# `Speed`

A speed: `30mph` or `100kmh`.

Use it for how fast something moves. A [`Length`](/docs/types/type-length.md) over a [`Duration`](/docs/types/type-duration.md) gives one. For data speed, use [`Bitrate`](/docs/types/type-bitrate.md).

## Shape

One value, with no fields to fill in. As text it reads `340 mps`.

## Example

```jadpo
Speed(30mph)
```

`text.parse(raw, as: Speed)` reads one from text that comes from outside. Text that isn't one at all fails `malformed`, such as `5`, `1.2.3` or `five`.

These operations give one: `units.of` and `units.sum`.

## Good to know

- It is held in metres per second to 6 places.
- `==` tells you whether two are the same, and `<` and `list.sort_by` compare and sort them.

## In JSON

An object of its parts, in a request, a response or `json.write`:

```json
{"value":"0.5","unit":"mps"}
{"value":"13.4112","unit":"mps"}
{"value":"340","unit":"mps"}
```

## Details

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

`Speed` is in the group Physical quantities, 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: `0.5 mps`, `13.4112 mps` and `340 mps`.

### How it is stored

It is stored in one column named for the field, of type `numeric(38, 6)`, holding the speed in metres per second (m/s). `13.4112 mps` is stored as `13.4112`.

### How it is shown to people

`text.format(value, locale:)` shows it to people in their locale. A call may also give `unit:`.

With `locale: Locale.en_gb`:

| Value | Shown |
|---|---|
| `0.5 mps` | `0.5 m/s` |
| `13.4112 mps` | `13.4112 m/s` |
| `340 mps` | `340 m/s` |

### Operations

- `units.in(value: Quantity, unit: QuantityUnit, scale: Int, rounding: Rounding) -> Decimal`: `value` in `unit`, a unit of its kind, as a `Decimal` of `scale` places rounded by `rounding`: `units.in(5km, LengthUnit.mile, scale: 2, rounding: Rounding.half_even)` is `3.11`.
- `units.of(value: Number, unit: QuantityUnit) -> Quantity`: `value` of `unit`, an `Int` or a `Decimal`, as the quantity of `unit`'s kind, converted exactly to the base unit and rounded half to even to the kind's scale: `units.of(5, LengthUnit.mile)`.
- `units.sum(items: List<Quantity>) -> Quantity`: The items added up, all of one kind; nothing for no items. A `Temperature` or a `SoundLevel` doesn't add to itself, so neither is summed.

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>