How to assign ?T to T in Motoko?

Developers
1

@NathanosDev I want to do the following assign in Motoko:

let a: ?Nat = ?3;
let b: Nat = a;   <----Error: Cannot produce expected type

How can I do that assignement?

2

You can unwrap the value: Option | Internet Computer

1 Like
3

it says " @deprecated Option.unwrap is unsafe and fails if the argument is null; it will be removed soon; use a switch or do? expression instead". How can I use switch?

4

You can do:

let a: ?Nat = ?3;
let b: ?Nat = a;

or unwrap it like this in one line

let a: ?Nat = ?3;
let ?b = a else return; // or 0 etc if outside function

That will then let you access unwrapped a as b

2 Likes
5

let ?b = a else return #err("item was null"); ← Note, the else has to return a “none” which can be a return, a trap, or a default value.

or

let b = switch(a){
  case(null) 0; //default
  case(?val) val;
};
2 Likes