Question

Let’s find files that not exist in the current directory:

filterM (\f -> return . not =<< doesFileExist f) files

And now I want to do it better with point notation:

filterM (liftM not . doesFileExist) files

Are there another approaches to do this? For example, later works not so good for composition of pure functions, need braces:

filterM (liftM (isExtensionPNG . not) . doesFileExist) files

Answer 1

You could define an infix operator to make it nicer to write:

infixr 9 .: -- same as .

(.:) :: Monad m => (b -> c) -> (a -> m b) -> a -> m c
f .: g = liftM f . g


filterM (isExtensionPNG .: not .: doesFileExist) files