On 1/13/06, Dave Whipp <[EMAIL PROTECTED]> wrote:
> I'm trying to work out if there's a clever perl6 way to write this using
> pattern matching:
>
> for @*ARGV -> "-f", $filename {
> $filename .= absolute_filename;
> }
There is, but it's a different kind of pattern matching:
if @*ARGV ~~ / <,> -f <,> (.*?) <,> / {
$0 .= absolute_filename;
}
Of course, that only works if you're allowed to modify the source
through the $# variables. I think you are, but I'm not certain.
This is a good example of how silly the current array matching
semantics are. You'd get a match for this call, because of
"carelessness" in writing the rule.
davesprog foo - f file
Because elements are implicitly concatenated together. The "correct"
way to write the rule is:
/ <,> - <!,> f <,> (.*?) <,> /
Isn't that dumb?
Here's what I counterpropose: matching against arrays treats each
element of the array as a single character. <()>-assertions make the
current character their topic. There is no <,>; it's implicit in the
character-element unification. Here's the new regex:
/ <( $_ eq '-f' )>. (.) /
This method makes more sense for a list of tokens, too.
We could shuffle around the special assertions to give shorthands for
these presumably common constructs:
<( ... )>.
<( $_ eq ... )>.
<( $_ ~~ /.../ )>.
The last one is the most important. It allows you to connect regexes
on a stream to regexes on an element of the stream. Possible
shorthands for those, respectively:
<+( ... )> # treating an assertion as a char class
<'...'> # hmm, already takes, maybe we could
# make a special case because it
# doesn't make much sense for
# nonstrings
</.../> # obvious
> Would this actually work, or would it stop at the first elem that
> doesn't match ("-f", ::Item)?
If by "stop" you mean "die", yes it would stop.
> Is there some way to associate alternate codeblocks for different
> patterns (i.e. local anonymous MMD)?
As Austin points out, that's called "given". I'll also point out that
if we have lookahead in "for" (a feature that I think is best reserved
for third-party modules):
for @*ARGV -> $cur, ?$next is rw {
if $cur eq '-f' {
$next .= absolute_filename;
}
}
Which isn't as slick as a pattern matching approach, but it gets the
job done without having to use indices.
Luke
