# `ExchangeRate`

A rate between two currencies at a moment in time.

Use it to record the rate you were given and to convert: `money.convert` applies one. Jadpo doesn't fetch rates, so your app gets them from a provider.

## Shape

| Field | Type | Required? |
|---|---|---|
| `from` | [`Currency`](/docs/types/type-currency.md) | Yes |
| `to` | [`Currency`](/docs/types/type-currency.md) | Yes |
| `rate` | `Decimal` | Yes |
| `at` | [`Instant`](/docs/types/type-instant.md) | Yes |

## Example

```jadpo
ExchangeRate { from: Currency.gbp to: Currency.eur rate: 1.17 at: "2026-10-18T09:00:00Z" }
```

`text.parse(raw, as: ExchangeRate)` reads one from text that comes from outside. Text that isn't one at all fails `malformed`, such as `GBP/EUR 1.17`, `GBP-EUR 1.17 2026-10-18T09:00:00Z` or `GBP/XYZ 1.17 2026-10-18T09:00:00Z`.

## Good to know

- It holds when the rate applied, so keep it with what you converted to show later how a figure was reached.
- `==` tells you whether two are the same. `<` and `list.sort_by` don't take it, so sort by one of its parts instead.

## In JSON

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

```json
{"from":"GBP","to":"EUR","rate":"1.17","at":"2026-10-18T09:00:00Z"}
{"from":"EUR","to":"JPY","rate":"162.4031","at":"2026-10-18T09:30:00Z"}
```

## Details

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

`ExchangeRate` is in the group Money and banking, 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: `GBP/EUR 1.17 2026-10-18T09:00:00Z` and `EUR/JPY 162.4031 2026-10-18T09:30:00Z`.

### How it is stored

It is stored in 4 columns, each named for the field, `_` and its suffix. This is `EUR/JPY 162.4031 2026-10-18T09:30:00Z`:

| Column | Type | Holds |
|---|---|---|
| `<field>_from` | `char(3)` | `'EUR'` |
| `<field>_to` | `char(3)` | `'JPY'` |
| `<field>_rate` | `numeric` | `162.4031` |
| `<field>_at` | `timestamptz(3)` | `'2026-10-18 09:30:00.000000+00'::timestamptz` |

### How it is shown to people

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

With `locale: Locale.en_gb`:

| Value | Shown |
|---|---|
| `GBP/EUR 1.17 2026-10-18T09:00:00Z` | `GBP/EUR 1.17 2026-10-18T09:00:00Z` |
| `EUR/JPY 162.4031 2026-10-18T09:30:00Z` | `EUR/JPY 162.4031 2026-10-18T09:30:00Z` |

### Operations

- `money.convert(value: Money, rate: ExchangeRate) -> Money`: `value` in `rate`'s `to` currency, from its `from` one: the major units times `rate.rate`, rounded half to even to a minor unit, so £10.50 at a `GBP/EUR` rate of 1.17 is €12.28. It fails with `currency_mismatch` when `value` isn't in `rate.from`, unless that is known: an inverted rate is then a compile error. The rate is one the app holds; nothing here fetches one.

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>