[parenscript-devel] What should PS LOOP do with null collections?

[Vladimir Sedach]

So what you're proposing is making the implicit sequence type be (or
array nil), where nil is treated like a zero-length array? I think
that's a good idea, it will make a lot of things simpler.

Vladimir

2010/7/12 Daniel Gackle <danielgackle at gmail.com>:
> I don't know if anyone other than us is using PS LOOP, but we use it
> heavily since our application is very loop-intensive. A continual source
> of bugs is looping over a null collection. Say BAR is null. The following
> will crash:
>   (ps (loop :for foo :in bar :do (baz foo)))
> ...because the code expands to include (length bar). This is particularly
> annoying if you're running the code in CL as well, where looping over
> a null collection is just fine.
> We've been dealing with this like so:
>   (ps (loop :for foo :in (or bar '()) :do (baz foo)))
> ... but this is ugly and more importantly very brittle, since it means
> tracking down every crash case-by-case.
> My question: what does everybody think about making PS LOOP
> accept null collections and just do nothing in that case? Something
> about this bothers me. It goes against the philosophy of PS, which
> declines to pretend that JS arrays are Lisp lists (something I fully agree
> with). But I'm inclined to make an exception in this case because the
> strict approach is so painful. And this of course raises the
> analogous question for any PS versions of MAPCAR, MAPC, etc.
> Daniel
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