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*From*: Hillel Wayne <hwayne@xxxxxxxxx>*Date*: Sat, 30 Jul 2022 02:19:49 -0500*References*: <c6213ec6-1261-4cc9-b646-9cda6c8d81d3n@googlegroups.com>*User-agent*: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0

Sum types are subtly different from union types. To represent union types in TLC, I usually do something like this:

`CONSTANT NotAnInt
ASSUME NotAnInt \notin Int
TypeInv ==
OptionalVal \in Int \union {NotAnInt}`

To represent optionals as a sum type, I'd instead used a tagged
struct:

`TypeInv ==
OptionVal \in [type: {"some"}, val: Nat] \union [type:
{"none"}]`

H

On 7/29/2022 7:37 PM, Brandon Barker
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**:**Re: [tlaplus] Modeling optional values - from a newbie***From:*Brandon Barker

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

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