# `Mass`

A mass or weight: `250g`, `2.5kg` or `500mcg`.

Use it for weights of goods and doses. For a dose of medicine with a route and timing, use [`DosageInstruction`](/docs/types/type-dosage-instruction.md).

## Shape

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

## Example

```jadpo
Mass(2.5kg)
```

`text.parse(raw, as: Mass)` 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 kilograms to 9 places, which is one microgram.
- `==` 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":"0.25","unit":"kg"}
{"value":"2.5","unit":"kg"}
{"value":"1000","unit":"kg"}
```

## Details

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

`Mass` 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: `0.25 kg`, `2.5 kg` and `1000 kg`.

### How it is stored

It is stored in one column named for the field, of type `numeric(38, 9)`, holding the mass in kilograms (kg). `2.5 kg` is stored as `2.5`.

### 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 |
|---|---|
| `0.25 kg` | `0.25 kg` |
| `2.5 kg` | `2.5 kg` |
| `1000 kg` | `1,000 kg` |

### 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>