@hackage endo0.3.0.1

Package defines extra functions for Data.Monoid.Endo data type, and also generic endomorphism folding machinery. Generic endomorphism folding can be used for various purposes, including as a builder.

Here is an example how to use it with optparse-applicative package:

data Verbosity = Silent | Normal | Verbose | Annoying
  deriving (Show)

data Config = Config Verbosity FilePath
  deriving (Show)

options :: Parser Config
options = runIdentityT $ runEndo defaultConfig <$> options'
  where
    -- All this IdentityT clutter is here to avoid orphan instances.
    options' :: IdentityT Parser (Endo Config)
    options' = foldEndo
        <*> outputOption     -- :: IdentityT Parser (Maybe (E Config))
        <*> verbosityOption  -- :: IdentityT Parser (Maybe (E Config))
        <*> annoyingFlag     -- :: IdentityT Parser (E Config)
        <*> silentFlag       -- :: IdentityT Parser (E Config)
        <*> verboseFlag      -- :: IdentityT Parser (E Config)

    defaultConfig :: Config
    defaultConfig = Config Normal ""

main :: IO ()
main = execParser (info options fullDesc) >>= print

ghci> :main -o an.out.put --annoying
Config Annoying "an.out.put"

For details how individual option parsers look like see module Data.Monoid.Endo.Fold which contains other examples as well as this one.