# Annotations

You can annotate functions and values with the fx.Annotate function before passing them to fx.Provide, fx.Supply, fx.Invoke, fx.Decorate, or fx.Replace.

This allows you to re-use a plain Go function to do the following without manually wrapping the function to use parameter or result objects.

# Annotating a function

Prerequisites

A function that:

does not accept a parameter object, when annotating with fx.ParamTags.
does not return a result object when annotating with fx.ResultTags.

Steps

Given a function that you're passing to fx.Provide, fx.Invoke, or fx.Decorate,
```
    fx.Provide(
      NewHTTPClient,
    ),
```

Wrap the function with fx.Annotate.

    fx.Provide(
      fx.Annotate(
        NewHTTPClient,
      ),
    ),

Inside fx.Annotate, pass in your annotations.

    fx.Provide(
      fx.Annotate(
        NewHTTPClient,
        fx.ResultTags(`name:"client"`),
      ),
    ),

This annotation tags the result of the function with a name.

Related resources

fx.Annotation (opens new window) holds a list of all supported annotations.

# Casting structs to interfaces

You can use function annotations to cast a struct value returned by a function into an interface consumed by another function.

Prerequisites

A function that produces a struct or pointer value.

func NewHTTPClient(Config) (*http.Client, error) {

A function that consumes the result of the producer.

func NewGitHubClient(client *http.Client) *github.Client {

Both functions are provided to the Fx application.

    fx.Provide(
      NewHTTPClient,
      NewGitHubClient,
    ),

Steps

Declare an interface that matches the API of the produced *http.Client.

type HTTPClient interface {
  Do(*http.Request) (*http.Response, error)
}

// This is a compile-time check that verifies
// that our interface matches the API of http.Client.
var _ HTTPClient = (*http.Client)(nil)

Change the consumer to accept the interface instead of the struct.
```
func NewGitHubClient(client HTTPClient) *github.Client {
```

Finally, annotate the producer with fx.As to state that it produces an interface value.

    fx.Provide(
      fx.Annotate(
        NewHTTPClient,
        fx.As(new(HTTPClient)),
      ),
      NewGitHubClient,
    ),

With this change,

the annotated function now only puts the interface into the container
the producer's API remains unchanged
the consumer is decoupled from the implementation and independently testable

← Result Objects Feeding →