Fx

# Frequently Asked Questions

This page contains answers to common questions and issues with using Fx.

# Does the order of fx.Options matter?

No, the order in which you provide Fx options to fx.Options, fx.New, fx.Module, and others does not matter.

Ordering of options relative to each other is as follows:

  • Adding values: Operations like fx.Provide and fx.Supply are run in dependency order. Dependencies are determined by the function parameters and results.

    // The following are all equivalent:
fx.Options(fx.Provide(ParseConfig, NewLogger))
fx.Options(fx.Provide(NewLogger, ParseConfig))
fx.Options(fx.Provide(ParseConfig), fx.Provide(NewLogger))
fx.Options(fx.Provide(NewLogger), fx.Provide(ParseConfig))

  • Consuming values: Operations like fx.Invoke and fx.Populate are run after their dependencies have been satisfied: after fx.Provides.

    Relative to each other, invokes are run in the order they were specified.

    fx.Invoke(a, b)
// a() is run before b()

    fx.Module hierarchies affect invocation order: invocations in a parent module are run after those of a child module.

    fx.Options(
  fx.Invoke(a),
  fx.Module("child", fx.Invoke(b)),
),
// b() is run before a()

  • Replacing values: Operations like fx.Decorate and fx.Replace are run after the Provide operations that they depend on, but before the Invoke operations that consume those values.

    Ordering of decorations relative to each other is determined by fx.Module hierarchies: decorations in a parent module are applied after those of a child module.

# Why does fx.Supply not accept interfaces?

This is a technical limitation of how reflection in Go works. Suppose you have:

var redisClient ClientInterface = &redis.Client{ ... }

When you call fx.Supply(redisClient), the knowledge that you intended to use this as a ClientInterface is lost. Fx has to use runtime reflection to inspect the type of the value, and at that point the Go runtime only tells it that it’s a *redis.Client.

You can work around this with the fx.Annotate function and the fx.As annotation.

fx.Supply(
  fx.Annotate(redisClient, fx.As(new(ClientInterface))),
)

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