[Haskell-cafe] Do people use visitor pattern in Haskell...
nicholls.mark at vimn.com
Fri Jun 19 15:48:46 UTC 2015
My initial guess would be no...its built into the language
I find myself tempted to use it (in order to match on a type that's the instance of a typeclass)...which makes me think I've missed something.
> data Foo = Foo
> data Bar = Bar
Ooo...this looks like an OO design pattern...surely wrong?
> class Wibble a where
> visit :: (Foo -> b) -> (Bar -> b) -> a -> b
> instance Wibble Foo where
> visit f _ x = f x
> instance Wibble Bar where
> visit _ f x = f x
I want a list of Wibbles....
(Wibble a) => [a] is clearly wrong...
I want [(Wibble a) =>a]
so I package it up?
> data WibblePackage where
> WibblePackage :: (Wibble a) => a -> WibblePackage
lets try this now...so pointless function across Wibbles in a list.
> fizzBuzz :: [WibblePackage] -> Integer
> fizzBuzz  = 0
> fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)
> help :: Integer
> help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]
OO nerds will get uptight about "closing" the Wibble typeclass....but I've managed to create a list of different types and have a mechanism for recovering the type.
How would a Haskell nerd do this? (I have looked, but they look more complicated....maybe to my OO eye).
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