Re: [Haskell-cafe] Applicative but not Monad

On Fri, Oct 30, 2009 at 5:59 PM, Luke Palmer wrote:

On Fri, Oct 30, 2009 at 10:39 AM, 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)

IIRC, that doesn't satisfy the associativity law, particularly when you are joining a list of lists of different lengths. 2 minutes of experimenting failed to find me the counterexample though.

Cool, thanks Luke, that explains why this is available in Stream, but not in ZipList too. Bob