Questions regarding Bounded

[Sven Panne]

Am Mi., 6. Mai 2020 um 21:13 Uhr schrieb David Feuer <david.feuer at gmail.com
>:

> The Bounded class is broken because it combines two distinct concepts:
> BoundedAbove and BoundedBelow.


Indeed, having that distinction would be useful. Just to add some
bikeshedding regarding the names:

   * HasInfimum / HasSupremum or
   * HasInitialObject / HasTerminalObject  or shorter
   * HasInitial / HasTerminal (maybe a bit too short)

I wouldn't be too surprised if Edward K. already has something like this on
Hackage. :-)


> If Maybe is to have instances similar
> to what it has now, they should surely look like
>
> instance BoundedBelow (Maybe a) where
>   minBound = Nothing
> instance BoundedAbove a => BoundedAbove (Maybe a) where
>   maxBound = Just maxBound
>

Well, choosing Nothing to be the infimum is rather arbitrary, you could as
well define:

instance BoundedBelow a => BoundedBelow (Maybe a) where
   minBound = Just minBound
instance BoundedAbove (Maybe a) where
   maxBound = Nothing

But the previous definition is probably a better fit given the Ord
instance, nevertheless. the choice *is* arbitrary. Perhaps Maybe should not
be an instance of either class?
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