|Perl: the Markov chain saw|
Preserving Calling Contextby TedYoung (Deacon)
|on Apr 18, 2006 at 18:03 UTC||Need Help??|
TedYoung has asked for the
wisdom of the Perl Monks concerning the following question:
So, I finally bought a copy of HOP. In it, the author uses anonymous subs as iterators. The details of this aren't too important for this discussion. He then introduces a function called igrep, which applies a filter to an iterator, much like grep, but one that is evaluated lazily, in the tradition of an iterator.
So, here is an example implementation:
The author later introduces some examples of iterators that return lists on each invocation, instead of scalars. To account for this, he starts using a hypothetical igrep_l function that works with lists instead of a scalar. Not hard to implement, but I feel it gets ugly to have different functions for different contexts, especially iterators whose return value is dependant on context.
So, finally the question: how can this igrep function effectively propagate the calling context to the iterator? Here is something that I cobbled together. I was wondering if anyone had any simpler ideas. Note: the sub passed to igrep will have @_ as the param (in a scalar context) or params (in a list context) just retuned from the iterator. $_ is aliased to the first element returned by the iterator for the simple cases where an iterator is always called in a scalar context, or always returns a scalar.
So, it seems like there is a lot of checks to wantarray here. An alternative would be to have a giant if (wantarray) dividing the sub into two implementations.
BTW, while reading this book made me think to ask it, I have actually had this problem come up in other domains. I can't remember where I have done before, but I do remember using a similar strategy.
Some other notes: I have intentionally not used strict or warning for the sake of brevity and clarity in these examples. Also, for the same reasons, I am not worried about the void context.
Well, I hope this question has interested more people than it has bored. :-)
Ted Young($$<<$$=>$$<=>$$<=$$>>$$) always returns 1. :-)