<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div>data WibblePackage = WPFoo Foo | WPBar Bar<br><br>Отправлено с iPhone</div><div><br>19 июня 2015 г., в 17:48, Nicholls, Mark <<a href="mailto:nicholls.mark@vimn.com">nicholls.mark@vimn.com</a>> написал(а):<br><br></div><blockquote type="cite"><div>
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<p class="MsoNormal">My initial guess would be no…its built into the language<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">But….<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">So…<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> data Foo = Foo<o:p></o:p></p>
<p class="MsoNormal">> data Bar = Bar<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Ooo…this looks like an OO design pattern…surely wrong?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> class Wibble a where<o:p></o:p></p>
<p class="MsoNormal">> visit :: (Foo -> b) -> (Bar -> b) -> a -> b<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> instance Wibble Foo where<o:p></o:p></p>
<p class="MsoNormal">> visit f _ x = f x<o:p></o:p></p>
<p class="MsoNormal">> instance Wibble Bar where<o:p></o:p></p>
<p class="MsoNormal">> visit _ f x = f x<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I want a list of Wibbles....<o:p></o:p></p>
<p class="MsoNormal">hmmm...<o:p></o:p></p>
<p class="MsoNormal">(Wibble a) => [a] is clearly wrong...<o:p></o:p></p>
<p class="MsoNormal">I want [(Wibble a) =>a]<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">so I package it up?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> data WibblePackage where<o:p></o:p></p>
<p class="MsoNormal">> WibblePackage :: (Wibble a) => a -> WibblePackage<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">lets try this now…so pointless function across Wibbles in a list.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> fizzBuzz :: [WibblePackage] -> Integer<o:p></o:p></p>
<p class="MsoNormal">> fizzBuzz [] = 0<o:p></o:p></p>
<p class="MsoNormal">> fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> help :: Integer<o:p></o:p></p>
<p class="MsoNormal">> help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">That works!<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">How would a Haskell nerd do this? (I have looked, but they look more complicated….maybe to my OO eye).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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