In which cases does astMapper(...).statement(...) return nil?

[gthb]

The return types of all methods on IAstPartialMapper include | nil. In what case might that be returned?

In my case I'm calling astMapper(...).statement(stmt) where stmt is a SelectFromStatement. Staring at AstDefaultMapper.selection, that returns assignChanged(val, ...) and assignChanged returns the same type as its first argument. So I fully expect to get a SelectFromStatement back. What's the nil there for? And if I do get a nil, how would I get at some more information about what went wrong?

[oguimbal]

Regarding the | nil part of your question

It's a convention that for the whole mapper. If you override any method, and this method returns nil , then that means that this statement is not to be taken into account anymore.

For instance, lets say that you are coding a request proxy, and you want to exclude all queries that are not selects.

You will be able to do something like that:

const mapper = astMapper(map => ({
        statem sent: s => s.type !== 'select' ? null : map.select(s),
    }));

const transformed = mapper.statement(parseFirst(sql));
if (!transformed) { 
   throw forbidden;
}
const newSql = toSql.statement(transformed);
// do whatever

This will not only filter statements like insert into xxx or truncate yyy, but will also remove statements which top-level seems to be a selection, but that might themselves contain non readonly statements like:

const transformed mapper.statement(`with ids as (select id from t1) delete from t1 where 1=1 returning id`);

// "transformed" will be null

Regarding the Statement instead of SelectFromStatement return type

Each method of the mapper returns the broadest type of statements that could be compatible with the input argument.

For instance, any expression mapper (like refs, binariess, etc) returns an Expr so you can have a mapper that transforms any kind of expression into another compatible expression.

Likewise, any top-level statement that returns data (select, with, delete, etc... are all able to return data) will be able to be mapped as any kind of data-returning statement.

That's what offers the most flexibility, while ensuring that types are guiding you to build correct statements.

Makes sense ?