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

10 Apr 2007


      Nicolas Frisby wrote:
...
...
Not portably.
stefan@stefans:~$ ghc-6.4.2 -e '(  ("foo"++) `Data.Monoid.mappend`
("bar"++) ) "END"'
"foobarEND"
stefan@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

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

Chris Kuklewicz