begin Brent Yorgey quotation: > On Sun, May 30, 2010 at 11:15:40AM -0700, Mike Dillon wrote: > > begin Michael Snoyman quotation: > > > http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/Control-Monad.html#v%3AliftM2 > > > > > > <file:///usr/share/doc/ghc6-doc/html/libraries/base-4.2.0.0/Control-Monad.html#v%3AliftM2>Strangely, > > > Hayoo didn't turn this one up... anyone know why? > > > > Hoogle finds it. I didn't think Hayoo was expected to do this sort of > > abstract type signature search: > > > > > > http://haskell.org/hoogle/?hoogle=Monad+m+%3D%3E+%28a+-%3E+a+-%3E+a%29+-%3E+m+a+-%3E+m+a+-%3E+m+a > > > > It comes up as the second hit on that search or the first hit on this > > one: > > > > > > http://haskell.org/hoogle/?hoogle=Monad+m+%3D%3E+%28a+-%3E+b+-%3E+c%29+-%3E+m+a+-%3E+m+b+-%3E+m+c > > > > That second search also shows zipWith in there; I never really thought > > about zipWith being like liftM2 for the list Monad. I don't believe > > that's actually true for the normal list Monad, but it should be true of > > an alternate list Monad along the lines of the Functor and Applicative > > instances for the ZipList newtype in Control.Applicative. > > As Max noted, ZipList is not a monad. However, you have the right > idea: zipWith is exactly liftA2 (the equivalent of liftM2 for > Applicatives) for ZipList.
Thanks to both you and Max. After you guys responded, I went back and found an interesting haskell-cafe discussion about how ZipList cannot be made into a Monad from August 2009. I should have realized that there was a reason there is no Monad instance defined along with ZipList. -md _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
