Re: [Haskell-cafe] Do people use visitor pattern in Haskell...

data WibblePackage = WPFoo Foo | WPBar Bar Отправлено с iPhone

19 июня 2015 г., в 17:48, Nicholls, Mark написал(а):

My initial guess would be no…its built into the language

But….

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.

So…

...

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.... hmmm... (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]

That works!

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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