fx.Self: a parameter to fx.As for providing a type as itself #1201
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We frequently see requests for folks who want to use `fx.As` to provide a type as another type, while also providing it as itself: * uber-go#1196 * uber-go#1148 * uber-go#1079 This is currently not possible because `fx.As` causes a constructor to no longer provide its original type. The workaround we often give is for folks to do something like: ```go fx.Provide( newConcreteType, func(ct *concreteType) Interface { return ct } ) ``` which is admitedly not very ergonomic, and leads to a common mistake of providing the cosntructor twice instead: ```go fx.Provide( newConcreteType, fx.Annotate(newConcreteType, fx.As(new(Interface))), ) ``` This PR adds `fx.Self()`, which returns a special value to indicate an `fx.As` should retain that original return type: ```go fx.Provide( newConcreteType, fx.As(fx.Self()), fx.As(new(Interface)), ) ``` This can be used as positional arguments to `fx.As` as well.
Codecov ReportAll modified and coverable lines are covered by tests ✅
Additional details and impacted files@@ Coverage Diff @@
## master #1201 +/- ##
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- Coverage 98.49% 98.46% -0.03%
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Files 34 34
Lines 2860 2676 -184
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- Hits 2817 2635 -182
+ Misses 36 34 -2
Partials 7 7 ☔ View full report in Codecov by Sentry. |
JacobOaks
requested review from
tchung1118 and
sywhang
and removed request for
tchung1118
tchung1118
reviewed
May 14, 2024
Addressed! |
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We frequently see requests for folks who want to use
fx.Asto provide a type as another type, while also providing it as itself. To name a few:This is currently not possible via strictly using
fx.As+ a single constructor, sincefx.Ascauses a constructor to no longer provide its original type. The workaround we often give is for folks to do something like this, which involves adding a second "constructor":which is admittedly not very ergonomic. A somewhat common pattern mistaken to be a workaround is providing the constructor twice instead:
This PR adds
fx.Self(), which returns a special value to indicate anfx.Asshould retain that original return type:As an alternative, I considered a new annotation altogether, named something like
fx.AlsoAs, but adding a special type that can be passed as an argument tofx.Asdirectly allows for more fine-tuned control over individual positional return values.For example, this function's return types can be easily expressed as
*asStringerandio.Writerusingfx.Self():Whereas something like
fx.AlsoAswouldn't provide the ability to skip over the first positional return value entirely.