Onus review · reporting

pub fn monthly_totals(
  db: sql.Db[ReadOnly, schema: "orders"],
  year: Int where 2000 <= it and it <= 2100
) -> Result[List[MonthlyTotal], sql.Error] may sql.read, alloc
  ensures forall t: MonthlyTotal in result: t.total_pence >= 0
{
  -- text is a const parameter (§3.8.1): std.sql's own parse_select runs at check time and rejects anything but a single SELECT.
  let stmt: sql.Select[MonthlyTotal] = sql.select[
    text: "select to_char(created, 'YYYY-MM') as month, sum(amount_pence) as total_pence from orders where extract(year from created) = $1 group by 1 order by 1"
  ](params: [sql.int(x: year)], row: MonthlyTotal)
  return sql.query(db: db, statement: stmt)
}

pub fn main(
  args: List[Text],
  env: io.Env,
  files: io.Files,
  net: io.Net
) -> Result[Unit, AppError] may io.env, io.file, io.net, sql.read, alloc {
  let cfg: config.Config = try config.load(env: env) else e: Config(detail: e)
  let reporting: sql.Db[ReadOnly] = try sql.connect(
    net: net,
    dsn: cfg.reporting_dsn,
    mode: ReadOnly
  ) else e: Storage(detail: e)
  let orders: sql.Db[ReadOnly, schema: "orders"] = sql.restrict(db: reporting, schema: "orders")
  let rows: List[MonthlyTotal] = try monthly_totals(db: orders, year: 2026)
    else e: Storage(detail: e)
  let out: io.File = try io.create(files: files, path: "report.csv") else e: Io(detail: e)
  for row: MonthlyTotal in rows {
    try io.write(file: out, text: row.month ++ "," ++ Int.to_text(x: row.total_pence) ++ "\n")
      else e: Io(detail: e)
  }
  return Ok(value: Unit)
}

pub fn load(env: io.Env) -> Result[Config, Error] may io.env, alloc {
  match io.get_env(env: env, name: "REPORTING_DSN") with
  | Some(value) -> return Ok(value: Config { reporting_dsn: value })
  | None -> return Err(error: Missing(key: "REPORTING_DSN"))
}