Why does order of fx.Invoke affect soft value group results?

[sywhang]

Asked by @jkanywhere in #911 (comment).

Please clarify, what is the designed behavior and why?

https://go.dev/play/p/umSD0spuotW demonstrates that the order of fx.Invoke arguments matters:
If the soft invokation is first, it receives an empty value group.

It is not obvious to me why the order should matter. Please help clarify why that is desirable.

**** soft before hard ****
[Fx] PROVIDE string[group = "strings"] <= fx.Annotate(main.run.func1(), fx.ResultTags(["group:"strings""])
[Fx] PROVIDE string[group = "strings"] <= fx.Annotate(main.run.func2(), fx.ResultTags(["group:"strings""])
[Fx] PROVIDE fx.Lifecycle <= go.uber.org/fx.New.func1()
[Fx] PROVIDE fx.Shutdowner <= go.uber.org/fx.(*App).shutdowner-fm()
[Fx] PROVIDE fx.DotGraph <= go.uber.org/fx.(*App).dotGraph-fm()
[Fx] INVOKE fx.Annotate(main.main.func1(), fx.ParamTags(["group:"strings,soft""])
Soft: []
[Fx] INVOKE fx.Annotate(main.main.func2(), fx.ParamTags(["group:"strings""])
Hard: [hello there]
[Fx] RUNNING

**** hard before soft ****
[Fx] PROVIDE string[group = "strings"] <= fx.Annotate(main.run.func1(), fx.ResultTags(["group:"strings""])
[Fx] PROVIDE string[group = "strings"] <= fx.Annotate(main.run.func2(), fx.ResultTags(["group:"strings""])
[Fx] PROVIDE fx.Lifecycle <= go.uber.org/fx.New.func1()
[Fx] PROVIDE fx.Shutdowner <= go.uber.org/fx.(*App).shutdowner-fm()
[Fx] PROVIDE fx.DotGraph <= go.uber.org/fx.(*App).dotGraph-fm()
[Fx] INVOKE fx.Annotate(main.main.func2(), fx.ParamTags(["group:"strings""])
Hard: [hello there]
[Fx] INVOKE fx.Annotate(main.main.func1(), fx.ParamTags(["group:"strings,soft""])
Soft: [hello there]
[Fx] RUNNING

Program exited.

[sywhang]

This isn't "by design" of soft value groups, but rather a limitation in Fx introduced by Invoke.

The order of Invokes change whether the constructors that provide the soft value group has already run or not. In your example, the "hard" running before soft will populate the soft value group dependency because the constructor that provides the soft value is forced to run.

This is one of the main reasons we recommend that Invokes be used sparingly.

Be deliberate in your choice to use fx.Invoke in your module. By design, Fx executes constructors added via fx.Provide only if the application consumes its result, either directly or indirectly, through another module, constructor, or invoke. On the other hand, functions added with fx.Invoke run unconditionally, and in doing so instantiate every direct and transitive value they depend on.