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
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