Can you elaborate on why Const is not a monad?
return x = Const x
fmap f (Const x) = Const (f x)
join (Const (Const x)) = Const x
What am I missing?
Tom Davie wrote:
Of note, there is a sensible monad instance for zip lists which I
*think* agrees with the Applicative one, I don't know why they're not
monads:
instance Monad (ZipList a) where
return = Ziplist . return
join (ZipList []) = ZipList []
join (ZipList (a:as)) = zlHead a `zlCons` join (map zlTail as)
I'll provide an alternative though, Const a is an applicative, but not a
monad.
Bob
On Fri, Oct 30, 2009 at 5:25 PM, Eugene Kirpichov <[email protected]
<mailto:[email protected]>> wrote:
Yes. ZipList.
http://en.wikibooks.org/wiki/Haskell/Applicative_Functors
2009/10/30 Yusaku Hashimoto <[email protected]
<mailto:[email protected]>>:
> Hello cafe,
> Do you know any data-type which is Applicative but not Monad?
>
> Cheers,
> -~nwn
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--
Eugene Kirpichov
Web IR developer, market.yandex.ru <http://market.yandex.ru>
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