Converting the array of values into the record

Then you should probably reduce to option<t>:

let make: array<ParserResult.t> => option<t> = lines =>
  lines->Array.reduce(None, (ledger, parserResult) =>
    ledger->Option.map(ledger => {
      let settings = switch parserResult {
      | Setting(DefaultCurrency(defaultCurrency)) => {...ledger.settings, defaultCurrency}
      // | others
      | _ => ledger.settings
      }

      {...ledger, settings}
    })
  )