[Haskell-cafe] A convenient way to deal with conditional function composition?

[Chris Kuklewicz]

Nicolas Frisby wrote:
>> Not portably.
>>
>> stefan at stefans:~$ ghc-6.4.2 -e '(  ("foo"++) `Data.Monoid.mappend`
>> ("bar"++) ) "END"'
>> "foobarEND"
>> stefan at stefans:~$ ghc-6.6 -e '(  ("foo"++) `Data.Monoid.mappend`
>> ("bar"++) ) "END"'
>> "fooENDbarEND"
>>
>>
>> -- 6.6 sources
>> instance Monoid b => Monoid (a -> b) where
>>         mempty _ = mempty
>>         mappend f g x = f x `mappend` g x
>>
>>
>> Stefan

Thanks for the reminder.  So the fixed 6.6 code is

> import Control.Monad(when)
> import Control.Monad.Writer(Writer,tell,execWriter)
> import Data.Monoid(Endo(..))
> 
> type Writes = Writer (Endo String) ()
> 
> data PieceType = Pawn | Other deriving (Eq,Show)
> type File = Int
> type Square = Int
> 
> data Move = Move {
>                  movePiece     :: PieceType,
>                  moveFile      :: Maybe File,
>                  moveTarget    :: Square,
>                  moveIsCapture :: Bool
>                  --movePromotion :: Maybe PieceType
>                }
>   deriving (Eq)
> 
> instance Show Move where showsPrec = showsPrec_Move
> 
> tShow :: Show a => a -> Writes
> tShow = tell . Endo . shows
> 
> tChar :: Char -> Writes
> tChar = tell . Endo . (:)
> 
> tString :: String -> Writes
> tString = tell . Endo . (++)
> 
> showsPrec_Move :: Int -> Move -> ShowS
> showsPrec_Move _ Move { movePiece     = p
>                       , moveFile      = f
>                       , moveTarget    = s
>                       , moveIsCapture = c } = appEndo . execWriter $ do
>   when (p/=Pawn) (tShow p)
>   maybe (return ()) tShow f
>   when c (tChar 'x')
>   tShow s
> 
> testMove = Move Other (Just 6) 10 True