[Haskell-cafe] name of this monadic combinator?

[Brent Yorgey]

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.

-Brent