How to write an expression rule that returns multiple values?

Just an aside...in a future release of Appian you'll be able to return data in 'map' format (this is already generated from a Decision Rule if you have multiple outputs of different data types) but you'll have access to the a!map() function natively, as per the following example:

a!map(
  myInteger: 99,
  myString: "Ipsum Lorem",
  myDecimal: 3.14159,
  myBoolean: true,
  myIntegerList: {3,5,7,9}
)

The above code returns the following output:

This will bust the current limitation where Expression Rules can only return one 'type' of data as their final result.

What will be the difference to the "{}" dictionary?

+1, curious what benefits this offers compared to essentially returning an "any type" dictionary.

That when extracting the values from the map the value won't be wrapped in a variant but will be natively of the type expected:

{
  myInteger: 99,
  myString: "Ipsum Lorem",
  myDecimal: 3.14159,
  myBoolean: true,
  myIntegerList: {3,5,7,9}
}.myInteger

returns:

whereas:

a!map(
  myInteger: 99,
  myString: "Ipsum Lorem",
  myDecimal: 3.14159,
  myBoolean: true,
  myIntegerList: {3,5,7,9}
).myInteger

returns:

According to the feature description:

"[this]...will help to reduce unexpected errors in your interfaces and rules. Maps will simplify your expressions and make it easier to work with other types."

That when extracting the values from the map the value won't be wrapped in a variant but will be natively of the type expected:

{
  myInteger: 99,
  myString: "Ipsum Lorem",
  myDecimal: 3.14159,
  myBoolean: true,
  myIntegerList: {3,5,7,9}
}.myInteger

returns:

whereas:

a!map(
  myInteger: 99,
  myString: "Ipsum Lorem",
  myDecimal: 3.14159,
  myBoolean: true,
  myIntegerList: {3,5,7,9}
).myInteger

returns:

According to the feature description:

"[this]...will help to reduce unexpected errors in your interfaces and rules. Maps will simplify your expressions and make it easier to work with other types."

oh, that sounds great then - i can finally get rid of this nervous twitch i've developed over time by being caught off guard by weird type mismatches

Hah, likewise! I can't promise you which release of Appian it'll arrive in but it's in progress.

Really nice little feature!

This will be nice to have! Bit curious, will process variables be able to hold data of this 'type'?

The feature description includes the following:

"Expose map type in type pickers (rule inputs, process variables, activity class parameters)"

...and, just to reiterate: whilst this is on the road-map there isn't a guarantee that this will arrive in any given release!