2011/03/04

Carp from somewhere else

Carp provides two main functions, carp and croak, as replacements for core Perl's warn and die. The functions from Carp report the file and line number slightly differently from core Perl; walking up the callstack looking for a different package name. The idea being these are used to report errors in values passed in to the function, typically bad arguments from the caller.

These functions use the dynamic callstack (as provided by caller()) at the time the message is warned (or thrown) to create their context. One scenario where this does not lead to sensible results is when the called function is internally implemented using a closure via some other package again.

Having thought about this one a bit, it seems what's required is to split collecting the callstack information, from creating the message. Collect the information in one function call, and pass it into the other.

This would be useful in CPS for example. Because the closures used in CPS::kseq or kpar aren't necessarily invoked during the dynamic scope of the function that lexically contains the code, a call to croak may not be able to infer the calling context. Even if they are, the presence of stack frames in the CPS package would confuse croak's scanning of the callstack. Instead, it would be better to capture the calling context using whence, and pass it into whoops if required for generating a message.

For example, this from IO::Async::Connector:
sub connect
{
my ( %params ) = @_;
...

my $where = whence;

kpar(
sub {
my ( $k ) = @_;
if( exists $params{host} and exists $params{service} ) {
my $on_resolve_error = $params{on_resolve_error} or whoops $where, "Expected 'on_resolve_error' callback";
...
}
These functions would be a fairly simple replacement of carp and croak; capture the callsite information at entry to a function, and pass it to the message warning function.

It does occur to me though, the code will be small and self-contained, and not specific to CPS. I think it ought to live in its own module somewhere - any ideas on a name?

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