Match – Jadpo

match chooses between the cases of a value. It must cover every case, and it works as a statement or as a value. Its arms use ->, the same arrow as result types.

example

function receipt(outcome: PaymentOutcome) -> ReceiptId? {    return match outcome {        PaymentOutcome.pending -> none        PaymentOutcome.paid(paid) -> paid.receipt_id        PaymentOutcome.declined -> none    }}function headline(listing: Listing) -> ProductName {    return match listing.subtitle {        some(subtitle) -> subtitle        none -> listing.name    }}action follow_up(todo: Todo) -> Unit {    match todo.status {        TodoStatus.open -> {            notify_owner(todo.id)        }        TodoStatus.done -> {}    }}

A match on a call that can fail names its success value and each failure (Handling a failure in match):

example

function lookup_or_default(key: LookupCode) fails LookupRejected -> LookupResult {    return match lookup(key) {        success(value) -> value        LookupMissing -> "not found"        LookupRefused -> reject LookupRejected    }}

Pattern	Matches
`Enum.variant`	a plain variant, or a variant whose fields this arm doesn't read
`Enum.variant(name)`	a variant with fields, naming them `name`; `name.field` has the field's type, `Enum.variant.field`
`some(name)`, `none`	a nullable value
`true`, `false`, `"text"`, `42`	a literal
`_`	anything else; only where the cases can't be listed, such as text and numbers
`success(name)`, `FailureName`	the result of a call that can fail: its value, or one of its failures

There is no _ on an enum, a Bool or a nullable value: every case is listed. So adding a variant to an enum makes every match that must handle it fail to compile until it does.

An arm is one of:

pattern -> expression;
pattern -> reject F, which maps the failure to another;
pattern -> propagate, which passes the failure on;
pattern -> { statements }.

A failure arm can recover with a value, map to another failure with reject, or pass the failure on with propagate. Every failure the call can produce needs an arm (Failures).

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