Some perplexities on S06
Today I was reading S06 from
http://dev.perl.org/perl6/doc/design/syn/S06.html, and I have some
perplexities on what's written there.
1) it's written:
$pair = :when;
doit $pair,1,2,3; # always a positional arg
doit *$pair,1,2,3; # always a named arg
but also:
To pass pairs out of hash without their being interpreted as named
parameters, use
doit %hash:p,1,2,3;
doit %hash{'b'}:p,1,2,3;
instead.
I interpret the last sentence as meaning that:
$hash=:a<2>; doit %hash;
would be equivalent to:
doit :a<2>
which contradicts what is written earlier... I'd assume I'd have to
write:
doit *(%hash);
for the pair to be used as a named argument. What am I missing?
2) Consider:
doit %*hash; # 1 arg, the global hash
doit *%hash; # n args, the key-value pairs in the hash
I think I'll use a lot of C-t while writing Perl6 code...
3) What does the second line in:
push @array, :a<1>;
say *pop(@array);
mean? I'd parse it as 'say (*pop)(@array)', that is, call the global
'pop' subroutine. But the text around this example seems to imply that
it is to be parsed as 'say *(pop(@array))', i.e. execute 'pop' and splat
the result. What gives?
4) It's written:
(Conjectural: Within the body you may also use exists on the
parameter name to determine whether it was passed. Maybe this will have
to be restricted to the ? form, unless we're willing to admit that a
parameter could be simultaneously defined and non-existant.)
Ok, this is 'conjectural', but... if i don't pass a parameter in an
invocation, and that parameter had a defined default, that it would be
both defined and non-passed... here I'm sure I'm missing something
5) while talking about pipes, it says that
(0..2; 'a'..'c') ==> my @;tmp;
for @;tmp.zip { say }
produces
[0,'a']
[1,'b']
etc. Right. But then it says:
for @;tmp.each -> $i, $a { say "$i: $a" }
as far as I understand, this should say
0: undef
'a': undef
etc., since 'each' is visiting the multidimensional array 'by rows',
and producing 1 value at a time, which when bound to the 2-ary signature
would leave the $a parameter without a value, and so undef (I'm assuming
that pointy sub positional parameters are implicitly optional, otherwise
that statement would be a run-time error). I'm pretty sure I don't know
what 'each' does...
6) It's written:
Every lexical scope gets its own implicitly declared @; variable,
which is the default receiver.
To me, that is a variable with a null name ('@;' being a twigil).
Should it not be @;_ ?
I'll continue reading...
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MMD and redefinition
In Perl5, if I do:
sub foo {return 1}
sub foo {return 2}
print foo();
I get a redefinition error, and a '2' on STDOUT. Can I assume this will
be the same in Perl6? i.e. can I write a test for pugs to check this?
Moreover:
sub foo(Num $a) {return 1}
sub foo(Str $a) {return 2}
print foo(1),foo('a');
should have the same effect? Printing 2 twice? I don't expect MMD on
non-multi subs.
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(multi)subroutine names
Say I have:
multi sub foo(Array $a,Int $b) {...}
multi sub foo(Hash %a, Int $b) {...}
and I want to (distinctly) wrap each multisub, say for testing, or AOP,
or whatever. How do I get the two different code references? As far as i
can gather from the Apocalipses and Synopses, there should be a way to
express the "long name", and use that to get the ref. Is there any
proposed syntax for this? (I ask about syntax because the semantic seems
pretty clear...)
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Re: (multi)subroutine names
Rod Adams wrote:
> It used to be
>
>&foo
>&foo
>
> but I seem to recall that there was a mild change that occurred. Or
> maybe I'm thinking about the adding of the colon for operators. I'm not
> certain, but it's something very close to the above.
Well, it doesn't seem ambiguous to me. Operators would be in the form
&infix:<-> . But then, I don't design grammars... If enough
people think this syntax could work, I'll add "unspecced" tests to pugs
for it.
> In my mind, the more interesting question is what does &foo without the
> <> specifiers return when foo is multi? I see the following three options:
>
> 1) undef/error, since there's no single sub called foo, and multi's need
> a postfix <>.
At first I thought this was the right way, but then...
> 2) the name 'foo', and then performs in effect call by name when
> deferenced, using the dereferencing scope to determine which flavors of
> foo are available at this time, and dispatching accordingly.
>
> 3) a dispatch table of all the foo's currently in scope at the time the
> reference is made.
...one of these is actually necessary. Example:
- I write a module that exports a 'foo' (a sub)
- You use my module, and use that '&foo' to get a ref, which you
subsequently use as you please
- I then change my module so that 'foo' is a multi-sub (this is an
implementation detail, not an API change)
- Your code *must not* break
so we can't return undef. Your #3 seem useful (for introspection, at
least), but what about:
multi sub foo(Array $a) {return 1}
multi sub foo(Hash $a) {return 2}
my $foo_A_H=&foo;
multi sub foo(Str $a) {return 3}
print $foo_A_H.('a');
should that print 3? Or coerce 'a' into ['a'] and print 1?
What about (thinking aloud):
my $foo_static=&foo;
my $foo_dynamic:=&foo;
where the first one has "snapshot" semantic, as in your #3, and the
second one has 'name' semantic, as in your #2, in the same way that:
my $a=1;
my $b=$a;
my $c:=$a;
$a=2;
print $b,$c;
prints 1 2 ?
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