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Can't find implementation of Ord a #5

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@williamdemeo

I am new to Idris, so forgive me if this a dumb question.

When doing Exercise 5 on page 101 of the TypeDD book, my solution (which is the same as the suggested solution in this repository) exhibits the following behavior when I launch idris ex_4_1.idr:

Type checking ./ex_4_1.idr
*ex_4_1> maxMaybe Nothing Nothing
Can't find implementation for Ord a

According to the definition of maxMaybe (included below for reference), it seems the answer should be Nothing. Is this an error? Do I not have Idris set up correctly?

maxMaybe : Ord a => Maybe a -> Maybe a -> Maybe a
maxMaybe Nothing Nothing = Nothing
maxMaybe Nothing (Just y) = Just y
maxMaybe (Just x) Nothing = Just x
maxMaybe (Just x) (Just y) = Just (max x y)

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