Re: [Haskell-cafe] Applicative but not Monad

What type is your "return"? On 30 Oct 2009, at 21:48, Dan Weston wrote:

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 mailto:ekirpichov@gmail.com> wrote: Yes. ZipList. http://en.wikibooks.org/wiki/Haskell/Applicative_Functors 2009/10/30 Yusaku Hashimoto mailto:nonowarn@gmail.com>: > Hello cafe, > Do you know any data-type which is Applicative but not Monad? > > Cheers, > -~nwn > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org mailto:Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Eugene Kirpichov Web IR developer, market.yandex.ru http://market.yandex.ru _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org mailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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