split function 'no instance for' error

Question

this function should tokenize a String:

    split s c xs i j =
        if head s == c
        then split s c (subStr s i j : xs) j (j + 1)
        else if j == length s
        then (subStr s i j) : xs
        else split s c xs i j + 1
    subStr s i j = take j(drop i s)

However I get the following error msg: No instance for (Num [[Char]]) arising from a use of 'split' Possible fix: add an instance declaration at (Num [[Char]])

Thanks.

Ok function is now:

split s c xs i j =
        if j == length s
        then (subStr s i j) : xs
        else if head (drop j s) == c
        then split s c (subStr s i j : xs) (j + 1) (j + 1)
        else split s c xs i (j + 1)

subStr s i j = take j(drop i s)

now when I apply the function with the following args: split "123,456,789" ',' [] 0 0 I get the result ["789", "456,789", "123"] what's happening here?

You should include your type signature for the function. It makes it a bit easier for us to tell what you're trying to do. but my initial guess is that you just need to parenthesize j+1 on the second to last line. — genisage
– genisage, Commented Jun 26, 2014 at 18:17
I agree with cdk that this function could be rewritten more clearly. But if you just want this one to work, make substr s i j = take (j-i) (drop i s) — genisage
– genisage, Commented Jun 26, 2014 at 19:48

cdk · Accepted Answer · 2014-06-26 19:17:47Z

2

The problem is on the line else split s c xs i j + 1. It seems like you're trying to add 1 to the result of split. You probably forgot parens around (j + 1)

I'll assume split returns a [String], yes?

Edit: Your function is difficult to follow. The strange ordering is probably a result of you prepending substrings on the 3rd line (subStr s i j) : xs.

Try rewriting your function using takeWhile, dropWhile :: (a -> Bool) -> [a] -> [a], or better yet, a proper string processing library like ByteString which provides

Data.ByteString.Char8.split :: (Char -> Bool) -> ByteString -> [ByteString]

edited Jun 26, 2014 at 19:17

answered Jun 26, 2014 at 18:20

cdk

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2 Comments

user2069328 Over a year ago

Thanks, having a problem on logic now: so the function looks like this: split s c xs i j = if j == length s then (subStr s i j) : xs else if head (drop j s) == c then split s c (subStr s i j : xs) (j + 1) (j + 1) else split s c xs i (j + 1) subStr s i j = take j(drop i s) When I apply: split "123,456,789" 'c' [] 0 0 I get the output ["789", "456, 789", "123"] have any idea what's going on in the middle there?

cdk Over a year ago

I cannot read your code as you've presented it. So no, I have no idea what's going on. I recommend accepting this answer (once you're able to) and asking a new question regarding your problem.

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