Brandon Allbery wrote:
Now, as to how useful it is... you'll note there isn't actually a lot of code out there that uses ShowS
I use it sometimes. ShowS was the first and simplest, "pretty-printer" library. A pretty-printer library is an alternative to big ugly monolithic expressions for creating output. Because it is based on function composition, ShowS makes it easier to write in a nice combinator style. All of the early pretty-printer libraries used function composition in this way. It appears early library writers assumed that because of its advantages, and also because of certain limitations of early compilers, everyone would always use ShowS instead of directly constructing Strings. So they used ShowS for the types of showHex and showOct. In reality ShowS never caught on, but it's not worth the trouble to change those original types now.
The simplistic show/read system uses it, but most other parsers and string generators use other mechanisms instead: Monad, Applicative, occasionally Arrow.
Recently the trend is to use "builders" based on Monoid for fast rendering libraries. That style has some of the advantages of ShowS, plus, unlike ShowS, it works equally well with ByteString and Text. While not the fastest or most featureful, ShowS still has the advantages of being really simple to use, available directly from the Prelude without any external library (and thus a standard style, even though it isn't as popular as it could be), and it generally has a pleasing simplifying effect on the surrounding code. It also works absolutely seamlessly with showHex and showOct :). Any big expression of type String that starts with "concat" followed by a long list of String expressions is a good candidate for using ShowS instead. Here is a simple example of how to use ShowS: showPerson :: String -> Int -> Int -> String showPerson name age shoeSize = concat [ "Name: ", name, ", Age: ", show age, ", Shoe size: ", show shoeSize] ... putStrLn $ showPerson name age shoe becomes, in the ShowS style: showsPerson :: String -> Int -> Int -> ShowS showsPerson name age shoeSize = ("Name: " ++) . (name ++) . (", Age: " ++) . shows age . (", Shoe size: " ++) . shows shoeSize ... putStrLn $ showsPerson name age shoe "" Regards, Yitz