diff --git a/src/ast.hpp b/src/ast.hpp
new file mode 100644
index 0000000..7b4e170
--- /dev/null
+++ b/src/ast.hpp
@@ -0,0 +1,79 @@
+#pragma once
+#include <string>
+#include <memory>
+#include "pattern.hpp"
+
+namespace lily {
+    struct ast {
+        virtual ~ast() = default;
+    };
+
+    typedef std::unique_ptr<ast> ast_ptr;
+
+    struct ast_num : ast {
+        int num;
+
+        ast_num(int i) : num(i) {}
+        ~ast_num() = default;
+    };
+
+    struct ast_var : ast {
+        std::string name;
+
+        ast_var(std::string n) :
+            name(std::move(n)) {}
+        ~ast_var() = default;
+    };
+
+    struct ast_app : ast {
+        ast_ptr left;
+        ast_ptr right;
+
+        ast_app(ast_ptr l, ast_ptr r) :
+            left(std::move(l)), right(std::move(r)) {}
+        ~ast_app() = default;
+    };
+
+    struct ast_op : ast {
+        enum class op {
+            add,
+            subtrct,
+            times,
+            divide
+        } op;
+        std::unique_ptr<ast> left;
+        std::unique_ptr<ast> right;
+
+        ast_op(enum op o, ast_ptr l, ast_ptr r) :
+            op(o), left(std::move(l)), right(std::move(r)) {}
+        ~ast_op() = default;
+    };
+
+    struct ast_let : ast {
+        std::string name;
+        ast_ptr expr;
+        ast_ptr in;
+
+        ast_let(std::string n, ast_ptr e, ast_ptr i) :
+            name(std::move(n)), expr(std::move(e)), in(std::move(i)) {}
+        ~ast_let() = default;
+    };
+
+    struct ast_letrec : ast_let {
+        ast_letrec(std::string n, ast_ptr e, ast_ptr i) :
+            ast_let(std::move(n), std::move(e), std::move(i)) {}
+        ~ast_letrec() = default;
+    };
+
+    struct ast_case : ast {
+        struct branch {
+            pattern_ptr pattern;
+            ast_ptr expr;
+        };
+
+        ast_ptr of;
+        std::vector<branch> branches;
+
+        ~ast_case() = default;
+    };
+}
diff --git a/src/pattern.hpp b/src/pattern.hpp
new file mode 100644
index 0000000..ad2d780
--- /dev/null
+++ b/src/pattern.hpp
@@ -0,0 +1,29 @@
+#pragma once
+#include <string>
+#include <memory>
+#include <vector>
+
+namespace lily {
+    struct pattern {
+        virtual ~pattern();
+    };
+
+    typedef std::unique_ptr<pattern> pattern_ptr;
+
+    struct pattern_var {
+        std::string name;
+
+        pattern_var(std::string n) :
+            name(std::move(n)) {}
+        ~pattern_var() = default;
+    };
+
+    struct pattern_cons {
+        std::string name;
+        std::vector<std::string> vnames;
+
+        pattern_cons(std::string n) :
+            name(std::move(n)) {}
+        ~pattern_cons() = default;
+    };
+}