Readable by people. Predictable for models.

// A "?" return type means: known, or abstained.
fn capital(country: Text) -> Text? {
  know atlas[country] => it
  else => abstain "no record for {country}"
}

match capital("Atlantis") {
  known v   => print(v)
  abstain r => print("no answer: {r}")
}

let atlas = source "atlas.csv"

fn capital(country: Text) -> Text? {
  know atlas[country] as row => row.capital grounded row
  else => abstain "no record for {country}"
}

match capital("Japan") {
  known v => print("{v} (from {v.grounds})")
  abstain r => print(r)
}
// Tokyo (from atlas.csv:2)

Tokyo (from atlas.csv:2)

tool order_status(id: OrderId) -> Status? {
  knows   "orders table, read-only"
  unknown "anything about shipping carriers"
}

agent support(question: Text) -> Reply? validate strict {
  use order_status
  // The reply must cite a grounded Status, or abstain.
  know order_status(parse_id(question)) as s => reply(s)
  else => abstain "cannot answer from available tools"
}

let c = capital("Atlantis")
print(c)

error[E012]: unhandled abstention
 --> bad.dv:2:7
  |
2 | print(c)
  |       ^ `c` is Text?, it may abstain
  |
help: match on it, or write `c else "n/a"` to supply a default