Well the default would still be replaced with something different depending on the attribute, so to me the contract stops being simple.

That said I'm ok to go with "$default", the expression resolver would treat it as any other string so there's no issue there.
Then I still have some questions:
 - what to do if the attribute has no default value, we can treat it as "undefined" but as Brian mentioned this should then fail in a nested expression; or we could treat it as empty string but that doesn't seem ok either
 - if we have that worked out, could we then not introduce "${foo.bar?}" as a shorthand for "${foo.bar:$default}"?
 - should we be concerned with edge cases like "${foo.bar:$def}ault"?

On Tue, Sep 21, 2021 at 3:10 PM David Lloyd <david.lloyd@redhat.com> wrote:
On Fri, Sep 17, 2021 at 3:25 PM Brian Stansberry <brian.stansberry@redhat.com> wrote:
On Fri, Sep 17, 2021 at 1:31 PM David Lloyd <david.lloyd@redhat.com> wrote:
On Fri, Sep 17, 2021 at 12:28 PM Michal Petrov <mpetrov@redhat.com> wrote:
Okay, that makes sense except for this bit:

> Especially strange would be if I had an expression like "${foo.bar?}baz" and the default value for the property gets put in;

For one this wouldn't work and I don't think we want it to work, the default value will be put in only if the attribute resolution is empty (or if it's the reserved string if we're going with that), so "${foo.bar?}baz" would resolve to "baz". We're not looking for a way to "inject" the default value, we want the attribute definition to use its default value because the user didn't provide anything (as far as the definition is concerned). As such the optional expression doesn't make sense in any kind of nested context.

Definitely understood. The use case seems pretty clear: as a user, I need to be able to explicitly specify the default value of a property in the event that another property cannot be expanded.

I think this is what you meant but just in case not.. its "I need to be able to explicitly specify the default value of a property *should be used* in the event that another property cannot be expanded."  For the case where the user wants to specify a fallback value, that's the existing ${foo.bar:baz}.

Yes, that is what I meant.

However, the proposed implementation *is* defined to use the existing string expression interpolation engine. The contract for the engine is clear: wherever there is an expression, that expression is lexicographically replaced with the exact replacement string. By stipulating a new modified contract based on the use case (specifically, that certain expressions must stand alone), you're introducing a new and arguably unexpected behavior to the existing contract, which brings complications in terms of usability (principle of least surprise for example) as well as implementation (an extra check based on the kind of expression found).

I'll need to check whether WildFly expression resolution results in ${foo:} returning an empty string. I thought it didn't but may be wrong. If so the '?' approach can be logically seen as saying resolve to 'null', not empty string. And then ${foo?}baz resolves to a conceptual null.append(baz) i.e. a failure. Any nesting that must use the 'null', e,g ${foo?:$baz}} must fail.

That might be an internally consistent rationalization, but does it really make sense to the user? The user contract of the expression resolver definitely does not include null values; it replaces a string expression with another string or else gives a validation error for the particular expression.

The caller of the expression resolution (the AttributeDefinition) can deal with the 'null' return, providing the default if one exists, or failing. (Or perhaps not setting the attribute to undefined. The situation is ambiguous.)

That still feels off to me, but at least I can see how it results in an understandable semantic.

I imagine you've had lots of debates in MP Config around these null vs empty questions, so I'm prepared to schooled. ;)

On the other hand, since the contract of the interpolation engine *is* already existent, if the expression engine is used for this purpose in an idiomatic manner (i.e. a special expression argument, which is directly supported already by the expression API), then the user can easily extrapolate that the default value expression is just another expression and can be used in the exact manner of any other expression, and as a result the user has slightly more flexibility than what was originally required (which is not necessarily a bad thing, as long as the implemented functionality can be concisely described in documentation).

I'm not clear on what you're suggesting here.

What I mean is, we have a simple contract for expressions that the user can understand: _this_ thing is replaced with _that_ thing. Is it so bad that they would gain the ability to decorate the default value string, if we used a sentinel variable name like "default"?

- DML • he/him