"Default values" for Haskell function?

Question

Is there any way that input values to a function can be pre-defined so that a user doesn't have to define them each time?

For example, assuming I had a function "zr" which returned a list zeros of size n, such that:

zr 1 = [0]
zr 5 = [0, 0, 0, 0, 0]

And so on.

My current way of implementing it is:

zr :: [Int] -> Int -> [Int]
zr x y
    | length x == y = x
    | otherwise = zr x++[y]

But this isn't particularly elegant as every time I call zr I need to include an empty list as a parameter:

zr [] 5

Any ideas?

Thanks!

Answer 1

I think what you're looking for is partial application:

zr' :: Int -> [Int]
zr' = zr []

If zr is a function that takes a list and an integer, then zr [] is a function that takes only an integer n and returns zr [] n.

In your case, zr is obviously a "temporary" function (you don't want anyone calling zr [1] 4 by accident), so you might as well define it locally:

zr :: Int -> [Int]
zr = zr' []
  where
     zr' :: [Int] -> Int -> [Int]
     zr' x y
         | length x == y = x
         | otherwise = zr' x++[0] y