What is the behavior of Error.reject

Developers
1

Try to use the Error from base library.

import Error "mo:base/Error";

actor Main {
    var c = 1;

    public func add1() : async Nat {
        c += 100;
        throw Error.reject("test"); 
    };

    public func add2() : async Nat {
        c += 100;
        assert(false);
        return c;
    };

    public func reset() : async Nat {
        c := 1;
        return c;
    };

    public query func getc() : async Nat {
        return c;
    };
};

here is my log:

sudo dfx build main
Password:
Building canisters...

sudo dfx canister --no-wallet install main -m=reinstall
Reinstalling code for canister main, with canister_id rwlgt-iiaaa-aaaaa-aaaaa-cai

dfx canister --no-wallet call main getc
(1)

dfx canister --no-wallet call main add2
The Replica returned an error: code 5, message: "Canister rwlgt-iiaaa-aaaaa-aaaaa-cai trapped explicitly: assertion failed at /Users/flyq/workspace/test/pubfunc/src/main.mo:13.9-13.22"
dfx canister --no-wallet call main getc
(1)

dfx canister --no-wallet call main add1
The Replica returned an error: code 4, message: "test"

dfx canister --no-wallet call main getc
(101)

assert correctly throw an exception and successfully rolled back the state, meets expectations.

But Error.reject throws an exception, but there is no rollback state.

2

That is the intended semantics, not a bug.

3
  1. If I want to throw an exception in Motoko, I want to be guaranteed the rollback state and throw the related information of the exception, what should I need to do?
  2. Is there more information about Error.reject? I find Overview of canister calling, but not about Error.
4

I found a little bit of info here but I don’t think it fully answers your questions.

1 Like
5

An uncaught/escaping throw expression maps to the System API call msg_reject, which has the property of committing state changes, just like msg_reply. So the high-level semantics is largely determined by that of the low-level platform.

Note that throws can be locally caught using try-catch expressions, and it would be weird if those caught throws reverted state.

1 Like