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

[Christopher Allen]

This is a pattern people usually refer to as the "existential typeclass
antipattern".

An article goes into more depth here:
https://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-typeclass/

Sometimes you need a GADT with an existential type and a typeclass, but it
doesn't occur as often as the equivalent does in OO.

On Fri, Jun 19, 2015 at 10:48 AM, Nicholls, Mark <nicholls.mark at vimn.com>
wrote:

>  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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-- 
Chris Allen
Currently working on http://haskellbook.com
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