Converting the array of values into the record

I would create default records and reduce the array:

let defaultSettings = {
  startDate: "2025-08-24",
  untilDate: "2025-08-25",
  defaultCurrency: Currency("EUR"),
  rates: Map.make(),
}

let defaultLedger = {
  settings: defaultSettings,
}

let make: array<ParserResult.t> => t = lines =>
  lines->Array.reduce(defaultLedger, (ledger, parserResult) => {
    let settings = switch parserResult {
    | Setting(DefaultCurrency(defaultCurrency)) => {...ledger.settings, defaultCurrency}
 // | others
    | _ => ledger.settings
    }

    {...ledger, settings}
  })

Playground example

You might want to make some fields optional instead:

type settings = {
  startDate?: string,
  untilDate?: string,
  defaultCurrency: currency,
  rates: Map.t<currency, float>,
}

Of course this comes at the cost of worse DX but you get the compiler to check if the dates are not just dummy ones.