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

[Nicholls, Mark]

The design pattern is very well known in the OO community…most of the use cases evaporate in Haskell….in some sense its part of the language…

Haskell is my 9th or 10th language, I can barely write “hello world” without the manual, so I’m still at times I’m still trying to discover the basic idioms…

How to put heterogenuos things in a list…and yet not loose all type information (I know about HList…but I think that’s for special occasions)


From: Haskell-Cafe [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Anupam Jain
Sent: 19 June 2015 5:04 PM
To: haskell-cafe Cafe
Subject: Re: [Haskell-cafe] Do people use visitor pattern in Haskell...

I discovered the same general pattern foroop

On Friday, June 19, 2015, Nicholls, Mark <nicholls.mark at vimn.com<mailto: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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