Result Objects¶

A result object is an object with the sole purpose of carrying results for a specific function or method.

As with parameter objects, the object is defined exclusively for a single function, and not shared with other functions.

In Fx, result objects contain exported fields exclusively, and are always tagged with fx.Out.

Related

Parameter objects are the parameter analog of result objects.

Using result objects¶

To use result objects in Fx, take the following steps:

Define a new struct type named after your constructor with a Result suffix. If the constructor is named NewClient, name the struct ClientResult. If the constructor is named New, name the struct Result. This naming isn't strictly necessary, but it's a good convention to follow.
```
type ClientResult struct {
}
```
Embed fx.Out into this struct.
```
type ClientResult struct {
   fx.Out
```
Use this new type as the return value of your constructor by value.
```
func NewClient() (ClientResult, error) {
```

Add values produced by your constructor as exported fields on this struct.

type ClientResult struct {
   fx.Out

   Client *Client
}

Set these fields and return an instance of this struct from your constructor.

func NewClient() (ClientResult, error) {
   client := &Client{
       // ...
   }
   return ClientResult{Client: client}, nil
}

Once you have a result object on a function, you can use it to access other advanced features of Fx:

Feeding value groups with result objects

Adding new results¶

You can add new values to an existing result object in a completely backwards compatible manner.

Take an existing result object.

type Result struct {
   fx.Out

   Client *Client
}

func New() (Result, error) {
   client := &Client{
       // ...
   }
   return Result{
       Client: client,
   }, nil

Add a new field to it for your new result.

type Result struct {
   fx.Out

   Client *Client
   Inspector *Inspector
}

In your constructor, set this field.

   return Result{
       Client: client,
       Inspector: &Inspector{
           // ...
       },
   }, nil