# Document a config struct

Add a table of the fields of a struct. The default of each field comes from the code, so the table cannot drift from it.

<!-- site:editor -->

<!-- docgen:code file=internal/cli/testdata/repo/v2/machine/machine.go symbol=Config -->

``` go
type Config struct {
    // Wait is how long the Lamp stays on. 0 means DefaultWait.
    Wait time.Duration
    // Tries is the number of blinks. 0 means 1, and it is at most MaxTries.
    Tries int
    // Name is a label.
    Name string
}
```

<!-- /docgen:code -->

<!-- docgen:code file=internal/cli/testdata/repo/v2/machine/machine.go symbol=NewLamp -->

``` go
func NewLamp(cfg Config) (*Lamp, error) {
    if cfg.Tries > MaxTries {
        return nil, &docgen.ConfigError{Field: "Tries"}
    }
    l := &Lamp{wait: cfg.Wait, tries: cfg.Tries}
    if l.wait == 0 {
        l.wait = DefaultWait
    }
    if cfg.Tries == 0 {
        l.tries = 1
    }
    return l, nil
}
```

<!-- /docgen:code -->

<!-- docgen:docgen root=internal/cli/testdata/repo module=v2 page=docs/machines/guide-config.md -->

#### Before

    # Settings

    <!-- docgen:config pkg=machine type=Config -->
    <!-- /docgen:config -->

#### After

    # Settings

    <!-- docgen:config pkg=machine type=Config -->
    | Field | Type | Default | Description |
    |---|---|---|---|
    | `Wait` | `time.Duration` | `DefaultWait` (250ms) | Wait is how long the Lamp stays on. 0 means DefaultWait. |
    | `Tries` | `int` | `1` | Tries is the number of blinks. 0 means 1, and it is at most MaxTries. |
    | `Name` | `string` | zero value | Name is a label. |
    <!-- /docgen:config -->

<!-- /docgen:docgen -->

<!-- /site:editor -->

Tip: apply each default in an `if` that compares the field with its zero value, such as `if cfg.Tries == 0 { l.tries = 1 }`. A default that the code computes in another way is not in the table.

For all the rules and options, see [`docgen:config`](../reference/markers.md#docgenconfig).