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*From*: Leslie Lamport <tlaplus.ll@xxxxxxxxx>*Date*: Fri, 29 Jul 2022 18:13:10 -0700 (PDT)*References*: <c6213ec6-1261-4cc9-b646-9cda6c8d81d3n@googlegroups.com>

The mathematical way to define None to be a value that is not a natural number is:

None == CHOOSE n : n \notin Nat

I believe that if you then create a TLA model for the spec, that model will substitute for None what is called a model value that TLC considers to be unequal to any value other than None.

Leslie

On Friday, July 29, 2022 at 5:37:19 PM UTC-7 brandon...@xxxxxxxxx wrote:

Some languages have optional data types (Scala's Option, Java's Optional, Haskell's Maybe, etc), the purpose of which is to obviate the need for null values. As a union type, it can be expressed like:Option a = Some a | None(so an Option of a type a is either a Some of a or it is None.I was trying to think of a good way to do this in an untyped, set-theoretic setting, while trying to avoid accidental equivalence with other values, and came up with something a bit convoluted and arbitrary:None == {{"a", {"fairly unique", {"id:", {"69a86178-6aba-445b-a74b-6769ff09cc29"}}}}}

some(x) == [some: x]

NatOption[x \in Nat] == [some: x] \union {None}This seemed to cause errors in my relatively small, nascent spec once I ran TLC. Unfortunately, I didn't seem to get a line # for the error:The exception was a java.lang.RuntimeException

: Attempted to compare the set {"69a86178-6aba-445b-a74b-6769ff09cc29"} with the value:

"id:"I can try to come up with a a minimal example to reproduce, but maybe it is better to ask how others would model optional values (and types)?

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**Follow-Ups**:**[tlaplus] Re: Modeling optional values - from a newbie***From:*Leslie Lamport

**References**:**[tlaplus] Modeling optional values - from a newbie***From:*Brandon Barker

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