[Haskell-beginners] Comments on Map/Reduce Code
byorgey at seas.upenn.edu
Tue Jul 17 15:28:02 CEST 2012
On Tue, Jul 17, 2012 at 03:01:39PM +0200, Thomas Bach wrote:
> On Sat, Jul 14, 2012 at 08:35:44PM -0400, Brent Yorgey wrote:
> > On Fri, Jul 13, 2012 at 06:02:02PM +0200, Thomas Bach wrote:
> > > The reducer still throws an error when piping in an empty
> > > newline. But, I'm not sure, what a proper solution for this could be.
> > The problem is your 'summation' function:
> > summation :: Num b => [(a, b)] -> (a, b)
> > In fact, it is impossible to implement something with this type which
> > works for all inputs. If you get the empty list as input, there is no
> > way to make up a value of type 'a' in the output tuple.
> It really is not that big of a deal. As I'd assume that Hadoop
> guarantees that at least one line will be passed to the reducer. But,
> just out of curiosity: wouldn't this be a case where monads are
> applied? Say, `Maybe'? So that the type becomes
> summation :: Num b => [(a, b)] -> Maybe (a, b)
Yes, wrapping the return type in Maybe could be a good idea indeed.
You say "wouldn't this be a case where monads are applied", but you
are jumping too far ahead: using Maybe in this way is a good idea no
matter whether you happen to know that Maybe is an instance of Monad
or not. Likewise, you could work with a function of this type without
using the Monad interface at all. Using the Monad interface might
simplify some of the code but it is by no means required.
More information about the Beginners