Dear Café, I'm trying to solve a couple of examples and exercises just for me. I've come to the point, when I'm trying working code manipulating lists rewrite to work on Foldable (etc.). Nevertheless, I must be doing some mistake, overlooking something, as simple code like this: chkDup [] = False chkDup (0:ns) = chkDup ns chkDup (n:ns) = elem n ns || chkDup ns which works fine, type-checks, can be used within other code, I'm trying to replace with more generic, but probably less efficient and wrong code: chkDup ns = fst $ foldr f (False,mempty) ns where f _ res@(True,_) = res f v res@(_,vs) = if v==0 then (False, vs) else (elem v vs, pure v <> vs) which does not even type-check. Nevertheless, the error message is not too helpful, searching the Internet just confirms it's wrong and that adding AllowAmbiguousTypes would not work. The error message is: helper.hs:49:1: error: • Could not deduce (Foldable f0) from the context: (Eq a, Num a, Foldable t, Foldable f, Applicative f, Monoid (f a)) bound by the inferred type for ‘chkDup’: forall a (t :: * -> *) (f :: * -> *). (Eq a, Num a, Foldable t, Foldable f, Applicative f, Monoid (f a)) => t a -> Bool at helper.hs:(49,1)-(53,80) The type variable ‘f0’ is ambiguous • In the ambiguity check for the inferred type for ‘chkDup’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes When checking the inferred type chkDup :: forall a (t :: * -> *) (f :: * -> *). (Eq a, Num a, Foldable t, Foldable f, Applicative f, Monoid (f a)) => t a -> Bool | 49 | chkDup ns = | ^^^^^^^^^^^... So is there a nicer and working way, how to change my simple example? Is there a way, how to make it compile? Or is it useless to do that, original function is just fine... Best regards, Dušan