Ashley Yakeley wrote: > Wolfgang Jeltsch wrote: > > > This brings me to another point. One year ago we had a discussion on The > > Haskell Mailing List concerning arrows. (The subject of the mails was just > > "arrows".) The point was that it seemed strange to me that first and second > > are included in the basic arrow class Arrow while left and right have their > > extra class ArrowChoice. Not only that it seemed strange to me but it made > > impossible to make Parser baseMonad an instance of Arrow. Parser baseMonad > > has nice implementations of pure and (>>>) but none of first or second. > > I agree. My own Arrow module hierarchy looks more or less like this: > > class Compositor comp where [...] > class (Compositor arrow) => Arrow arrow where [...] > class (Arrow arrow) => ProductArrow arrow where [...] > class (Arrow arrow) => CoproductArrow arrow where [...] > class (ProductArrow arrow,CoproductArrow arrow) => FullArrow arrow > instance (ProductArrow arrow,CoproductArrow arrow) => FullArrow arrow > class (Arrow arrow) => ArrowFix arrow where [...] > class (FullArrow arrow) => ApplyArrow arrow where [...]
On that topic, see below for what mine looks like (from HXML, <URL: http://www.flightlab.com/~joe/hxml/ >). I started off with Hughes' conventions, but for some reason could never remember the difference between &&& and ***, or between ||| and +++. I found &&&, >&<, |||, >|< to have better mnemonic value. This also frees up +++ for ArrowPlus, which -- in HXML applications -- is frequently used and should thus be easy to type. When using the ArrowChoice operators, I kept tripping over all the 'Either' coproduct types, so added some syntactic sugar (borrowed from HaXML): data Choice a = a :> a class (Arrow a) => ArrowChoice a where [ ... ] ( ?>) :: (b -> Bool) -> Choice (a b c) -> a b c (>?>) :: a b Bool -> Choice (a b c) -> a b c I found "p ?> f :> g" much more pleasant to use. (I also like the idea of splitting the product operators out of the base Arrow class -- will consider doing that in my library). -- infixr 5 +++ infixr 3 >&<, &&& infixr 2 >|<, |||, ?>, >?>, :> infixl 1 >>> class Arrow a where arr :: (b -> c) -> a b c (>>>) :: a b c -> a c d -> a b d apfst :: a b c -> a (b,x) (c,x) apsnd :: a b c -> a (x,b) (x,c) (>&<) :: a b c -> a d e -> a (b,d) (c,e) (&&&) :: a b c -> a b d -> a b (c,d) liftA2 :: (b -> c -> d) -> a e b -> a e c -> a e d aConst :: c -> a b c idArrow :: a b b -- Minimal implementation: arr, >>>, apfst or >&< data Choice a = a :> a class (Arrow a) => ArrowChoice a where apl :: a b c -> a (Either b d) (Either c d) apr :: a b c -> a (Either d b) (Either d c) (>|<) :: a b c -> a d e -> a (Either b d) (Either c e) (|||) :: a b c -> a d c -> a (Either b d) c ( ?>) :: (b -> Bool) -> Choice (a b c) -> a b c (>?>) :: a b Bool -> Choice (a b c) -> a b c -- Minimal implementation: >|< or apl class (Arrow a) => ArrowApply a where app :: a (a b c,b) c class (Arrow a) => ArrowZero a where aZero :: a b c aMaybe :: a (Maybe c) c aGuard :: (b -> Bool) -> a b b class (Arrow a) => ArrowPlus a where (+++) :: a b c -> a b c -> a b c --Joe English [EMAIL PROTECTED] _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
