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P5caller cpan:ELIZABETH last updated on 2018-05-19


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P5caller - Implement Perl 5's caller() built-in


use P5caller;

sub foo { bar }
sub bar { say caller[3] }        # foo

sub baz { say caller(:scalar) }  # GLOBAL
my $package = caller


This module tries to mimic the behaviour of the caller function of Perl 5 as closely as possible.


caller EXPR
caller  Returns the context of the current pure perl subroutine call. In
        scalar context, returns the caller's package name if there is a
        caller (that is, if we're in a subroutine or "eval" or "require")
        and the undefined value otherwise. caller never returns XS subs
        and they are skipped. The next pure perl sub will appear instead
        of the XS sub in caller's return values. In list context, caller

            # 0         1          2
            ($package, $filename, $line) = caller;

        With EXPR, it returns some extra information that the debugger
        uses to print a stack trace. The value of EXPR indicates how many
        call frames to go back before the current one.

            #  0         1          2      3            4
            ($package, $filename, $line, $subroutine, $hasargs,

            #  5          6          7            8       9         10
            $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
             = caller($i);

        Here, $subroutine is the function that the caller called (rather
        than the function containing the caller). Note that $subroutine
        may be "(eval)" if the frame is not a subroutine call, but an
        "eval". In such a case additional elements $evaltext and
        $is_require are set: $is_require is true if the frame is created
        by a "require" or "use" statement, $evaltext contains the text of
        the "eval EXPR" statement. In particular, for an "eval BLOCK"
        statement, $subroutine is "(eval)", but $evaltext is undefined.
        (Note also that each "use" statement creates a "require" frame
        inside an "eval EXPR" frame.) $subroutine may also be "(unknown)"
        if this particular subroutine happens to have been deleted from
        the symbol table. $hasargs is true if a new instance of @_ was set
        up for the frame. $hints and $bitmask contain pragmatic hints that
        the caller was compiled with. $hints corresponds to $^H, and
        $bitmask corresponds to "${^WARNING_BITS}". The $hints and
        $bitmask values are subject to change between versions of Perl,
        and are not meant for external use.

        $hinthash is a reference to a hash containing the value of "%^H"
        when the caller was compiled, or "undef" if "%^H" was empty. Do
        not modify the values of this hash, as they are the actual values
        stored in the optree.

        Furthermore, when called from within the DB package in list
        context, and with an argument, caller returns more detailed
        information: it sets the list variable @DB::args to be the
        arguments with which the subroutine was invoked.

        Be aware that the optimizer might have optimized call frames away
        before "caller" had a chance to get the information. That means
        that caller(N) might not return information about the call frame
        you expect it to, for "N > 1". In particular, @DB::args might have
        information from the previous time "caller" was called.

        Be aware that setting @DB::args is best effort, intended for
        debugging or generating backtraces, and should not be relied upon.
        In particular, as @_ contains aliases to the caller's arguments,
        Perl does not take a copy of @_, so @DB::args will contain
        modifications the subroutine makes to @_ or its contents, not the
        original values at call time. @DB::args, like @_, does not hold
        explicit references to its elements, so under certain cases its
        elements may have become freed and reallocated for other variables
        or temporary values. Finally, a side effect of the current
        implementation is that the effects of "shift @_" can normally be
        undone (but not "pop @_" or other splicing, and not if a reference
        to @_ has been taken, and subject to the caveat about reallocated
        elements), so @DB::args is actually a hybrid of the current state
        and initial state of @_. Buyer beware.


In Perl 5, caller can return an 11 element list. In the Perl 6 implementation only the first 4 elements are the same as in Perl 5: package, filename, line, subname. The fifth element is actually the Sub or Method object and as such provides further introspection possibilities not found in Perl 5.

As there is no such thing as scalar or list context in Perl 6, one must specify a :scalar named parameter to emulate the scalar context return value.


Elizabeth Mattijsen

Source can be located at: . Comments and Pull Requests are welcome.


Copyright 2018 Elizabeth Mattijsen

Re-imagined from Perl 5 as part of the CPAN Butterfly Plan.

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.