On Fri, Jan 05, 2007 at 08:47:18PM +0000, Luke Palmer wrote:
: I propose that we remove the following two lines from the smart match
: table in S03:
:
: Hash Any hash entry existence exists $_{$x}
: Array Any array contains item* any($_) === $x
:
: These are the two lines with Any on the right side. I want to remove
: these so that we can have an extensible ~~ operator.
:
: You can think of $x ~~ $y as saying "$x matches $y", i.e. $y is some
: data pattern and we are asking if $x conforms to it. I want to be
: able to add my own kinds of patterns, and entries with Any in the
: right side undermine this.
It's much worse than just those two entries, because any entry not marked
with * is currently considered reversible. However, I think you're
the path toward sanity.
: Why? Let's say I created a pattern which implemented another textual
: matcher alongside Perl's regexes, which was more convenient for some
: things (I shouldn't have to convince you that this is possible and
: useful). Let's call it Irregex. I want it to work just like Perl's
: regexes; i.e. I want it so that if you ask whether an array matches
: it, it will match against the elements of the array as though they
: were characters. So I just overload ~~ to work on (Array, Irregex)
: and that overrules (Array, Any) so there is no MMD ambiguity.
:
: However! Let's say I'm using a payroll library (or something, sorry
: for the mundane examples; I'm trying to keep it realistic) which sends
: a notification if any user's status matches a pattern, and it is
: implemented like so:
:
: method notify(Any|Junction $pattern) {
: if @.users>>.<status> ~~ $pattern {
: # send notification
: }
: }
:
: This is perfectly fine, elegant code given the ~~ table. But it
: breaks since we have added our new pattern type! It doesn't check if
: any user's status matches, it checks if the users' statuses'
: concatenated considered as atoms matches, totally wrong!
I'm not so worried about extensibility as I am about consistency between
compile time and run time, such that the optimizer can optimize without
surprising the user. But this also argues against any anonymous Any on
the right. If all the when expressions are Num, the optimizer wants to
be able to evaluate the given in a numeric context whether or not you
put + on the front of it.
: So, I'm essentially asking for ~~ to be singly-dispatched based on its
: right argument, which gets to pick how to interpret the left one. In
: order to encourage this usage, I propose a design like so:
:
: role Pattern {
: method match($item) {...}
: }
: # Code, Bool, Undef, Whatever, Num, Junction, Str, Hash, ... ...
: all do Pattern
:
: sub *infix:<~~> ($item, Pattern $pat) {
: $pat.match($item);
: }
One little problem here is that .match is currently defined the reverse
of that for the method form of m//.
: So to add a new pattern, you just implement the Pattern role, and you are
: safe.
:
: If you are very attached do those two rows of the smart match table,
: we can reimplement them as Pattern objects, at the expense of some
: unreadability:
:
: given @array {
: when 42 { ... }
: }
:
: Becomes:
:
: given @array {
: when contains 42 { ... }
: }
:
: etc.
Rather than special casing array matching, I'd say we want a general form
for explicitly reversing the figure/ground. If .match demands a pattern on
the left, then
given @array {
when .match(42) {...}
}
will already reverse the test to mean 42 ~~ @array. As I say, we need to
figure out what to do with "foo".match(/bar/) though.
Larry
