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

[Maxime Henrion]

Chris Kuklewicz wrote:
> 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

Thanks a lot for all the nice answers, guys.

I have a few remaining questions if you don't mind though.  Should
I expect significant performance reduction by using the Writer monad
here, as opposed to the version I wrote?  And, most importantly,
I'd like to know how *you* would write this if you had to :-).
Would you juse the Writer monad version?

Thanks,
Maxime