diff --git a/03/ast.cpp b/03/ast.cpp
index 734a54a..f244ffa 100644
--- a/03/ast.cpp
+++ b/03/ast.cpp
@@ -1 +1,88 @@
 #include "ast.hpp"
+
+std::string op_name(binop op) {
+    switch(op) {
+        case PLUS: return "+";
+        case MINUS: return "-";
+        case TIMES: return "*";
+        case DIVIDE: return "/";
+    }
+    throw 0;
+}
+
+type_ptr ast_int::typecheck(type_mgr& mgr, const type_env& env) const {
+    return type_ptr(new type_base("Int"));
+}
+
+type_ptr ast_lid::typecheck(type_mgr& mgr, const type_env& env) const {
+    return env.lookup(id);
+}
+
+type_ptr ast_uid::typecheck(type_mgr& mgr, const type_env& env) const {
+    return env.lookup(id);
+}
+
+type_ptr ast_binop::typecheck(type_mgr& mgr, const type_env& env) const {
+    type_ptr ltype = left->typecheck(mgr, env);
+    type_ptr rtype = right->typecheck(mgr, env);
+    type_ptr ftype = env.lookup(op_name(op));
+    if(!ftype) throw 0;
+
+    type_ptr place_a = mgr.new_type();
+    type_ptr place_b = mgr.new_type();
+    type_ptr place_c = mgr.new_type();
+    type_ptr arrow_one = type_ptr(new type_arr(place_b, place_c));
+    type_ptr arrow_two = type_ptr(new type_arr(place_a, arrow_one));
+
+    mgr.unify(arrow_two, ftype);
+    mgr.unify(place_a, ltype);
+    mgr.unify(place_b, rtype);
+    return place_c;
+}
+
+type_ptr ast_app::typecheck(type_mgr& mgr, const type_env& env) const {
+    type_ptr ltype = left->typecheck(mgr, env);
+    type_ptr rtype = right->typecheck(mgr, env);
+
+    type_ptr place_a = mgr.new_type();
+    type_ptr place_b = mgr.new_type();
+    type_ptr arrow = type_ptr(new type_arr(place_a, place_b));
+    mgr.unify(arrow, ltype);
+    mgr.unify(place_a, rtype);
+    return place_b;
+}
+
+type_ptr ast_case::typecheck(type_mgr& mgr, const type_env& env) const {
+    type_ptr case_type = of->typecheck(mgr, env);
+    type_ptr branch_type = mgr.new_type();
+
+    for(auto& branch : branches) {
+        type_env new_env = env.scope();
+        branch->pat->match(case_type, mgr, new_env);
+        type_ptr curr_branch_type = branch->expr->typecheck(mgr, new_env);
+        mgr.unify(branch_type, curr_branch_type);
+    }
+
+    return branch_type;
+}
+
+void pattern_var::match(type_ptr t, type_mgr& mgr, type_env& env) const {
+    env.bind(var, t);
+}
+
+void pattern_constr::match(type_ptr t, type_mgr& mgr, type_env& env) const {
+    type_ptr constructor_type = env.lookup(constr);
+    if(!constructor_type) throw 0;
+
+    for(int i = 0; i < params.size(); i++) {
+        type_arr* arr = dynamic_cast<type_arr*>(constructor_type.get());
+        if(!arr) throw 0;
+
+        env.bind(params[i], arr->left);
+        constructor_type = arr->right;
+    }
+
+    mgr.unify(t, constructor_type);
+    type_base* result_type = dynamic_cast<type_base*>(constructor_type.get());
+    if(!result_type) throw 0;
+}
diff --git a/03/ast.hpp b/03/ast.hpp
index e1629cd..c658be7 100644
--- a/03/ast.hpp
+++ b/03/ast.hpp
@@ -2,15 +2,20 @@
 #include <memory>
 #include <vector>
 #include "type.hpp"
+#include "env.hpp"
 
 struct ast {
     virtual ~ast() = default;
+
+    virtual type_ptr typecheck(type_mgr& mgr, const type_env& env) const = 0;
 };
 
 using ast_ptr = std::unique_ptr<ast>;
 
 struct pattern {
     virtual ~pattern() = default;
+
+    virtual void match(type_ptr t, type_mgr& mgr, type_env& env) const = 0;
 };
 
 using pattern_ptr = std::unique_ptr<pattern>;
@@ -37,6 +42,9 @@ using constructor_ptr = std::unique_ptr<constructor>;
 
 struct definition {
     virtual ~definition() = default;
+    
+    virtual void typecheck_first(type_mgr& mgr, type_env& env) = 0;
+    virtual void typecheck_second(type_mgr& mgr, const type_env& env) const = 0;
 };
 
 using definition_ptr = std::unique_ptr<definition>;
@@ -53,6 +61,8 @@ struct ast_int : public ast {
 
     explicit ast_int(int v)
         : value(v) {}
+
+    type_ptr typecheck(type_mgr& mgr, const type_env& env) const;
 };
 
 struct ast_lid : public ast {
@@ -60,6 +70,8 @@ struct ast_lid : public ast {
 
     explicit ast_lid(std::string i)
         : id(std::move(i)) {}
+
+    type_ptr typecheck(type_mgr& mgr, const type_env& env) const;
 };
 
 struct ast_uid : public ast {
@@ -67,6 +79,8 @@ struct ast_uid : public ast {
 
     explicit ast_uid(std::string i)
         : id(std::move(i)) {}
+
+    type_ptr typecheck(type_mgr& mgr, const type_env& env) const;
 };
 
 struct ast_binop : public ast {
@@ -76,6 +90,8 @@ struct ast_binop : public ast {
 
     ast_binop(binop o, ast_ptr l, ast_ptr r)
         : op(o), left(std::move(l)), right(std::move(r)) {}
+
+    type_ptr typecheck(type_mgr& mgr, const type_env& env) const;
 };
 
 struct ast_app : public ast {
@@ -84,6 +100,8 @@ struct ast_app : public ast {
 
     ast_app(ast_ptr l, ast_ptr r)
         : left(std::move(l)), right(std::move(r)) {}
+
+    type_ptr typecheck(type_mgr& mgr, const type_env& env) const;
 };
 
 struct ast_case : public ast {
@@ -92,6 +110,8 @@ struct ast_case : public ast {
 
     ast_case(ast_ptr o, std::vector<branch_ptr> b)
         : of(std::move(o)), branches(std::move(b)) {}
+
+    type_ptr typecheck(type_mgr& mgr, const type_env& env) const;
 };
 
 struct pattern_var : public pattern {
@@ -99,6 +119,8 @@ struct pattern_var : public pattern {
 
     pattern_var(std::string v)
         : var(std::move(v)) {}
+
+    void match(type_ptr t, type_mgr& mgr, type_env& env) const;
 };
 
 struct pattern_constr : public pattern {
@@ -107,6 +129,8 @@ struct pattern_constr : public pattern {
 
     pattern_constr(std::string c, std::vector<std::string> p)
         : constr(std::move(c)), params(std::move(p)) {}
+
+    void match(type_ptr t, type_mgr&, type_env& env) const;
 };
 
 struct definition_defn : public definition {
@@ -114,10 +138,16 @@ struct definition_defn : public definition {
     std::vector<std::string> params;
     ast_ptr body;
 
+    type_ptr return_type;
+    std::vector<type_ptr> param_types;
+
     definition_defn(std::string n, std::vector<std::string> p, ast_ptr b)
         : name(std::move(n)), params(std::move(p)), body(std::move(b)) {
 
     }
+
+    void typecheck_first(type_mgr& mgr, type_env& env);
+    void typecheck_second(type_mgr& mgr, const type_env& env) const;
 };
 
 struct definition_data : public definition {
@@ -126,4 +156,7 @@ struct definition_data : public definition {
 
     definition_data(std::string n, std::vector<constructor_ptr> cs)
         : name(std::move(n)), constructors(std::move(cs)) {}
+
+    void typecheck_first(type_mgr& mgr, type_env& env);
+    void typecheck_second(type_mgr& mgr, const type_env& env) const;
 };
diff --git a/03/clean.sh b/03/clean.sh
index b987cf1..7580487 100755
--- a/03/clean.sh
+++ b/03/clean.sh
@@ -1 +1 @@
-rm -f parser.o parser.cpp parser.hpp stack.hh scanner.cpp scanner.o type.o a.out
+rm -f parser.o parser.cpp parser.hpp stack.hh scanner.cpp scanner.o type.o env.o ast.o definition.o a.out
diff --git a/03/compile.sh b/03/compile.sh
index 7c28115..e67846b 100755
--- a/03/compile.sh
+++ b/03/compile.sh
@@ -1,6 +1,9 @@
 bison -o parser.cpp -d parser.y
 flex -o scanner.cpp scanner.l
-g++ -c -o scanner.o scanner.cpp
-g++ -c -o parser.o parser.cpp
-g++ -c -o type.o type.cpp
-g++ main.cpp parser.o scanner.o type.o
+g++ -g -c -o scanner.o scanner.cpp
+g++ -g -c -o parser.o parser.cpp
+g++ -g -c -o type.o type.cpp
+g++ -g -c -o env.o env.cpp
+g++ -g -c -o ast.o ast.cpp
+g++ -g -c -o definition.o definition.cpp
+g++ -g main.cpp parser.o scanner.o type.o env.o ast.o definition.o
diff --git a/03/definition.cpp b/03/definition.cpp
new file mode 100644
index 0000000..69b3196
--- /dev/null
+++ b/03/definition.cpp
@@ -0,0 +1,48 @@
+#include "ast.hpp"
+
+void definition_defn::typecheck_first(type_mgr& mgr, type_env& env) {
+    return_type = mgr.new_type();
+    type_ptr full_type = return_type;
+
+    for(auto it = params.rbegin(); it != params.rend(); it++) {
+        type_ptr param_type = mgr.new_type();
+        full_type = type_ptr(new type_arr(param_type, full_type));
+        param_types.push_back(param_type);
+    }
+
+    env.bind(name, full_type);
+}
+
+void definition_defn::typecheck_second(type_mgr& mgr, const type_env& env) const {
+    type_env new_env = env.scope();
+    auto param_it = params.begin();
+    auto type_it = param_types.rbegin();
+
+    while(param_it != params.end() && type_it != param_types.rend()) {
+        new_env.bind(*param_it, *type_it);
+        param_it++;
+        type_it++;
+    }
+
+    type_ptr body_type = body->typecheck(mgr, new_env);
+    mgr.unify(return_type, body_type);
+}
+
+void definition_data::typecheck_first(type_mgr& mgr, type_env& env) {
+    type_ptr return_type = type_ptr(new type_base(name));
+
+    for(auto& constructor : constructors) {
+        type_ptr full_type = return_type;
+
+        for(auto& type_name : constructor->types) {
+            type_ptr type = type_ptr(new type_base(type_name));
+            full_type = type_ptr(new type_arr(type, full_type));
+        }
+
+        env.bind(constructor->name, full_type);
+    }
+}
+
+void definition_data::typecheck_second(type_mgr& mgr, const type_env& env) const {
+    // Nothing
+}
diff --git a/03/main.cpp b/03/main.cpp
index 6a72a95..43c1b4f 100644
--- a/03/main.cpp
+++ b/03/main.cpp
@@ -8,8 +8,32 @@ void yy::parser::error(const std::string& msg) {
 
 extern std::vector<definition_ptr> program;
 
+void typecheck_program(const std::vector<definition_ptr>& prog) {
+    type_mgr mgr;
+    type_env env;
+
+    type_ptr int_type = type_ptr(new type_base("Int")); 
+    type_ptr binop_type = type_ptr(new type_arr(
+                int_type,
+                type_ptr(new type_arr(int_type, int_type))));
+
+    env.bind("+", binop_type);
+    env.bind("-", binop_type);
+    env.bind("*", binop_type);
+    env.bind("/", binop_type);
+
+    for(auto& def : prog) {
+        def->typecheck_first(mgr, env);
+    }
+
+    for(auto& def : prog) {
+        def->typecheck_second(mgr, env);
+    }
+}
+
 int main() {
     yy::parser parser;
     parser.parse();
+    typecheck_program(program);
     std::cout << program.size() << std::endl;
 }
diff --git a/03/parser.y b/03/parser.y
index d8c45b7..fdb72c9 100644
--- a/03/parser.y
+++ b/03/parser.y
@@ -32,7 +32,7 @@ extern yy::parser::symbol_type yylex();
 %define api.value.type variant
 %define api.token.constructor
 
-%type <std::vector<std::string>> lowercaseParams
+%type <std::vector<std::string>> lowercaseParams uppercaseParams
 %type <std::vector<definition_ptr>> program definitions
 %type <std::vector<branch_ptr>> branches
 %type <std::vector<constructor_ptr>> constructors
@@ -71,6 +71,11 @@ lowercaseParams
     | lowercaseParams LID { $$ = std::move($1); $$.push_back(std::move($2)); }
     ;
 
+uppercaseParams
+    : %empty { $$ = std::vector<std::string>(); }
+    | uppercaseParams UID { $$ = std::move($1); $$.push_back(std::move($2)); }
+    ;
+
 aAdd
     : aAdd PLUS aMul { $$ = ast_ptr(new ast_binop(PLUS, std::move($1), std::move($3))); }
     | aAdd MINUS aMul { $$ = ast_ptr(new ast_binop(MINUS, std::move($1), std::move($3))); }
@@ -102,7 +107,7 @@ case
     ;
 
 branches
-    : branches COMMA branch { $$ = std::move($1); $1.push_back(std::move($3)); }
+    : branches branch { $$ = std::move($1); $1.push_back(std::move($2)); }
     | branch { $$ = std::vector<branch_ptr>(); $$.push_back(std::move($1));}
     ;
 
@@ -129,7 +134,7 @@ constructors
     ;
 
 constructor
-    : UID lowercaseParams
+    : UID uppercaseParams
         { $$ = constructor_ptr(new constructor(std::move($1), std::move($2))); }
     ;
 
diff --git a/03/type.cpp b/03/type.cpp
index 91f87d3..f370a91 100644
--- a/03/type.cpp
+++ b/03/type.cpp
@@ -70,7 +70,7 @@ void type_mgr::unify(type_ptr l, type_ptr r) {
     throw 0;
 }
 
-void type_mgr::bind(std::string s, type_ptr t) {
+void type_mgr::bind(const std::string& s, type_ptr t) {
     type_var* other = dynamic_cast<type_var*>(t.get());
     
     if(other && other->name == s) return;
diff --git a/03/type.hpp b/03/type.hpp
index ab358a4..e63040e 100644
--- a/03/type.hpp
+++ b/03/type.hpp
@@ -40,5 +40,5 @@ struct type_mgr {
 
     void unify(type_ptr l, type_ptr r);
     type_ptr resolve(type_ptr t, type_var*& var);
-    void bind(std::string s, type_ptr t);
+    void bind(const std::string& s, type_ptr t);
 };