A (contrived) use for NullaryTypeClasses

Main.hs

{-# LANGUAGE NullaryTypeClasses, Rank2Types #-}
import Prelude hiding (log)
import Unsafe.Coerce
import System.IO.Unsafe


class NeedsInit

provideInit :: (NeedsInit => a) -> (() -> a)
provideInit = unsafeCoerce

log :: NeedsInit => String -> a -> a
log msg x = unsafePerformIO $ do
    -- imagine this was an ffi call to some logging library
    putStrLn msg
    return x

initLog :: (NeedsInit => Int) -> Int
initLog x = let x' = provideInit x () in unsafePerformIO $ do
    -- imagine this was initializing the logging library
    putStrLn "init"
    return x'

test :: NeedsInit => Int
test = log "left" 2 + log "right" 3

-- No instance for NeedsInit
-- main = print test

-- typechecks!
main = print (initLog test)