Use heskell to construct a function lengthNum so that it will only return the length of a list of numbers. (i.e. it will not work for any other type of list, e.g. a list of String).

Here's an implementation of `lengthNum` function in Haskell that only works for lists of numbers:

lengthNum :: Num a => [a] -> Int
lengthNum xs = length xs

The function takes a list of type `[a]`, where `a` is any numeric type, and returns its length as an `Int`. Since the input type is constrained to only numeric types, it will not work for any other type of list, such as a list of `String`. The `Num` typeclass constraint ensures that only numeric types are allowed as inputs.