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*From*: Brandon Barker <brandon.barker@xxxxxxxxx>*Date*: Fri, 29 Jul 2022 17:37:19 -0700 (PDT)

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}

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:"

: 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**:**Re: [tlaplus] Modeling optional values - from a newbie***From:*Hillel Wayne

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

**Re: [tlaplus] Modeling optional values - from a newbie***From:*Felipe Oliveira Carvalho

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