# `TraceParent`

A `traceparent` header's value, which links a request to its trace.

Use it to pass a trace to or from another service, or read it from a request. For each part alone, use [`TraceId`](/docs/types/type-trace-id.md) and [`SpanId`](/docs/types/type-span-id.md).

## Shape

One value, held as text in one set form, with no parts: `00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-00`.

## Example

```jadpo
TraceParent("00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01")
```

`TraceParent(raw)` checks the text, and text that isn't one breaks the `format` rule.

`text.parse(raw, as: TraceParent)` reads one from text that comes from outside. Text that isn't one at all fails `malformed`.

## Good to know

- `==` tells you whether two are the same. `<` and `list.sort_by` don't take it.

## In JSON

A string, in a request, a response or `json.write`:

```json
"00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01"
"00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-00"
```

## Details

<details>
<summary>Text form, storage, how it is shown, and operations</summary>

`TraceParent` is in the group Software, and has been in Jadpo since 0.1.0.

### Text form

`text.from` writes one standard text form, the same form `text.parse` reads back: `00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01` and `00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-00`.

### How it is stored

It is stored in one column named for the field, of type `text`. `00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-00` is stored as `'00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-00'`.

### How it is shown to people

`text.format(value, locale:)` shows it to people in their locale.

With `locale: Locale.en_gb`:

| Value | Shown |
|---|---|
| `00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01` | `00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01` |
| `00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-00` | `00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-00` |

### Operations

It also takes `text.parse`, `text.from`, `text.format`, `json.read`, `json.write` and `==`. [How every standard type behaves](/docs/every-standard-type-meets-one-contract.md) says how they work.

</details>