# `Length`

A length or distance: `5km`, `45cm`, `3inch` or `2metre`.

Use it for any distance, in any unit. It keeps your sums correct across units, so `1metre + 3cm` just works. For a box's three sides, use [`Dimensions`](/docs/types/type-dimensions.md).

## Shape

One value, with no fields to fill in. As text it reads `1000 metre`.

## Example

```jadpo
Length(5km)
```

`text.parse(raw, as: Length)` 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: `geo.distance`, `geo.length`, `units.of` and `units.sum`.

## Good to know

- `m` is minutes, so a metre is `metre`. Inches are `inch`.
- A `Length` times a `Length` gives an [`Area`](/docs/types/type-area.md).
- `units.of(5, LengthUnit.mile)` builds one from a number, and `units.in` reads a number back.
- It is held in metres 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":"-1.5","unit":"metre"}
{"value":"0.0254","unit":"metre"}
{"value":"1000","unit":"metre"}
```

## Details

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

`Length` 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: `-1.5 metre`, `0.0254 metre` and `1000 metre`.

### How it is stored

It is stored in one column named for the field, of type `numeric(38, 6)`, holding the length in metres (m). `0.0254 metre` is stored as `0.0254`.

### 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 |
|---|---|
| `-1.5 metre` | `-1.5 m` |
| `0.0254 metre` | `0.0254 m` |
| `1000 metre` | `1,000 m` |

### Operations

- `geo.distance(from: Point, to: Point) -> Length`: The geodesic distance between two points on the ellipsoid, rounded half to even to the micrometre.
- `geo.length(line: T) -> Length`: A `LineString`'s or `MultiLineString`'s length: each edge measured as `distance` measures it, and summed.
- `geo.within(a: T, b: U, distance: Length?) -> Bool`: `geo.contains(b, a)`; or, with `distance:`, whether two points are at most that far apart, as `geo.distance` measures them.
- `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>