<<.operator-example 1 "A B C D :intersection[enlist[C D E F]]">>
<<.operator-example 2 "A B C D :intersection[enlist[A B C D]]">>
<<.operator-example 3 "A B C D :intersection[enlist[E F G H]]">>
<<.operator-example 4 ":intersection[enlist[A B C D]]" "Using intersection in the first filter run is of little use. The result will always be empty.">>
!! Difference between `+` and `:intersection`
The `+` prefix should be thought of as an "AND" in formal logic, e.g. "give me all titles that satisfy condition A ''and'' condition B". But it's not suitable for all cases; if condition B uses a filter operator that replaces its input, then it will be difficult to use the `+` prefix. For example, if you wanted to find out what tags two tiddlers have in common, you might try to write a filter expression like:
But that won't work, because the second filter run will end up throwing away its input and replacing it with an input consisting of the single title `[[compare Operator]]`. So the result you'd get from that filter expression would be just the tags of the `compare Operator` tiddler.
For cases like this, the `:intersection` prefix is what you need. It takes the filter output so far, //sets it aside//, and starts the next filter run with all tiddler titles as input. Then once the latest filter run has completed, it takes the latest output, compares it to the set-aside output, and produces a new output that contains only titles that appeared in both the set-aside output and the latest output. So to get only the tags that the `field Operator` and `compare Operator` tiddlers have in common, you would write a filter expression like this:
<<.operator-example 7 "[[field Operator]tags[]] +[[compare Operator]tags[]]" """The "field Operator" tiddler's tags are lost, so only the "compare Operator" tiddler's tags are returned""">>
<<.operator-example 8 "[[field Operator]tags[]] :intersection[[compare Operator]tags[]]" """Returns the tags that both tiddlers have in common""">>
