Make strings work like printf.

output.txt

23. Hello, World!

PrintfString.hs

{-# LANGUAGE OverloadedStrings, FlexibleInstances, UndecidableInstances #-}
{-# OPTIONS_GHC -fno-warn-orphans #-}

import Data.String ( IsString(..) )
import Text.Printf ( PrintfType, printf )

instance (PrintfType a) => IsString a where
    fromString s = printf s

main :: IO ()
main = do
    let x = 23 :: Int
        s = "World!" :: String
    "%d. Hello, %s\n" x s

The associated blog post is here.