That is just because you are calling min and max against the Maybe rather than on the values inside of your maybes. Max is working because there is an instance of Ord for Maybe and Nothing > Just n > Just ( n + 1 ) This is certainly not the most elegant solution ( I am a beginner, too ) but here is what I would do: instance Monoid Stats where  mempty  = Stats 0 Nothing Nothing 0  mappend (Stats sm1 mn1 mx1 len1) (Stats sm2 mn2 mx2 len2) =    Stats    (sm1 + sm2)    (chooseMaybe min mn1 mn2)    (chooseMaybe max mx1 mx2)    (len1 + len2) chooseMaybe _ Nothing Nothing   = Nothing chooseMaybe _ (Just a) Nothing  = Just a chooseMaybe _ Nothing  (Just b) = Just b chooseMaybe f (Just a) (Just b) = Just $ f a b Hopefully this quick answer can get you on your way to solving your problem and we can both learn a better way of doing it when someone optimises my solution. ;) On Mon, Dec 12, 2011 at 7:37 AM, goodman.m.w@gmail.com wrote:

Thanks for the reply, Brent!

I have a question about the "later" I referred to:

On Sun, Dec 11, 2011 at 7:19 AM, Brent Yorgey wrote:

...

On Sat, Dec 10, 2011 at 07:21:58PM -0800, goodman.m.w@gmail.com wrote:

...

... later I will add support for other statistics, like max, min, sum, prod, etc.

Specifically, how max and min work with the Monoid implementation.

...

Let's add a Monoid instance for Stats, which specifies how two Stats objects should be combined:

 instance Monoid Stats where    mempty = Stats 0 0    mappend (Stats sm1 len1) (Stats sm2 len2) = Stats (sm1 + sm2) (len1 + len2)

Monoids are new to me, but I just read a bit about them in Real World Haskell, and what you have makes sense. mempty is the identity function of Stats, and mappend is the associative binary operator. And thus far these properties hold true for the definition of Stats. If I add entries for min and max, it seems like it could still be valid. Of course, for an empty list of numbers, whereas sum can be 0.0 and len can be 0, there is no corresponding value for max and min, so I make them Maybe Double with an initial value of Nothing, and modify your code as follows:

data Stats = Stats { sm :: Double, mn, mx :: Maybe Double, len :: Int }             deriving Show

instance Monoid Stats where  mempty = Stats 0.0 Nothing Nothing 0  mappend (Stats sm1 mn1 mx1 len1) (Stats sm2 mn2 mx2 len2)         = Stats (sm1 + sm2) (min mn1 mn2) (max mx1 mx2) (len1 + len2)

mkStats x = Stats x (Just x) (Just x) 1

This compiles and works for max, but min does not work. I tested in ghci, and:

Prelude> min (Just 0.1) (Nothing) Nothing Prelude> max (Just 0.1) (Nothing) Just 0.1

Which explains the problem. Any idea for the solution?

-- -Michael Wayne Goodman

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