diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5e8713f..97b1d8b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,5 +2,5 @@ cmake_minimum_required(VERSION 3.0)
 project(lily)
 
 set(CMAKE_CXX_STANDARD 14)
-add_executable(lily src/main.cpp src/parser.cpp src/parser.c src/type.cpp src/type_manager.cpp src/ast.cpp src/type_checker.cpp)
+add_executable(lily src/main.cpp src/parser.cpp src/parser.c src/type.cpp src/type_manager.cpp src/ast.cpp src/type_checker.cpp src/pattern.cpp)
 target_include_directories(lily PUBLIC src)
diff --git a/src/ast.cpp b/src/ast.cpp
index c011bd5..3caa55c 100644
--- a/src/ast.cpp
+++ b/src/ast.cpp
@@ -65,6 +65,32 @@ namespace lily {
     }
 
     type* ast_case::check(type_manager& mgr, std::shared_ptr<type_env> env) {
-        throw error("unimplemented");
+        type* ctype = of->check(mgr, env);
+        type* pattern_type = nullptr;
+        type* branch_type = nullptr;
+        for(auto& branch : branches) {
+            auto new_env = std::make_shared<type_env>();
+            new_env->set_parent(env.get());
+            type* new_type = branch.pattern->check_modifying(mgr, new_env);
+
+            if(!pattern_type) {
+                pattern_type = new_type;
+            } else {
+                if(!pattern_type->unify_with(new_type))
+                    throw error("incompatible pattern types");
+            }
+            if(!pattern_type->unify_with(ctype))
+                throw error("pattern type doesn't match case value type");
+
+            type* new_branch_type = branch.expr->check(mgr, new_env);
+            if(!branch_type) {
+                branch_type = new_branch_type;
+            } else {
+                if(!branch_type->unify_with(new_branch_type))
+                    throw error("branches have incompatible types");
+            }
+        }
+
+        return branch_type;
     }
 }
diff --git a/src/main.cpp b/src/main.cpp
index d40831d..e6267a9 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -10,7 +10,8 @@ int main() {
                 "data IntList = { Nil, Cons(Int, IntList) }\n"
                 "defn other x y = { 3 }\n"
                 "defn add x y = { x + y }\n"
-                "defn ones = { Cons 1 ones }");
+                "defn ones = { Cons 1 ones }\n"
+                "defn len l = { case l of { Nil -> { 0 } Cons(x, xs) -> { 1 + len xs } } }");
     } catch(lily::error& e) {
         std::cout << e.message << std::endl;
     }
diff --git a/src/parser.cpp b/src/parser.cpp
index 2b2dfbe..8cbe3f2 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -95,10 +95,10 @@ namespace lily {
 
             pgs_tree* patterns = PGS_TREE_NT_CHILD(*tree, 2);
             while(PGS_TREE_NT_COUNT(*patterns) > 1) {
-                new_pattern->vnames.push_back(tree_str(PGS_TREE_NT_CHILD(*tree, 0), source));
-                patterns = PGS_TREE_NT_CHILD(*tree, 2);
+                new_pattern->vnames.push_back(tree_str(PGS_TREE_NT_CHILD(*patterns, 0), source));
+                patterns = PGS_TREE_NT_CHILD(*patterns, 2);
             }
-            new_pattern->vnames.push_back(tree_str(PGS_TREE_NT_CHILD(*tree, 0), source));
+            new_pattern->vnames.push_back(tree_str(PGS_TREE_NT_CHILD(*patterns, 0), source));
             return ptr;
         }
     }
diff --git a/src/pattern.cpp b/src/pattern.cpp
new file mode 100644
index 0000000..7dbc7b8
--- /dev/null
+++ b/src/pattern.cpp
@@ -0,0 +1,32 @@
+#include "pattern.hpp"
+#include "error.hpp"
+#include "type_checker.hpp"
+#include "type_manager.hpp"
+
+namespace lily {
+    type* pattern_var::check_modifying(type_manager& mgr, std::shared_ptr<type_env> env) {
+        type* parameter = mgr.create_type<type_parameter>();
+        env->set_type(name, parameter);
+        return parameter;
+    }
+
+    type* pattern_cons::check_modifying(type_manager& mgr, std::shared_ptr<type_env> env) {
+        type* constructor_type = env->get_identifier_type(name);
+        int arity = constructor_type->arity();
+        if(arity != vnames.size())
+            throw error("invalid number of arguments to constructor");
+
+        if(arity == 0) {
+            return constructor_type;
+        } else {
+            type_func* func_type;
+            type* current = constructor_type;
+            int index = 0;
+            while((func_type = dynamic_cast<type_func*>(current))) {
+                env->set_type(vnames[index++], func_type->left);
+                current = func_type->right;
+            }
+            return current;
+        }
+    }
+}
diff --git a/src/pattern.hpp b/src/pattern.hpp
index ba0629b..4c1153d 100644
--- a/src/pattern.hpp
+++ b/src/pattern.hpp
@@ -4,8 +4,13 @@
 #include <vector>
 
 namespace lily {
+    class type_manager;
+    class type_env;
+    struct type;
+
     struct pattern {
         virtual ~pattern() = default;
+        virtual type* check_modifying(type_manager& mgr, std::shared_ptr<type_env> env) = 0;
     };
 
     typedef std::unique_ptr<pattern> pattern_ptr;
@@ -16,6 +21,7 @@ namespace lily {
         pattern_var(std::string n) :
             name(std::move(n)) {}
         ~pattern_var() = default;
+        type* check_modifying(type_manager& mgr, std::shared_ptr<type_env> env);
     };
 
     struct pattern_cons : pattern {
@@ -25,5 +31,6 @@ namespace lily {
         pattern_cons(std::string n) :
             name(std::move(n)) {}
         ~pattern_cons() = default;
+        type* check_modifying(type_manager& mgr, std::shared_ptr<type_env> env);
     };
 }
diff --git a/src/type.hpp b/src/type.hpp
index 564a720..398d384 100644
--- a/src/type.hpp
+++ b/src/type.hpp
@@ -13,6 +13,7 @@ namespace lily {
     struct type {
         virtual ~type() = default;
         virtual bool unify_with(type* other) = 0;
+        virtual int arity() { return 0; }
     };
 
     struct type_internal : type {
@@ -49,6 +50,7 @@ namespace lily {
         type_func(type* l, type* r) :
             left(l), right(r) {}
         bool unify_with(type* other);
+        int arity() { return right->arity() + 1; }
     };
 }