feature CtQuery#select(Filter)

[pvojtechovsky]

Actually Filter can be used as mapping function this way:

Filter filter = new SomeFilter();
element.map(new CtFunction<R, Boolean>() {
	@Override
	public Boolean apply(R input) {
		return filter.matches(input);
	}
}).failurePolicy(QueryFailurePolicy.IGNORE);

There are these problems:

• I needed 2 minutes to write it and 1h to found that failurePolicy must be added too - and as a author I know the queries the best - so current way how legacy filters can be used is ugly and error prone.
• the failurePolicy is set to whole query, but it should be set only to the query step which uses the filter. Internally there is already a CtQueryImpl#stepFailurePolicy, which should be therefore called instead, but then the code would be even more ugly.

So this PR allows to write:

element.filter(new SomeFilter());

[monperrus]

do we need it in CtQueryable?

[pvojtechovsky]

you are right. We do not need it. Thanks!

[monperrus]

you convinced me.

However, I would name the new method select (as in some other languages) to avoid any ambiguity with filterChildren.

Would you add an explanation in filter.md?

[pvojtechovsky]

Done

documentation in filter.md

Yes, I will do it, but I suggest to add it later in another PR. Because

• there is conflict with PR early terminating query feature: early terminating query #1138, which already changes filter.md
• there are other PRs which are waiting for this PR.