anonymous-functions, their closures, can now be sent over broker.
In order to send an anonymous function the receiver must have parsed
a definition of the functon, but it need not to have been evaluated.
See testing/btest/language/closure-sending.zeek for an example of how
this can be done.
This also sends their closures as well as the closures of regular
functions.
This allows anonymous functions in Zeek to capture their closures.
they do so by creating a copy of their enclosing frame and joining
that with their own frame.
There is no way to specify what specific items to capture from the
closure like C++, nor is there a nonlocal keyword like Python.
Attemptying to declare a local variable that has already been caught
by the closure will error nicely. At the worst this is an inconvenience
for people who are using lambdas which use the same variable names
as their closures.
As a result of functions copying their enclosing frames there is no
way for a function with a closure to reach back up and modify the
state of the frame that it was created in. This lets functions that
generate functions work as expected. The function can reach back and
modify its copy of the frame that it is captured in though.
Implementation wise this is done by creating two new subclasses in
Zeek. The first is a LambdaExpression which can be thought of as a
function generator. It gathers all of the ingredients for a function
at parse time, and then when evaluated creats a new version of that
function with the frame it is being evaluated in as a closure. The
second subclass is a ClosureFrame. This acts for most intents and
purposes like a regular Frame, but it routes lookups of values to its
closure as needed.
This commit marks (hopefully) ever one-parameter constructor as explicit.
It also uses override in (hopefully) all circumstances where a virtual
method is overridden.
There are a very few other minor changes - most of them were necessary
to get everything to compile (like one additional constructor). In one
case I changed an implicit operation to an explicit string conversion -
I think the automatically chosen conversion was much more convoluted.
This took longer than I want to admit but not as long as I feared :)
