could 'given' blocks have a return value?
Mark Stosberg
Sometimes I use 'given' blocks to set a value. To save repeating myself
on the right hand side of the given block, I found I kept want to do this:
my $foo = given { }
...and have whatever value that was returned from when {} or default {}
populate $foo.
It turns out pugs already allow this, through the trick of wrapping the
given block in an anonymoose sub...which is then immediately executed:
my $rm = sub { given $rm_param {
when Code { $rm_param(self) }
when Hash { %rm_param<run_mode> }
default { self.query.param($rm_param) }
}}();
Not only do you get implicit matching on the left side, you get implicit
return values on the right!
I'd just like to be able to clean that up a little to:
my $rm = given $rm_param {
when Code { $rm_param(self) }
when Hash { %rm_param<run_mode> }
default { self.query.param($rm_param) }
};
Mark
Jonathan Lang
>
> Sometimes I use 'given' blocks to set a value. To save repeating myself
> on the right hand side of the given block, I found I kept want to do this:
>
> my $foo = given { }
>
> ...and have whatever value that was returned from when {} or default {}
> populate $foo.
Isn't it still the case that the last expression evaluated within a
closure is returned by the closure? And isn't a given block just a
fancy kind of closure?
The question is whether or not a code block can be used where the
parser expects an expression; for instance, could one say:
my $foo = if condition {"BAR"} else {"BAZ"};
?
I'm no expert, but it occurs to me that allowing this could be a
parsing nightmare.
ISTR a programming construct along the lines of "eval" that is
effectively shorthand for "sub { ... }()".
> It turns out pugs already allow this, through the trick of wrapping the
> given block in an anonymoose sub...which is then immediately executed:
>
> my $rm = sub { given $rm_param {
> when Code { $rm_param(self) }
> when Hash { %rm_param<run_mode> }
> default { self.query.param($rm_param) }
> }}();
>
> Not only do you get implicit matching on the left side, you get implicit
> return values on the right!
>
> I'd just like to be able to clean that up a little to:
>
> my $rm = given $rm_param {
> when Code { $rm_param(self) }
> when Hash { %rm_param<run_mode> }
> default { self.query.param($rm_param) }
> };
So what happens if you forget to include a default in the given?
--
Jonathan "Dataweaver" Lang
Agent Zhang
>
> Sometimes I use 'given' blocks to set a value. To save repeating myself
> on the right hand side of the given block, I found I kept want to do this:
>
> my $foo = given { }
>
According to S04, given {} is at statement level, so you can't use it
directly as an expression. But Perl 6 always allow you to say
my $foo = do given {...}
As well as
my $foo = do if foo {...} else {...}
>
> my $rm = sub { given $rm_param {
> when Code { $rm_param(self) }
> when Hash { %rm_param<run_mode> }
> default { self.query.param($rm_param) }
> }}();
>
Apparently ``do'' is a simplified version for this. :)
Cheers,
Agent
Trey Harris
> my $rm = sub { given $rm_param {
> when Code { $rm_param(self) }
> when Hash { %rm_param<run_mode> }
> default { self.query.param($rm_param) }
> }}();
This is eerily like Contextual::Return, which made me wonder if it's even
required in Perl 6.
Obviously we can do
return do given want {
when :($) { ... }
...
};
But "return do given want" flows so badly, I desperately want some sugar
for this. Is there some and I missed it?
Trey
Mark Stosberg
>
> According to S04, given {} is at statement level, so you can't use it
> directly as an expression. But Perl 6 always allow you to say
>
> my $foo = do given {...}
>
> As well as
>
> my $foo = do if foo {...} else {...}
I confirmed this both work now with pugs!
I think the 'do given' case is useful enough to document
more officially along with 'given'.
If you are thinking about the if/else case, ?? ... !! would be a
simpler way to write it.
Mark
Damian Conway
> This is eerily like Contextual::Return, which made me wonder if it's
> even required in Perl 6.
>
> Obviously we can do
>
> return do given want {
> when :($) { ... }
> ...
> };
>
> But "return do given want" flows so badly, I desperately want some sugar
> for this. Is there some and I missed it?
No, the sugariest you'll find at the moment is probably:
return want.rw ?? $lvalue
:: want.count == 2 ?? (7,11)
:: want.item ?? 42
:: want.list ?? 1..10
:: die "Bad context;
but that still doesn't have the Awesome Power of C::R. ;-)
The simple answer is that C::R is probably the first module I will port to
Perl 6 (since it's the module I find myself using the most nowadays). The Perl
6 version will be cleaner and more efficient that the Perl 5 version, since
I'll be able to take advantage of macros.
Damian
Juerd
> return want.rw ?? $lvalue
> :: want.count == 2 ?? (7,11)
> :: want.item ?? 42
> :: want.list ?? 1..10
> :: die "Bad context;
s:g/::/!!/ # :)
Juerd
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