I recently published oset-0.1.1.0 in order to get around some (perceived, by me) shortcomings of ordered-containers. Mainly I was frustrated by the lack of Semigroup and Monoid instances.

In retrospect, I think I should have thought some more before ploughing ahead and implementing them the way I did, mainly because there are ( think) at least two valid, and obvious, possible instances of these type classes for an ordered set. Specifically:

• Left-preserving, or left-biased, append: in which duplicate elements on the right-hand side of the append are discarded
• Right-preserving, or right-biased, append: in which duplicates elements on the left-hand side of the append are discarded

Therefore, I’m planning to remove the existing Semigroup and Monoid instances and replace them with newtype wrappers providing the desired behaviours. Here’s a sketch of what I intend to do:

leftPreservingAppend and rightPreservingAppend are not the real implementations but are provided for illustrative purposes only.

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