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

[Christopher Allen]

Sorry for adding to the dogpile, should've guessed others would point to
the same resource :)

On Fri, Jun 19, 2015 at 11:11 AM, Christopher Allen <cma at bitemyapp.com>
wrote:

> 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
>



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