Regarding return values, Mutative has the same behavior as Immer.

An draft function returned a new value and modified its draft. Either return a new value or modify the draft.

For example,

expect(()=>{conststate=create({a:1},(draft)=>{draft.a=2;return{        ...draft,};});}).toThrowError();

However, there is an irrational aspect to this approach. As long as a modified draft can be finalized, it should be allowed to return any value.

Therefore, we are considering allowing Mutative to support draft functions returning any value.