Haskell - Instance of Num Char required for definition of a function

Question

I want to write a simple recursive function with 3 input parameters and 1 output parameter:

getLengthOfNumber :: (String, Int, Int) -> Int
getLengthOfNumber (n, i, res)
    | isCharDigit(n!!i+1) = getLengthOfNumber (n, i+1, res+1)
    | otherwise = res

Why does Hugs throw me the Error "Instance of Num Char required for definition of getLengthOfNumber"?

Answer 1

!! has higher priority than + , so n!!i+1 is parsed as (n !! i) + 1 , which tries to add one to an element of a string, which only works if Char is a number. You should instead write n !! (i+1) n !! (i+1) .

Answer 2

Because it interprets:

n!!i+1

as:

(n!!i) + 1

It thus first obtains the i -th character of the string, and then aims to add 1 to it. Now in Haskell one can define a custom number type. So here you could add that character and 1 together, but only if that character is a number.

But that being said, the above will not work even by fixing the brackets. You did not instruct when to stop iterating: for the string "123" , it would eventually obtain the last character, and then raise an "index too large" error when it reaches the end of the string. Furthermore !! is not efficient: it requires O(k) to access the k -th element, making this algorithm a quadratic one.

What you here can use is takeWhile :: (a -> Bool) -> [a] -> [a] and length :: [a] -> Int : first take the longest prefix of character, and then take the length of that list, like:

getLengthOfNumber :: String -> Int
getLengthOfNumber = length . takeWhile isCharDigit