[Haskell-cafe] Ambiguous types

[Dušan Kolář]

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

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