Fix function return type checking.

src/main.cpp

@@ -8,6 +8,7 @@ int main() {
                 "data Bool = { True, False }\n"
                 "data Color = { Red, Black }\n"
                 "data IntList = { Nil, Cons(Int, IntList) }\n"
+                "defn other x y = { 3 }\n"
                 "defn add x y = { x + add x x }");
     } catch(lily::error& e) {
         std::cout << e.message << std::endl;

src/parser.cpp

@@ -248,6 +248,7 @@ namespace lily {
         // Each function has an environment, which will be used
         // as base for type checking.
         std::map<std::string, std::shared_ptr<type_env>> function_envs;
+        std::map<std::string, type*> function_output_types;
         auto base_env = std::make_shared<type_env>();
 
         // First step is to collect all function types.
@@ -261,6 +262,7 @@ namespace lily {
             // Create the return type parameter
             type* return_type = type_mgr.create_type<type_parameter>();
             type* current_type = return_type;
+            function_output_types[pair.first] = return_type;
 
             // Create type parameters for every variable
             // We also want to place them in a local environment
@@ -279,7 +281,10 @@ namespace lily {
 
         // Now that we have collected the functions, check their bodies.
         for(auto& pair : functions) {
-            pair.second.body->check(type_mgr, function_envs[pair.first]);
+            type* body_type =
+                pair.second.body->check(type_mgr, function_envs[pair.first]);
+            if(!function_output_types[pair.first]->unify_with(body_type))
+                throw error("unable to unify function type");
         }
     }
 }