Usability problem with open types, optional/default fields and mapping constructors

[jclark]

Consider:

type Person record {
  string name;
  string country?;
}

Person p = { name: "James Clark", county: "Thailand" };

This will compile despite country being misspelled, because the record is open and the country field is not required. There would be the same problem if country was required, but had a default value. This seems very much not OK to me.

[jclark]

I think there is a potential solution which is to do something similar to what we do with the dot operator and assignment. We do not allow p.county = "Thailand";, and I think we can use a similar rule here. The rule would be something like this: when the contextually expected type is a record type, you can only use identifiers for field names that are mentioned in the type. For other fields, you would have to use a string, e.g. if you wanted a county field you would have to do

Person p = { name: "James Clark", "county": "Bangkok" };

just as you have to use p["county"] = "Bangkok"; to do the assignment.

[sanjiva]

I agree this is a serious usability problem and +1 for the proposed approach for solving it.

[hasithaa]

We have implemented this for JBallerina 1.0.0-Alpha

[jclark]

Is it working OK?

[hasithaa]

Yes, This change is a good one. It really helps to detect typos quickly.