On Sun, Sep 6, 2009 at 06:15, Philip Potter<philip.g.pot...@gmail.com> wrote:
> Dear all,
>
> I have a subroutine which takes an indirect filehandle and a Graph
> object as its two inputs. I'd like to check that I've been given the
> correct types in the subroutine, so that if I get it wrong I get a
> sensible error message. Here's what I've tried so far:
>
> sub writefsg {
> my ($filename,$graph) = @_;
> if (!ref($graph) || !$graph->isa('Graph')) { croak
> q{Pgp::Graph::FSG::writefsg didn't get a Graph object}; }
> # do work
> return;
> }
>
> This test works if there is no second argument or if it is an integer,
> string, or object reference; but if I pass in an array reference I get
> "'Can't call method "isa" on unblessed reference". Technically this
> does the right thing -- dies -- but reports the wrong error message,
> so is somewhat suboptimal.
snip
It sounds like you want the blessed function from [Scalar::Util][1]:
croak "Pgp::Graph::FSG::writefsg didn't get a Graph object"
unless blessed $graph and $graph->isa('Graph');
snip
> What is the idiomatic perl for checking that an argument is what it
> should be? Does it vary depending on whether you expect a number, a
> string, an array reference, a hash reference, a filehandle, or an
> object reference? Where can I find a discussion of passing subroutine
> arguments, including discussion of checking number and type?
snip
If you are using an OO style then you should really look into
[Moose][2]. It is Perl 6's object module backported to Perl 5. I
would also suggest using [MooseX::Declare][3] which changes the way
Perl 5 parses code to allow a simple declarative style of creating
classes:
#warning untested code to give you an idea of what it looks like
class Point::2D {
has x => (is => "rw", isa => "Int", required => 1);
has y => (is => "rw", isa => "Int", required => 1);
method _find_square (Point::2D $other) {
return
($self->x - $other->x) ** 2 +
($self->y - $other->y) ** 2;
}
method find_distance (Point::2D $other) {
return sqrt $self->_find_square($other)
}
}
class Point::3D extends Point::2D {
has z => (is => "rw", isa => "Int", required => 1);
#wrap Point::2D's _find_square and add our functionality
around _find_square (Point::3D $other) {
return $self->$orig + ($self->z - $other->z) ** 2;
}
}
1. http://perldoc.perl.org/Scalar/Util.html#blessed-EXPR
2. http://search.cpan.org/dist/Moose/lib/Moose/Manual.pod
3. http://search.cpan.org/dist/MooseX-Declare/lib/MooseX/Declare.pm
--
Chas. Owens
wonkden.net
The most important skill a programmer can have is the ability to read.
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