[Edinburgh-pm] parallel semantics in perl

Aaron Crane perl at aaroncrane.co.uk
Tue Dec 14 02:49:41 PST 2010

Wim Vanderbauwhede <wim.vanderbauwhede at gmail.com> wrote:
> Maybe I could have a pragma "seq"
> instead, as the default would be to try parallel evaluation.
> Pragmas are lexically scoped, aren't they?

Yes; `perldoc perlpragma`.  (Technically, you can write pragma-like
modules which have a non-lexical effect; but I don't think that's what
you were asking.)

> So I could write
> { use seq qw<foreach>;
>    foreach my $i (1..$n) {
>    }
> }
> and only the foreach in that block will be sequential, right?


> This is not quite the same as I have in my current language, I would
> need attribute support on blocks for that -- anyone ever mentioned
> something like that? That would be ideal:
>    foreach my $i (1..$n) :seq {
>    }
> would be neat, but AFAICT these are only supported on subs...

I don't think I've ever seen a proposal for that.  It looks initially
tempting; there's certainly a syntactic slot available where an
attribute-like thing could go.  It's hard to see how you'd make it
work for statement-modifier `for`, though.

> I didn't know about qw<:all>. Are there others apart from :all, (I
> just came across a mention of ":default") or can I implement my own?

If you implement your own `import` routine, you can treat the
arguments to `use` in any way that makes sense for your module; see
`perldoc perlmod`.  Colon-prefixed words are treated by Exporter as
identifying "tags" for arbitrary sets of names; `:all` is
conventionally a tag identifying everything you're willing to export.

Aaron Crane ** http://aaroncrane.co.uk/

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