[Haskell-beginners] Nested folds

[Francesco Ariis]

Hello Pietro,

Il 13 dicembre 2020 alle 10:39 Pietro Grandinetti ha scritto:
> Hello,
> I have a piece of code to represents Sentences, Paragraphs and the Content of an article. I added functions to count the words, code below. My questions:
> 
> […]
> 
> I also have two more practical questions on the following two functions:
> 
> makeSentence :: String -> Sentence
> makeSentence x = x::Sentence

You can omit the `:: Sentence` part, since it is specified in the
signature above. You can omit the whole function itself to be fair,
Sentence is a type synonym!

> sentCharCount :: Sentence -> Int
> sentCharCount x = length $ filter (/= ' ') x

You can write this point-free like this

    sentCharCount :: Sentence -> Int
    sentCharCount = length . filter (/= ' ')

In this example you can regard `$` as «evaluate everything on the right
before anything else», so

    length $ filter (/= ' ')
    ^^^^^^   ^^^^^^^^^^^^^^^
      |            |
      |            |
      |            +-- this has type :: [Char] -> [Char]
      |
      +-- length does not work on `[Char] -> [Char]`

`.` instead is appropriate

    λ> :t (.)
    (.) :: (b -> c) -> (a -> b) -> a -> c

Does this clear your doubts?
—F