Btw, just to give some context:
The reason we need sentinels at all is because our APIs don't like to wrap
the happy case.
As an example:
So in Python, when you do {'a': 'foo'}.get('a') you get 'foo' directly.
That's convenient.
In eg Haskell the equivalent operation gives you a wrapped 'foo'.
ghci> :m + Data.Map
ghci> Data.Map.lookup "a" (Data.Map.fromList [("a", "foo")])
Just "foo"
The benefit is that you can tell these two cases apart:
ghci> Data.Map.lookup "b" (Data.Map.fromList [("c", Nothing)])
Nothing
ghci> Data.Map.lookup "c" (Data.Map.fromList [("c", Nothing)])
Just Nothing
The drawback is that you have to unwrap your happy case (you have to remove
the `Just`.)
Many Python functions side-step the need for a sentinel by using
exceptions: the result of {'some-key': None}['some-key'] is unambigous, but
needs exception handling.
Using common sentinels would be totally besides the point: what if you want
to stick these common sentinels in eg a dict (or any other structure whose
operations sometimes use sentinels)?
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