Recursive function in haskell to capitilize each word by in a list of strings-CodePudding

I need to writea function titleRec :: [String] -> [String] which, given a list of words, capitalises them as a title should be capitalised. The proper capitalisation of a title (for our purposes) is as follows: The first word should be capitalised. Any other word should be capitalised if it is at least four letters long.

I came as far as,

capitalisedRec :: String -> String
capitalisedRec [] = []
capitalisedRec (x:xs) = toUpper x : lowered xs 
                       where 
                        lowered [] = []
                        lowered (x:xs) = toLower x : lowered xs

capitilized2 :: String -> String
capitilized2 xs = [toLower x | x <-xs]

titleRec :: [String] -> [String]
titleRec [] = []
titleRec (x:xs) = capitalisedRec x : adjust xs
                 where
                    adjust [] = []
                    adjust (y:ys) = [if (length y >= 4) then capitalisedRec y else capitilized2 y | y<- ys]

But when I run it on a list of strings, for example titleRec ["tHe", "bOSun", "ANd", "thE", "BriDGe"] it results in ["The","and","the","Bridge"], basically skips over the second word. I don't understand why.

CodePudding user response：

Your code reads:

adjust (y:ys) = 
  [ if (length y >= 4) 
    then capitalisedRec y
    else capitilized2 y 
  | y <- ys]

but the first y is not used later on, discarding it! The latter y is unrelated, and shadows the first one.

A minimal modification fix would be:

adjust (y:ys) = 
  [ if (length y2 >= 4)
    then capitalisedRec y2
    else capitilized2 y2 
  | y2 <- y:ys]

I would strongly recommend that you always keep warnings enabled using -Wall. Doing so, GHC would warn about y being unused (and about the shadowing as well).