How to abstract over a “back and forth” transformation?

Question

Consider this example (from https://codereview.stackexchange.com/questions/23456/crtitique-my-haskell-function-capitalize):

import Data.Char

capWord [] = []
cap         


        

Answer 1

It's indeed not flexible enough! How would you lift a function to work on a line-by-line basis? You're going to need a newtype wrapper for that! Like so

newtype LineByLine = LineByLine { unLineByLine :: String }

instance Lift LineByLine [String] where
    up = lines . unLineByLine
    down = LineByLine . unlines

But now there is no good reason to prefer the word-by-word version over the line-by-line one.

I would just use unwords . map f . words, to me that's the idiomatic "Apply f to all the words and put them back together". If you do this more often, consider writing a function.