I tried to write a generalized maximum function similar to the one in Prelude. My first naiv approach looked like this:
maximum' :: (F.Foldable a, Ord b) => a b -> Maybe b
maximum' mempty = Nothing
maximum' xs = Just $ F.foldl1 max xs
However, when I test it it always returns Nothing regardless of the input:
> maximum' [1,2,3]
> Nothing
Now I wonder whether it's possible to obtain the empty value of a Monoid type instance. A test function I wrote works correctly:
getMempty :: (Monoid a) => a -> a
getMempty _ = mempty
> getMempty [1,2,3]
> []
I had already a look at these two questions but I didn't figure out how the answers solve my problem:
Write a Maximum Monoid using Maybe in Haskell
Haskell Pattern Matching on the Empty Set
How would I rewrite the maximum' function to get it to work ?