[Lens] Formula: String / boolean literals

[flash1293]

Currently Lens formula is limited to number values only. However, there are some use cases where working with strings, dates or booleans would be helpful. One part of this is described in #94603 and consists out of checking the return types of the AST nodes, but it would also be necessary to specify literals in some way.

Implementation

This is difficult because in tinymath quoted strings can be variable names as well. Also, true and false are not reserved and might be variables as well.

Option 1

Introduce an espace-hatch for the grammar, for example const(<constant value here>). This is on tiny-math side, not Lens:

• const(true) becomes a literal true value during evaluation
• const(false) becomes a literal false value during evaluation
• const(abc) becomes a literal "abc" string value during evaluation
• const("quoted string") becomes a literal "quoted string" string value during evaluation
• For consistency: const(123) becomes a literal 123 number value during evaluation
• const(add(1,2)) is a syntax error and fails during parsing - the argument of const can only be a literal

This is a breaking change for tiny-math, but a very minor one.

Option 2

Work around this on Lens side - for each variable name which is not a field name, a variable with its name as value is created for the evaluation context. E.g. for if(last_value(status) == "ONLINE", average(memory), 0) - the math execution has the following variables: { ONLINE: ONLINE }

This would work without changing anything in tinymath, result in a cleaner syntax but it makes it harder to provide a good error message because we don't know whether the user is typing a string literal or a field name.

Use case

Only doing a calculation if an entity is currently in a certain state, e.g. average used memory of servers which are online right now: if(last_value(status) == "ONLINE", average(memory), 0)

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

I agree that option 2 is cleaner, and I think we can build this into the validation.

We should also add a null or undefined value, since that matters a lot in some chart types.

[ghudgins]

+1 https://discuss.elastic.co/t/boolean-comparison-using-lens-formula/285872

[markov00]

In order to provide better transparency of priorities, issues that will not be prioritized within the next 24 months are being closed.

Tracking request in Lens general improvements ice box, solvable possibly with ESQL #184648