{-# LANGUAGE CPP #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}

#if MIN_VERSION_transformers(0,4,0)
-- ExceptT was introduced in transformers == 0.4.0.0 and it deprecated ErrorT.
-- That is also the reason why ErrorT instance is not provided.
#define HAVE_EXCEPTT
#endif

-- |
-- Module:       $HEADER$
-- Description:  Generic folding for various endomorphism representations.
-- Copyright:    (c) 2014-2016, Peter Trško
-- License:      BSD3
--
-- Maintainer:   peter.trsko@gmail.com
-- Stability:    experimental
-- Portability:  CPP, DeriveDataTypeable, DeriveGeneric, FlexibleInstances,
--               NoImplicitPrelude, RankNTypes, TypeOperators, TypeFamilies
--
-- Generic folding for various endomorphism representations.
module Data.Monoid.Endo.Fold
    (
    -- * Usage Examples
    --
    -- | Examples in this section were taken from real live production code,
    -- but they were tamed down a little.

    -- ** Basic Idea
    -- $basicIdea

    -- ** Working With Corner Cases
    -- $workingWithCornerCases

    -- ** Using With Lenses
    -- $lenses

    -- ** Other Usage
    -- $otherUsage

    -- ** Using With optparse-applicative
    -- $optparseApplicativeExample

    -- * Generic Endomorphism Folding
      foldEndo
    , dualFoldEndo

    -- ** Type Classes
    , FoldEndoArgs(..)
    , AnEndo(..)

    -- ** Type Wrappers
    , WrappedFoldable(..)

    -- * Utility Functions and Types
    , (:->)
    , (&$)
    , (<&$>)
    , embedEndoWith
    , embedDualEndoWith
    )
  where

import Control.Applicative
    ( Applicative(pure)
#ifndef HAVE_FUNCTOR_CONST_MODULE
    , Const(Const)
#endif
    )
import Control.Monad (Monad(return))
import Data.Either (Either(Right))
import Data.Function ((.), id)
import Data.Functor (Functor(fmap))
#ifdef HAVE_FUNCTOR_CONST_MODULE
import Data.Functor.Const (Const(Const))
#endif
import Data.Functor.Identity (Identity(Identity))
import Data.Maybe (Maybe(Just))
import Data.Monoid (Dual(Dual), Endo, Monoid(mempty), (<>))
import System.IO (IO)

#ifdef HAVE_EXCEPTT
import Control.Monad.Trans.Except (ExceptT)
#endif
import Control.Monad.Trans.Identity (IdentityT)
import Control.Monad.Trans.List (ListT)
import Control.Monad.Trans.Maybe (MaybeT)
import Control.Monad.Trans.Reader (ReaderT)
import Control.Monad.Trans.RWS (RWST)
import qualified Control.Monad.Trans.RWS.Strict as Strict (RWST)
import Control.Monad.Trans.State (StateT)
import qualified Control.Monad.Trans.State.Strict as Strict (StateT)
import Control.Monad.Trans.Writer (WriterT)
import qualified Control.Monad.Trans.Writer.Strict as Strict (WriterT)
import Data.Functor.Compose (Compose)
import Data.Functor.Product (Product)

import Data.Monoid.Endo.AnEndo
    ( AnEndo(..)
    , WrappedFoldable(..)
    , embedDualEndoWith
    , embedEndoWith
    )


-- | Fold all variously represented endomorphisms in to one endomorphism.
--
-- Order in which endomorphisms are folded is preserved:
--
-- >>> foldEndo (Endo (1:)) [(2:), (3:)] `appEndo` []
-- [1,2,3]
--
-- For numbers it would look like:
--
-- >>> foldEndo (Endo (+1)) [(+2), (*3)] `appEndo` 1
-- 6
--
-- Above can be seen as:
--
-- >>> (+1) . (+2) . (*3) $ 1
-- 6
foldEndo :: FoldEndoArgs args => args
foldEndo = foldEndoArgs mempty

-- | Same as 'foldEndo', but folds endomorphisms in reverse order.
--
-- Following are the same examples as for 'foldEndo' function. Please, note the
-- differences in results.
--
-- Order in which endomorphisms are folded is reversed:
--
-- >>> dualFoldEndo (Endo (1:)) [(2:), (3:)] `appEndo` []
-- [3,2,1]
--
-- For numbers it would look like:
--
-- >>> dualFoldEndo (Endo (+1)) [(+2), (*3)] `appEndo` 1
-- 12
--
-- Above can be seen as:
--
-- >>> (*3) . (+2) . (+1) $ 1
-- 12
dualFoldEndo :: FoldEndoArgs args => args
dualFoldEndo = dualFoldEndoArgs mempty

-- {{{ FoldEndoArgs Type Class ------------------------------------------------

-- | Class of arguments for 'foldEndo' and its dual 'dualFoldEndo' functions.
--
-- Note that results are instances of this ('FoldEndoArgs') class and
-- endomorphism representations are instances of 'AnEndo' type class.
class FoldEndoArgs a where
    -- | Extracts type of a value that is modified by the result.
    type ResultOperatesOn a

    -- | Result type of the whole endomorphism folding. It can be used to
    -- restrict the result of 'foldEndo' and 'dualFoldEndoArgs'. Example:
    --
    -- @
    -- -- Type restricted version of 'foldEndo' that forces the result of the
    -- -- whole folding machinery to be \"'Endo' Int\".
    -- myFoldEndo
    --     :: ('Result' args ~ 'Endo' Int, 'FoldEndoArgs' args)
    --     => args -> args
    -- myFoldEndo = 'foldEndo'
    -- @
    type Result a

    foldEndoArgs     ::       Endo (ResultOperatesOn a)  -> a
    dualFoldEndoArgs :: Dual (Endo (ResultOperatesOn a)) -> a
#ifdef HAVE_MINIMAL_PRAGMA
    {-# MINIMAL foldEndoArgs, dualFoldEndoArgs #-}
#endif

-- | Recurse along 'FoldEndoArgs' instances if first argument is 'AnEndo'.
-- This instance is actually what makes 'foldEndo' and 'dualFoldEndo'
-- variadic-like.
instance
    ( AnEndo a
    , FoldEndoArgs r
    , EndoOperatesOn a ~ ResultOperatesOn r
    ) => FoldEndoArgs (a -> r)
  where
    type ResultOperatesOn (a -> r) = ResultOperatesOn r
    type Result (a -> r) = Result r
    foldEndoArgs     e e' = foldEndoArgs     (e <> anEndo    e')
    dualFoldEndoArgs e e' = dualFoldEndoArgs (e <> aDualEndo e')

instance FoldEndoArgs (Endo a) where
    type ResultOperatesOn (Endo a) = a
    type Result (Endo a) = Endo a
    foldEndoArgs              = id
    dualFoldEndoArgs (Dual e) = e

-- | This basically discards result of folding, in example:
--
-- >>> foldEndo ('n':) ('o':) :: Const () (Endo String)
-- Const ()
instance (Monoid c, FoldEndoArgs r) => FoldEndoArgs (Const c r) where
    type ResultOperatesOn (Const c r) = ResultOperatesOn r
    type Result (Const c r) = Const c (Result r)
    foldEndoArgs     _ = Const mempty
    dualFoldEndoArgs _ = Const mempty

instance FoldEndoArgs r => FoldEndoArgs (Either e r) where
    type ResultOperatesOn (Either e r) = ResultOperatesOn r
    type Result (Either e r) = Either e (Result r)
    foldEndoArgs     = Right . foldEndoArgs
    dualFoldEndoArgs = Right . dualFoldEndoArgs

instance FoldEndoArgs r => FoldEndoArgs (Identity r) where
    type ResultOperatesOn (Identity r) = ResultOperatesOn r
    type Result (Identity r) = Identity (Result r)
    foldEndoArgs     = Identity . foldEndoArgs
    dualFoldEndoArgs = Identity . dualFoldEndoArgs

-- | Allows endomorphism folding for endomorphisms wrapped inside 'IO' monad.
-- Examples:
--
-- @
-- 'foldEndo' \<*\> ((<>) \<$\> getLine) \<*\> ((<>) \<$\> getLine)
--     :: ('FoldEndoArgs' r, 'ResultOperatesOn' r ~ String) => IO r
-- @
--
-- In the next example, prefix @ghci\>@ indicates GHCi prompt, @ghci|@ is GHCi
-- continuation prompt, @\<\<\<@ indicates user input and @\>\>\>@ GHCi output.
-- Also, @:{@ and @:}@ is GHCi's way of starting and ending multiline mode,
-- respectively.
--
-- @
-- ghci\> :{
-- ghci| 'Control.Monad.Endo.runEndo' \"\" '<&$>' 'foldEndo'
-- ghci|     \<*\> ((++) \<$\> getLine)
-- ghci|     \<*\> ((++) \<$\> getLine)
-- ghci| :}
-- \<\<\< alpha
-- \<\<\< bet
-- \>\>\> \"alphabet\"
-- @
instance FoldEndoArgs r => FoldEndoArgs (IO r) where
    type ResultOperatesOn (IO r) = ResultOperatesOn r
    type Result (IO r) = IO (Result r)
    foldEndoArgs     = pure . foldEndoArgs
    dualFoldEndoArgs = pure . dualFoldEndoArgs

instance FoldEndoArgs r => FoldEndoArgs (Maybe r) where
    type ResultOperatesOn (Maybe r) = ResultOperatesOn r
    type Result (Maybe r) = Maybe (Result r)
    foldEndoArgs     = Just . foldEndoArgs
    dualFoldEndoArgs = Just . dualFoldEndoArgs

-- {{{ Transformers -----------------------------------------------------------

-- {{{ Functor Transformers ---------------------------------------------------

instance
    (Applicative f, Applicative g, FoldEndoArgs r)
    => FoldEndoArgs (Compose f g r)
  where
    type ResultOperatesOn (Compose f g r) = ResultOperatesOn r
    type Result (Compose f g r) = Compose f g (Result r)
    foldEndoArgs     = pure . foldEndoArgs
    dualFoldEndoArgs = pure . dualFoldEndoArgs

instance
    (Applicative f, Applicative g, FoldEndoArgs r)
    => FoldEndoArgs (Product f g r)
  where
    type ResultOperatesOn (Product f g r) = ResultOperatesOn r
    type Result (Product f g r) = Product f g (Result r)
    foldEndoArgs     = pure . foldEndoArgs
    dualFoldEndoArgs = pure . dualFoldEndoArgs

-- }}} Functor Transformers ---------------------------------------------------

-- {{{ Monad Transformers -----------------------------------------------------

-- | This instance can be used in cases when there is no 'FoldEndoArgs'
-- instance for a specific 'Applicative' functor. Example:
--
-- @
-- 'Control.Monad.Trans.Identity.runIdentityT' $ 'foldEndo'
--     \<*\> 'IdentityT' parseSomething
--     \<*\> 'IdentityT' parseSomethingElse
-- @
instance (Applicative f, FoldEndoArgs r) => FoldEndoArgs (IdentityT f r) where
    type ResultOperatesOn (IdentityT f r) = ResultOperatesOn r
    type Result (IdentityT f r) = IdentityT f (Result r)
    foldEndoArgs     = pure . foldEndoArgs
    dualFoldEndoArgs = pure . dualFoldEndoArgs

#ifdef HAVE_EXCEPTT
instance
    ( Monad m
#ifndef HAVE_APPLICATIVE_MONAD
    , Functor m
#endif
    , FoldEndoArgs r
    ) => FoldEndoArgs (ExceptT e m r)
  where
    type ResultOperatesOn (ExceptT e m r) = ResultOperatesOn r
    type Result (ExceptT e m r) = ExceptT e m (Result r)
    foldEndoArgs     = pure . foldEndoArgs
    dualFoldEndoArgs = pure . dualFoldEndoArgs
#endif

instance (Applicative f, FoldEndoArgs r) => FoldEndoArgs (ListT f r) where
    type ResultOperatesOn (ListT f r) = ResultOperatesOn r
    type Result (ListT f r) = ListT f (Result r)
    foldEndoArgs     = pure . foldEndoArgs
    dualFoldEndoArgs = pure . dualFoldEndoArgs

instance
    ( Monad m
#ifndef HAVE_APPLICATIVE_MONAD
    , Functor m
#endif
    , FoldEndoArgs r
    ) => FoldEndoArgs (MaybeT m r) where
    type ResultOperatesOn (MaybeT m r) = ResultOperatesOn r
    type Result (MaybeT m r) = MaybeT m (Result r)
    foldEndoArgs     = return . foldEndoArgs
    dualFoldEndoArgs = return . dualFoldEndoArgs

instance (Applicative f, FoldEndoArgs r) => FoldEndoArgs (ReaderT r' f r) where
    type ResultOperatesOn (ReaderT r' f r) = ResultOperatesOn r
    type Result (ReaderT r' f r) = ReaderT r' f (Result r)
    foldEndoArgs     = pure . foldEndoArgs
    dualFoldEndoArgs = pure . dualFoldEndoArgs

instance
    ( Monad m
#ifndef HAVE_APPLICATIVE_MONAD
    , Functor m
#endif
    , Monoid w
    , FoldEndoArgs r
    ) => FoldEndoArgs (RWST r' w s m r)
  where
    type ResultOperatesOn (RWST r' w s m r) = ResultOperatesOn r
    type Result (RWST r' w s m r) = RWST r' w s m (Result r)
    foldEndoArgs     = return . foldEndoArgs
    dualFoldEndoArgs = return . dualFoldEndoArgs

instance
    ( Monad m
#ifndef HAVE_APPLICATIVE_MONAD
    , Functor m
#endif
    , Monoid w
    , FoldEndoArgs r
    ) => FoldEndoArgs (Strict.RWST r' w s m r)
  where
    type ResultOperatesOn (Strict.RWST r' w s m r) = ResultOperatesOn r
    type Result (Strict.RWST r' w s m r) = Strict.RWST r' w s m (Result r)
    foldEndoArgs     = return . foldEndoArgs
    dualFoldEndoArgs = return . dualFoldEndoArgs

instance (Monad m, FoldEndoArgs r) => FoldEndoArgs (StateT s m r) where
    type ResultOperatesOn (StateT s m r) = ResultOperatesOn r
    type Result (StateT s m r) = StateT s m (Result r)
    foldEndoArgs     = return . foldEndoArgs
    dualFoldEndoArgs = return . dualFoldEndoArgs

instance (Monad m, FoldEndoArgs r) => FoldEndoArgs (Strict.StateT s m r) where
    type ResultOperatesOn (Strict.StateT s m r) = ResultOperatesOn r
    type Result (Strict.StateT s m r) = Strict.StateT s m (Result r)
    foldEndoArgs     = return . foldEndoArgs
    dualFoldEndoArgs = return . dualFoldEndoArgs

instance
    (Applicative f, FoldEndoArgs r, Monoid w) => FoldEndoArgs (WriterT w f r)
  where
    type ResultOperatesOn (WriterT w f r) = ResultOperatesOn r
    type Result (WriterT w f r) = WriterT w f (Result r)
    foldEndoArgs     = pure . foldEndoArgs
    dualFoldEndoArgs = pure . dualFoldEndoArgs

instance
    (Applicative f, FoldEndoArgs r, Monoid w)
    => FoldEndoArgs (Strict.WriterT w f r)
  where
    type ResultOperatesOn (Strict.WriterT w f r) = ResultOperatesOn r
    type Result (Strict.WriterT w f r) = Strict.WriterT w f (Result r)
    foldEndoArgs     = pure . foldEndoArgs
    dualFoldEndoArgs = pure . dualFoldEndoArgs

-- }}} Monad Transformers -----------------------------------------------------

-- }}} Transformers -----------------------------------------------------------

-- }}} FoldEndoArgs Type Class ------------------------------------------------

-- {{{ Utility Functions and Types --------------------------------------------

-- | Type alias that restricts type of endomorphism folding result, and it
-- looks similar to @->@. Example of creating version of 'foldEndo' with
-- specific result:
--
-- @
-- foldToEndoString :: 'FoldEndoArgs' args => args ':->' 'Endo' String
-- foldToEndoString = 'foldEndo'
-- @
--
-- >>> foldToEndoString ("foo" <>) ("bar" <>) `appEndo` "baz"
-- "foobarbaz"
--
-- Following type signatures for 'foldEndoArgs' are equivalent:
--
-- @
-- 'FoldEndoArgs' args => args ':->' 'Endo' String
-- ('FoldEndoArgs' args, 'Result' args ~ 'Endo' String) => args
-- @
type args :-> r = (Result args ~ r) => args

-- | Variant of function @('Data.Function.$') :: (a -> b) -> a -> b@, from
-- "Data.Function" module, but with fixity as
-- @('Data.Function.&') :: a -> (a -> b) -> b@ function from "Data.Function"
-- module (available in base since version 4.8.0.0).
(&$) :: (a -> b) -> a -> b
f &$ a = f a
infixl 1 &$

-- | Variant of function
-- @('Data.Functor.<$>') :: 'Data.Functor.Functor' f => (a -> b) -> a -> b@
-- from "Data.Functor" module, but with fixity as '&$' function.
(<&$>) :: Functor f => (a -> b) -> f a -> f b
(<&$>) = fmap
infixl 1 <&$>

-- }}} Utility Functions and Types --------------------------------------------

-- $basicIdea
--
-- Lets define simple application @Config@ data type as:
--
-- @
-- data Verbosity = Silent | Normal | Verbose | Annoying
--   deriving (Show)
--
-- data Config = Config
--     { _verbosity :: Verbosity
--     , _outputFile :: FilePath
--     }
--   deriving (Show)
-- @
--
-- Now lets define setters for @_verbosity@ and @_outputFile@:
--
-- @
-- setVerbosity :: Verbosity -> 'Data.Monoid.Endo.E' Config
-- setVerbosity b cfg = cfg{_verbosity = b}
--
-- setOutputFile :: FilePath -> 'Data.Monoid.Endo.E' Config
-- setOutputFile b cfg = cfg{_outputFile = b}
-- @
--
-- Note that 'Data.Monoid.Endo.E' is defined in "Data.Monoid.Endo" module and
-- it looks like:
--
-- @
-- type 'Data.Monoid.Endo.E' a = a -> a
-- @
--
-- Its purpose is to simplify type signatures.
--
-- Now lets get to our first example:
--
-- @
-- example1 :: 'Data.Monoid.Endo.E' Config
-- example1 = 'Data.Monoid.appEndo' '$' 'foldEndo'
--     '&$' setVerbosity Annoying
--     '&$' setOutputFile \"an.out.put\"
-- @
--
-- Above example shows us that it is possible to modify @Config@ as if it was a
-- monoid, but without actually having to state it as such. In practice it is
-- not always possible to define it as 'Monoid', or at least as a @Semigroup@.
-- Endomorphism are monoids under composition, therefore they are what usually
-- works in situations when the modified data type can not be instantiated as a
-- monoid.

-- $workingWithCornerCases
--
-- In real applications corner cases arise quite easily, e.g.
-- 'System.IO.FilePath' has one pathological case, and that is \"\". There
-- is a lot of ways to handle it. Here we will concentrate only few basic
-- techniques to illustrate versatility of our approach.
--
-- @
-- -- | Trying to set output file to \"\" will result in keeping original
-- -- value.
-- setOutputFile2 :: FilePath -> 'Data.Monoid.Endo.E' Config
-- setOutputFile2 \"\" = id
-- setOutputFile2 fp = setOutputFile fp
--
-- example2 :: 'Data.Monoid.Endo.E' Config
-- example2 = 'Data.Monoid.appEndo' $ 'foldEndo'
--     '&$' setVerbosity Annoying
--     '&$' setOutputFile2 \"an.out.put\"
-- @
--
-- Same as above, but exploits @instance 'AnEndo' a => 'AnEndo' 'Maybe' a@:
--
-- @
-- setOutputFile3 :: FilePath -> Maybe ('Data.Monoid.Endo.E' Config)
-- setOutputFile3 "" = Nothing
-- setOutputFile3 fp = Just $ setOutputFile fp
--
-- example3 :: 'Data.Monoid.Endo.E' Config
-- example3 = 'Data.Monoid.appEndo' $ 'foldEndo'
--     '&$' setVerbosity Annoying
--     '&$' setOutputFile3 \"an.out.put\"
-- @
--
-- Great thing about 'Maybe' is the fact that it has
-- 'Control.Applicative.Alternative' and 'Control.Monad.MonadPlus' instances.
-- Using 'Control.Monad.guard' may simplify @setOutputFile3@ in to definition
-- like following:
--
-- @
-- setOutputFile3':: FilePath -> Maybe ('Data.Monoid.Endo.E' Config)
-- setOutputFile3' fp = setOutputFile fp 'Data.Functor.<$' 'Control.Monad.guard' (not (null fp))
-- @
--
-- Following example uses common pattern of using 'Either' as error reporting
-- monad. This approach can be easily modified for arbitrary error reporting
-- monad.
--
-- @
-- setOutputFile4 :: FilePath -> Either String ('Data.Monoid.Endo.E' Config)
-- setOutputFile4 "" = Left \"Output file: Empty file path.\"
-- setOutputFile4 fp = Right $ setOutputFile fp
--
-- example4 :: Either String ('Data.Monoid.Endo.E' Config)
-- example4 = 'Data.Monoid.appEndo' '<&$>' 'foldEndo'
--     'Control.Applicative.<*>' 'pure' (setVerbosity Annoying)
--     'Control.Applicative.<*>' setOutputFile4 \"an.out.put\"
-- @
--
-- Notice, that above example uses applicative style. Normally, when using this
-- style for setting record values, one needs to keep in sync the order of
-- constructor arguments, and order of operations. Using 'foldEndo' (and its
-- dual 'dualFoldEndo') doesn't have this restriction.

-- $lenses
--
-- Instead of setter functions one may want to use lenses. In this example we
-- use types from <http://hackage.haskell.org/package/lens lens package>, but
-- definitions use function from
-- <http://hackage.haskell.org/package/between between package>:
--
-- @
-- verbosity :: Lens' Config Verbosity
-- verbosity = _verbosity 'Data.Function.Between.~@@^>' \\s b -> s{_verbosity = b}
--
-- outputFile :: Lens' Config FilePath
-- outputFile = _outputFile 'Data.Function.Between.~@@^>' \\s b -> s{_outputFile = b}
-- @
--
-- Now setting values of @Config@ would look like:
--
-- @
-- example5 :: 'Data.Monoid.Endo.E' Config
-- example5 = 'Data.Monoid.appEndo' $ 'foldEndo'
--     '&$' verbosity  .~ Annoying
--     '&$' outputFile .~ \"an.out.put\"
-- @

-- $otherUsage
--
-- Probably one of the most interesting things that can be done with this
-- module is following:
--
-- @
-- instance 'AnEndo' Verbosity where
--     type 'EndoOperatesOn' Verbosity = Config
--     'anEndo' = Endo . set verbosity
--
-- newtype OutputFile = OutputFile FilePath
--
-- instance 'AnEndo' OutputFile where
--     type 'EndoOperatesOn' OutputFile = Config
--     'anEndo' (OutputFile fp) = 'Endo' $ outputFile .~ fp
--
-- example6 :: 'Data.Monoid.Endo.E' Config
-- example6 = 'Data.Monoid.appEndo' $ 'foldEndo'
--     '&$' Annoying
--     '&$' OutputFile \"an.out.put\"
-- @

-- $optparseApplicativeExample
--
-- This is a more complex example that defines parser for
-- <http://hackage.haskell.org/package/optparse-applicative optparse-applicative>
-- built on top of some of the above definitions:
--
-- @
-- options :: Parser Config
-- options = 'Control.Monad.Trans.Identity.runIdentityT' $ 'Control.Monad.Endo.runEndo' defaultConfig \<$\> options'
--   where
--     -- All this IdentityT clutter is here to avoid orphan instances.
--     options' :: 'IdentityT' Parser ('Endo' Config)
--     options' = 'foldEndo'
--         \<*\> outputOption     -- :: IdentityT Parser (Maybe (E Config))
--         \<*\> verbosityOption  -- :: IdentityT Parser (Maybe (E Config))
--         \<*\> annoyingFlag     -- :: IdentityT Parser (E Config)
--         \<*\> silentFlag       -- :: IdentityT Parser (E Config)
--         \<*\> verboseFlag      -- :: IdentityT Parser (E Config)
--
--     defaultConfig :: Config
--     defaultConfig = Config Normal \"\"
--
-- main :: IO ()
-- main = execParser (info options fullDesc) \>\>= print
-- @
--
-- Example of running above @main@ function:
--
-- >>> :main -o an.out.put --annoying
-- Config {_verbosity = Annoying, _outputFile = "an.out.put"}
--
-- Parsers for individual options and flags are wrapped in 'IdentityT', because
-- there is no following instance:
--
-- @
-- instance 'FoldEndoArgs' r => 'FoldEndoArgs' (Parser r)
-- @
--
-- But there is:
--
-- @
-- instance ('Applicative' f, 'FoldEndoArgs' r) => 'FoldEndoArgs' ('IdentityT' f r)
-- @
--
-- Functions used by the above code example:
--
-- @
-- outputOption :: 'IdentityT' Parser (Maybe ('Data.Monoid.Endo.E' Config))
-- outputOption =
--     IdentityT . optional . option (set outputFile \<$\> parseFilePath)
--     $ short \'o\' \<\> long \"output\" \<\> metavar \"FILE\"
--         \<\> help \"Store output in to a FILE.\"
--   where
--     parseFilePath = eitherReader $ \\s ->
--         if null s
--             then Left \"Option argument can not be empty file path.\"
--             else Right s
--
-- verbosityOption :: 'IdentityT' Parser (Maybe ('Data.Monoid.Endo.E' Config))
-- verbosityOption =
--     'IdentityT' . optional . option (set verbosity \<$\> parseVerbosity)
--     $ long \"verbosity\" \<\> metavar \"LEVEL\" \<\> help \"Set verbosity to LEVEL.\"
--   where
--     verbosityToStr = map toLower . Data.showConstr . Data.toConstr
--     verbosityIntValues = [(show $ fromEnum v, v) | v <- [Silent .. Annoying]]
--     verbosityStrValues =
--         ("default", Normal) : [(verbosityToStr v, v) | v <- [Silent .. Annoying]]
--
--     parseVerbosityError = unwords
--         [ "Verbosity can be only number from interval"
--         , show $ map fromEnum [minBound, maxBound :: Verbosity]
--         , "or one of the following:"
--         , concat . intersperse ", " $ map fst verbosityStrValues
--         ]
--
--     parseVerbosity = eitherReader $ \s ->
--         case lookup s $ verbosityIntValues ++ verbosityStrValues of
--             Just v  -> Right v
--             Nothing -> Left parseVerbosityError
--
-- annoyingFlag :: 'IdentityT' Parser ('Data.Monoid.Endo.E' Config)
-- annoyingFlag = 'IdentityT' . flag id (verbosity .~ Annoying)
--     $ long \"annoying\" \<\> help \"Set verbosity to maximum.\"
--
-- silentFlag :: 'IdentityT' Parser ('Data.Monoid.Endo.E' Config)
-- silentFlag = 'IdentityT' . flag id (verbosity .~ Silent)
--     $ short 's' \<\> long "silent" \<\> help \"Set verbosity to minimum.\"
--
-- verboseFlag :: 'IdentityT' Parser ('Data.Monoid.Endo.E' Config)
-- verboseFlag = 'IdentityT' . flag id (verbosity .~ Verbose)
--     $ short 'v' \<\> long \"verbose\" \<\> help \"Be verbose.\"
-- @