[Haskell-beginners] Explanation of composite with foldl

[Lawrence Bottorff]

I see this on the Haskell 99 questions
<https://wiki.haskell.org/99_questions/Solutions/2> which will return the
second-from-last element of a list

lastbut1 :: Foldable f => f a -> a
lastbut1 = fst . foldl (\(a,b) x -> (b,x)) (err1,err2)
  where
    err1 = error "lastbut1: Empty list"
    err2 = error "lastbut1: Singleton"

I understand how the code works, but not the significance of the type
declaration. That looks like a type class. It works without it, I believe.
Why have we used Foldable type class?

Likewise with this

lastbut1safe :: Foldable f => f a -> Maybe a
lastbut1safe = fst . foldl (\(a,b) x -> (b,Just x)) (Nothing,Nothing)

What's happening with the type definition?

LB
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