Not having a thing is not the same as having an empty maybe-thing
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You aren't talking about the phone number that you may or may not have (the thing/non-thing). You're talking about its meta-state. Which is to say, you've encoded Maybe.
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The premise was 'having' and questions involving 'having/not' are fundamental, not meta, and more primitive than the composite conveyance of 'answer' or 'don't know'. Does the license 'have' a maybe phone number, maybe cat picture etc, in the same way it actually 'has' my d.o.b?
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Sure, but neither of those questions are asking for the phone number itself. The first question is a pure function that returns a boolean. The second is a pure function that returns some data structure like a set.
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It sounds like the issue is this: Maybe can only represent two cases, when in fact there are three. Is there a phone # on my license? 1. No, the field does not exist 2. The field exists, but it's blank 3. Yes
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A pair of a boolean indicating presence/absence and a value which is absent when boolean is false is trivially isomorphic to Maybe. Only the latter is much more convenient and the former can be much more efficient.
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