Pattern Matching

Pattern matching lets you branch on values and variants with a sequence of case arms. It is useful both in ordinary control flow and in error recovery.

Match a value directly

Use match when you want to inspect one value and choose a result from several cases. Literal patterns match exact values, and _ acts as the fallback arm.

const status = 200

const label = match status
  case 200 => "ok"
  case 404 => "not found"
  case _ => "unexpected"
end

@println(label)

Pattern matching in error recovery

Pattern matching is especially useful in catch blocks, where case is ... lets you branch on specific error variants and recover with different values.

error DataError {
  Missing(path: String)
  Permission
}

def describe(path: String) -> String
  try read(path) catch err
    case is DataError.Missing
      `missing:${err.path}`
    case is DataError.Permission
      "denied"
    case _
      "unexpected"
  end
end