How to delete spaces in string and write first letter uppercase?

Question

I am trying to delete every space between words and uppercase every letter of a word with foldr.

I have tried it with map:

deleteSpaces:: String -> String
deleteSpaces word = (filter(not . isSpace) .unwords . map (\(x:xs) -> (toUpper x):xs) . words) word

it works.

But with foldr I always get an error:

deleteSpacesF :: String -> String
deleteSpacesF word = (filter(not . isSpace) . unwords . foldr (\(x:xs) acc -> (toUpper x) : xs : acc) [] . words) word

I have also tried (\x acc -> (toUpper (head x)):(tail x):acc)

The error is:

• Couldn't match type ‘Char’ with ‘[Char]’ Expected: String Actual: Char • In the first argument of ‘(:)’, namely ‘(toUpper x)’ In the expression: (toUpper x) : xs : acc In the first argument of ‘foldr’, namely ‘(\ (x : xs) acc -> (toUpper x) : xs : acc)’

Answer 1

x is a Char, not a String, so head x and tail x, make no sense. You use x as the head and xs as the tail, so.

Here it thus looks like:

deleteSpacesF :: String -> String
deleteSpacesF = concat . foldr (\(x:xs) -> ((toUpper x : xs) :)) [] . words

you can also omit the concat with:

deleteSpacesF :: String -> String
deleteSpacesF = foldr (\(x:xs) -> (toUpper x :) . (xs ++)) [] . words