Proposal: make assisted arguments explicit

kotlin-inject’s way of injecting dependencies with assisted arguments is really clever but I’ve been thinking it’s very implicit and hidden for readers. Thoughts on making it explicit in some manner? Using dagger-like @Assisted annotation is one option, but I’m not sure if you’d want that. It’ll also remove the restriction of keeping assisted arguments at the end of a constructor, which is another hidden restriction.

[evant]

Ok here’s my proposal:

Introduce an @Assisted annotation to make assisted arguments explicit

problem

It can be not obvious which arguments are assisted or which are expected to be provided by the graph. This can make it confusing on why you are unable to inject a certain type. While there is extra flexibility with this on choosing which args you want to inject or not at the usage site, this feature isn’t often used.

solution

All assisted arguments must be annotated with an @Assisted annotation. This will limit the usage site to providing exactly those parameters in the same order they are declared. For example with the declaration:

@Inject class AssistedClass(@Assisted arg1: One , arg2: Two, @Assisted arg3: Three)

the only allowed way to inject that class is

@Inject Usage(createAssistedClass: (One, Three) -> AssistedClass)

these will fail to compile with an error about missing or mismatched args

@Inject Usage(createAssistedClass: (One) -> AssistedClass) // missing arg
@Inject Usage(createAssistedClass: (Three, One) -> AssistedClass) // mismatched args
@Inject Usage(createAssistedClass: AssistedClass) // missing args

migration

When this feature is implemented the behavior will be selected based on if there’s any args annotated with @Assisted. If so the new strategy will be used, otherwise a warning will be printed to suggest annotating the arg. This warning will then become an error in a future release.

[evant]

You mean something like this? 😉 (in reality I’d probably go for something a bit different since the lists aren’t expected to be super long but yea)