Improved error message and created document for known bugs.

src/TypeChecker/Bugs.md

@@ -0,0 +1,43 @@
+## Bugs
+
+### Polymorphic type variables are global?
+
+This doesn't work (occurs check failed, can't unify `(a -> a) = a`
+```hs
+data Maybe ('a) where {
+    Nothing : Maybe ('a)
+    Just : 'a -> Maybe ('a)
+};
+
+id : 'a -> 'a ;
+id x = x ;
+
+main : Maybe ('a -> 'a) ; 
+main = Just id;
+```
+
+But this does
+```hs
+data Maybe ('a) where {
+    Nothing : Maybe ('a)
+    Just : 'a -> Maybe ('a)
+};
+
+id : 'b -> 'b ;
+id x = x ;
+
+main : Maybe ('a -> 'a) ; 
+main = Just id;
+```
+
+### The function f is not carried into the case-expression
+
+Code example that does not type check
+```hs
+fmap : ('a -> 'b) -> Maybe ('a) -> Maybe ('b) ;
+fmap f x =
+    case x of {
+        Just x => Just (f x) ;
+        Nothing => Nothing
+    }
+```

src/TypeChecker/TypeChecker.hs

@@ -272,7 +272,7 @@ unify t0 t1 = case (trace ("LEFT: " ++ show t0) t0, trace ("RIGHT: " ++ show t1)
 occurs :: Ident -> Type -> Infer Subst
 occurs _ (TPol _) = return nullSubst
 occurs i t = if S.member i (free t)
-                then throwError "Occurs check failed"
+                then throwError $ unwords ["Occurs check failed, can't unify", printTree (TPol i), "with", printTree t]
                 else return $ M.singleton i t
 
 -- | Generalize a type over all free variables in the substitution set