To slow check for right input-CodePudding

drop1 n xs | n<0 = error "Use positive numbers"
drop1 _ [] = error "Empty list" 
drop1 0 xs = xs
drop1 n xs = drop1 (n-1)(tail1 xs)

My drop1 function checks every time. What can I do to check this only 1 first time? How can I modify this part: drop1 n xs | n<0 = error "Use positive numbers" ?

CodePudding user response：

You can work with a helper function:

drop1 :: Int -> [a] -> [a]
drop1 n
  | n < 0 = error "Use positive numbers"
  | otherwise = go n
  where go _ [] = error "Empty list"
        go 0 xs = xs
        go k (_:xs) = go (k-1) xs

The drop1 thus checks once if the value is less than 0, whereas go only checks for 0 and values larger than 0.

Probably you want to swap the two clauses about go _ [] = error "Empty list" and go 0 xs = xs, since dropping two items of a list containing two items probably should return an empty list, and not an error.