Non type-variable argument in the constraint error on Haskell map function

Question

I am writing a function called mapper2 that applies a function to two lists:

mapper2 :: (a-> b -> c) -> [a] -> [b] -> [c]
mapper2 f (         


        

Answer 1

Look at mapper2's type.

mapper2 :: (a -> b -> c) -> [a] -> [b] -> [c]

Now look at the type of the function you're passing in.

(\x -> x * 2) :: Num a => a -> a

The mapper2 function expects a function of two arguments to be passed in, but your lambda only takes one argument.