Parse escalation condition

[yhabteab]

fixes #34

[julianbrost]

The combination of filter evaluation and error handling can result in some strange behavior. Take incident_age>=InvalidDurationLiteral as an example. This will be evaluated as EvalExists("incident_age") && !EvalLess("incident_age", "InvalidDurationLiteral"). Here, EvalLess will encounter an error and return false, making the overall expression return true.

Two options for addressing that:

1. Make all the evaluation return (bool, error).
2. After parsing, go over all conditions contained in it, and verify that if column is incident_age, the value must be a valid duration literal and if the column is incident_severity, the value must be a valid severity name. That would remove the possibility of an invalid literal showing up during evaluation.

[julianbrost]

I think it would be better to pass an error through the whole filter evaluation, i.e. also add error to the return type of Filter.Eval(...), otherwise you will still end up with strange behavior for !(incident_age<InvalidDurationLiteral) (that one might look constructed but with more available filter variables in the future, that might happen more easily in realistic examples).

[yhabteab]

otherwise you will still end up with strange behavior for !(incident_age<InvalidDurationLiteral)

So what would happen in that case? I don't get it. This expression would be parsed as None(&LessThan(...)) and in Filter.LessThan#Eval() the error is being caught and handled accordingly.

[julianbrost]

in Filter.LessThan#Eval() the error is being caught and handled accordingly.

In the current implementation, this means it is handled just like a valid expression that was false. (Which means with a negation somewhere, this can become true again)

So what would happen in that case?

I'd expect that if any sub-expression was invalid, i.e. returned an error, the whole filter evaluation returns an error. When handling the overall filter, this will probably be handled similar to a return value, but with an additional "you're doing something broken here" log message.

[julianbrost]

@yhabteab Please have a look at the changes I added on top.

[yhabteab]

4119344 🤦

[julianbrost]

Is that supposed to say that I can merge this PR? 😅