"you don't have a function a: X x Y -> Z, you've got a function a: X -> (Y => Z)"

Haskell has products. What's the problem with f':

ghci> f x y = x + y

ghci> f' (x,y) = x + y

ghci> f 1 2

3

ghci> f' (1,2)

3

ghci> :t f'

f' :: Num a => (a, a) -> a

ghci> :t f

f :: Num a => a -> a -> a