+
+enum NodeType
+{
+    Expression = 0,
+    Extern,
+    FnDecl,
+    FnCall,
+    VarDecl,
+    COUNT_NODES,
+};
+
+class Node
+{
+public:
+    virtual NodeType GetType() const = 0;
+    virtual ~Node() {}
+};
+
+#define NODE_TYPE(x) \
+    NodeType GetType() const override { return (x); }
+                     
+
+class ExpressionNode : public Node
+{
+public:
+    ExpressionNode()
+        : m_left(nullptr), m_right(nullptr) {}
+    ~ExpressionNode() override {
+        delete m_left;
+        delete m_right;
+    }
+
+    NODE_TYPE(Expression)
+public:
+    Node* left() const { return m_left; }
+    Node* right() const { return m_right; }
+private:
+    Node* m_left;
+    Node* m_right;
+};
+
+class ExternNode : public Node
+{
+public:
+    // TODO: support multiple extern symbols
+    ExternNode(char* symbol)
+        : m_symbol(symbol) {}
+    ~ExternNode() override {
+        delete m_symbol;
+    }
+
+    NODE_TYPE(Extern)
+private:
+    char* m_symbol;
+};
+
+class FnDeclNode : public Node
+{
+public:
+    // TODO: support parameters
+    FnDeclNode(char* name, Node* body)
+        : m_name(name), m_body(body) {}
+    ~FnDeclNode() override {
+        delete m_name;
+        delete m_body;
+    }
+
+    NODE_TYPE(FnDecl)
+private:
+    char* m_name;
+    Node* m_body;
+};
+
+class FnCallNode : public Node
+{
+public:
+    // TODO: support multiple arguments
+    FnCallNode(char* name, Node* arg)
+        : m_name(name), m_arg(arg) {}
+    ~FnCallNode() override {
+        delete m_name;
+        delete m_arg;
+    }
+
+    NODE_TYPE(FnCall)
+private:
+    char* m_name;
+    Node* m_arg;
+};
+
+class VarDeclNode : public Node
+{
+public:
+    VarDeclNode(char* name, Node* value)
+        : m_name(name), m_value(value) {}
+    ~VarDeclNode() override {
+        delete m_name;
+        delete m_value;
+    }
+
+    NODE_TYPE(VarDecl)
+private:
+    char* m_name;
+    Node* m_value;
+};
+
+class AstParser
+{
+public:
+    AstParser(Lexer* lexer)
+        : m_lexer(lexer) {}
+public:
+    Node* ParseStatement()
+    {
+        while (m_lexer->NextToken())
+        {
+            auto token = m_lexer->token();
+            switch(token.token)
+            {
+            case Id:
+            {
+                if (strcmp(token.string, "extern") == 0)
+                {
+                    // Extern
+                    m_lexer->Eat(Id);
+                    return new ExternNode(m_lexer->token().string);
+                }
+                else if (strcmp(token.string, "fn") == 0)
+                {
+                    // Function Declaration
+                    m_lexer->Eat(Id);
+                    char* name = strdup(m_lexer->token().string);
+                    m_lexer->Eat('(');
+                    // TODO: parse parameters
+                    m_lexer->Eat(')');
+                    m_lexer->Eat('{');
+                    // TODO: parse function body
+                    m_lexer->Eat('}');
+                    return new FnDeclNode(name, nullptr);
+                }
+            }
+            break;
+            }
+        }
+
+        // TODO: report parse error
+        return nullptr;
+    }
+private:
+    Lexer* m_lexer;
+};
diff --git a/include/lexer.hpp b/include/lexer.hpp
new file mode 100644
index 0000000..612e955
--- /dev/null
+++ b/include/lexer.hpp
@@ -0,0 +1,136 @@
+#pragma once
+#include <iostream>
+#include <cctype>
+
+#include "string.hpp"
+
+enum TokenType
+{
+    Eof = 256,
+    Id,
+    IntLiteral,
+
+    Unknown,
+};
+
+struct Token
+{
+    TokenType token;
+    long int_number;
+    // null-terminated
+    char* string;
+    long line_number;
+    long offset_start;
+    long offset_end;
+public:
+    Token(TokenType t) : token(t) {}
+    Token(TokenType t, long lnumber, long soffset, long eoffset)
+        : token(t), line_number(lnumber), offset_start(soffset), offset_end(eoffset) {}
+    Token() : token(Unknown) {}
+};
+
+class Lexer
+{
+public:
+    const Token& token() { return m_token; }
+public:
+    Lexer(char* filename, StringView code)
+        : m_filename(filename), m_code(code) {}
+
+    Lexer(const Lexer&) = delete;
+    Lexer(Lexer&& other)
+    {
+        m_code = other.m_code;
+        other.m_code = StringView();
+    }
+public:
+    bool NextToken()
+    {
+        if (m_pos >= m_code.size)
+        {
+            m_token = Token(TokenType::Eof);
+            return false;
+        }
+
+        char c = m_code.data[m_pos++];
+
+        while(std::isspace(c)) {
+            if (c == '\n')
+            {
+                m_line++;
+                m_last_newline = m_pos;
+            }
+            c = m_code.data[m_pos++];
+        }
+
+        if (std::isalpha(c) != 0)
+        {
+            StringBuilder s;
+            long offset_start = m_pos - m_last_newline;
+            s.PushChar(c);
+            // id
+            while (std::isalpha(m_code.data[m_pos]) != 0)
+            {
+                s.PushChar(m_code.data[m_pos++]);
+            }
+            s.PushChar('\0');
+            m_token = Token(Id, m_line, offset_start, m_pos - m_last_newline);
+            m_token.string = s.data;
+            return true;
+        }
+
+        if (std::isdigit(c) != 0)
+        {
+            StringBuilder s;
+            long offset_start = m_pos - m_last_newline;
+            bool hex = c == '0' && m_code.data[m_pos] == 'x';
+            s.PushChar(c);
+            // integer (could be hex)
+            while (std::isdigit(m_code.data[m_pos]) != 0 || (hex && std::isalpha(m_code.data[m_pos]) != 0))
+            {
+                s.PushChar(m_code.data[m_pos++]);
+            }
+            s.PushChar('\0');
+            m_token = Token(IntLiteral, m_line, offset_start, m_pos - m_last_newline);
+            m_token.int_number = std::strtol(s.data, nullptr, hex ? 16 : 10);
+            m_token.string = s.data;
+            return true;
+        }
+        
+        m_token = Token((TokenType)c, m_line, m_pos - m_last_newline, m_pos - m_last_newline + 1);
+        return true;
+    }
+
+    void Eat(TokenType expected)
+    {
+        if (!NextToken())
+        {
+            fprintf(stderr, "%s:%d:%d: ERROR: expected %ld, but got EOF", m_filename, 0, 0, expected);
+            Exit(1);
+        }
+        if (token().token != expected)
+        {
+            fprintf(stderr, "%s:%d:%d: ERROR: expected %ld, but got %ld", m_filename, token().line_number, token().offset_start, expected);
+            Exit(1);
+        }
+    }
+
+    void Eat(char expected)
+    {
+        Eat((TokenType)expected);
+    }
+private:
+    void Exit(int status)
+    {
+        std::exit(status);
+    }
+private:
+    StringView m_code;
+    char* m_filename;
+
+    Token m_token;
+
+    size_t m_pos = 0;
+    size_t m_line = 1;
+    size_t m_last_newline = 0;
+};
\ No newline at end of file
diff --git a/include/string.hpp b/include/string.hpp
new file mode 100644
index 0000000..7cc7ed6
--- /dev/null
+++ b/include/string.hpp
@@ -0,0 +1,47 @@
+#pragma once
+#include <cstddef>
+
+struct StringView
+{
+    size_t size;
+    const char* data;
+public:
+    StringView()
+    {
+        data = nullptr;
+        size = 0;
+    }
+
+    StringView(const char* data, size_t size)
+    {
+        this->data = data;
+        this->size = size;
+    }
+};
+
+struct StringBuilder
+{
+    size_t size;
+    size_t capacity;
+    char* data;
+public:
+    StringBuilder()
+    {
+        size = 0;
+        capacity = 10;
+        data = (char*)malloc(capacity * sizeof(char));
+    }
+private:
+    void ensureSize(size_t newSize)
+    {
+        if (newSize <= capacity) return;
+        capacity = capacity + (capacity / 2);
+        data = (char*)realloc(data, capacity * sizeof(char));
+    }
+public:
+    void PushChar(char c)
+    {
+        ensureSize(size + 1);
+        data[size++] = c;
+    }
+};
\ No newline at end of file
diff --git a/src/main.cpp b/src/main.cpp
index fcf145f..771705c 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1,8 +1,55 @@
 #include <iostream>
+#include <fstream>
 #include <print>
+#include "lexer.hpp"
+#include "ast.hpp"
+
+void dump_tokens(const char* filename, Lexer* lexer)
+{
+    while (lexer->NextToken()) {
+        std::print("{}:{}:{}: ", filename, lexer->token().line_number, lexer->token().offset_start);
+        if (lexer->token().token == Id)
+            std::println("id = {}", lexer->token().string);
+        else if (lexer->token().token == IntLiteral)
+            std::println("int = {}", lexer->token().int_number);
+        else 
+            std::println("token = {}", (char)lexer->token().token);
+    }
+}
 
 int main(int argc, char** argv)
 {
-    std::println("Hello, World!");
+    for (int i = 0; i < argc; ++i) {
+        std::println("arg#{}: {}", i, argv[i]);
+    }
+    char* filename;
+    if (argc > 1) {
+        filename = (++argv)[0];
+    } else {
+        fprintf(stderr, "ERROR: Input file is required.\n");
+        return 1;
+    }
+
+    std::ifstream f(filename);
+    if (!f.is_open()) {
+        fprintf(stderr, "ERROR: Failed to open input file: %s\n", filename);
+        return 1;
+    }
+
+    std::string content((std::istreambuf_iterator<char>(f)), std::istreambuf_iterator<char>());
+
+    f.close();
+
+    // std::println("{}", content);
+
+    Lexer lexer(filename, StringView(content.c_str(), content.size()));
+    
+    // dump_tokens(filename, &lexer);
+
+    AstParser parser(&lexer);
+
+    auto node = parser.ParseStatement();
+    ExternNode* extrn = reinterpret_cast<ExternNode*>(node);
+
     return 0;
 }
diff --git a/test b/test
new file mode 100755
index 0000000..c35eebf
Binary files /dev/null and b/test differ
diff --git a/test.asm b/test.asm
new file mode 100644
index 0000000..235c87e
--- /dev/null
+++ b/test.asm
@@ -0,0 +1,16 @@
+format ELF64
+
+section ".text" executable
+
+public main
+
+extrn 'putchar' as __putchar
+putchar = PLT __putchar
+
+main:
+    mov rdi, 69
+    call putchar
+    mov rdi, 10
+    call putchar
+    mov rax, 0
+    ret
diff --git a/test.o b/test.o
new file mode 100644
index 0000000..ff075db
Binary files /dev/null and b/test.o differ