Сannot producer expected type

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2
  1. You need to switch and pattern-match on the optional value:
switch result {
  case (?val) { .../* val : Text here*/... };
  case null { ... };
}

If you have multiple of such tests, take a look at the do? construct, which comes in handy and e.g. allows collapsing multiple switches into one:

let result = do? { List.last(list1)! + List.last(list2)! + 1 };
switch result {
  case (?n) { ... handle result ... }
  case null { ... handle error ... }
}

The ! operator inside a do? block converts ?T to T and returns null to the enclosing do? otherwise.

  1. There are library functions for pretty-printing values of number types:
import Nat "mo:base/Nat";
let str = Nat.toText(1);
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