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