How to convert a symbolic reference to a hard reference

princepawn has asked for the wisdom of the Perl Monks concerning the following question:

I am reading in a configuration file which has a string representing the package-qualified name of a DBI exception handler.

DBI does not accept symbolic references for its HandleError attribute and that is what this string is intended to handle.

Therefore, I must give DBI a hard reference and would like to know the most direct way of doing so. Below almost works, but I get a funny error message in my dummy example.

the general idea

*Config::DBI::error_handler = eval "&$symbolic_ref";

DBI->connect($DSN, $User, $Pass, 
      { HandleError => Config::DBI::error_handler } );
[download]

the dummy code

package M;

sub x {
  print "sub __PACKAGE__x";
}

sub y {
  print "sub __PACKAGE__y";
}

1;

package N;

$s = 'M::y';

*M::x = eval "&$s";

M::x();
[download]

but it gives a bizarre errmsg:


 ~/hacks/config-dbi/scripts $ perl symref.pl
Undefined subroutine &main::1 called at symref.pl line 19.
sub __PACKAGE__y        ~/hacks/config-dbi/scripts $
[download]

Comment on How to convert a symbolic reference to a hard reference Select or Download Code

Replies are listed 'Best First'.
Re: How to convert a symbolic reference to a hard reference by broquaint (Abbot) on Nov 06, 2003 at 00:19 UTC
Creating symbolic references to a subroutine is fine, just not accessing them symbolically e.g `use strict; sub foo { print "in foo(@_)"; } my $f = \&{"foo"}; $f->("var"); ## or more succinctly (\&{"foo"})->("temp"); __output__ in foo(var) in foo(temp)` [download] And the bizarre error is due to `eval` trying to execute the return of `M::y()`, which is the return value of `print`. HTH `_________ broquaint`	[reply] [d/l]
Re: How to convert a symbolic reference to a hard reference by blokhead (Monsignor) on Nov 06, 2003 at 00:18 UTC
Close. `&x` does not return a reference to the subroutine at `x`, it returns the result of that subroutine (ignoring prototypes and passing along `@_`). For a reference, you need `\&x`. Changing `M::x = eval "&$s";` [download] to this `M::x = eval "\\&$s";` [download] causes your dummy code to work as expected. blokhead	[reply] [d/l] [select]
Re: How to convert a symbolic reference to a hard reference by revdiablo (Prior) on Nov 06, 2003 at 00:47 UTC
I see your question has been answered directly by a few other fine Monks, but I thought I might add something. If you'd like to get a coderef without using `eval`, you can use `can`. For example: `sub foo { print "Foo!\n"; } my $pkg = "main"; my $sub = "foo"; my $cr = $pkg->can($sub) or die "Can't find '$sub' in package '$pkg'\n"; $cr->();` [download]	[reply] [d/l] [select]
Re: Re: How to convert a symbolic reference to a hard reference by ysth (Canon) on Nov 06, 2003 at 03:24 UTC
Very nice, thanks. You don't even have to split the package off: `$sub = "X::foo"; $cr = main::->can($sub);` [download] You can just use any existing package name. (I say `main::` instead of `main->` in case there is a &main sub.) If you are downvoting this for other than lack of interest, thanks for sending me a message or reply to let me know why so I can do better (or just stay quiet) next time.	[reply] [d/l] [select]
Re: How to convert a symbolic reference to a hard reference by ysth (Canon) on Nov 06, 2003 at 00:26 UTC
Your `eval "&$s"` creates a call to the sub, not a reference to it. The return from the sub is the return value of the print (which is 1 because the print succeeded). Try `eval "\\&$s"` instead, or avoid the eval altogether with `*M:x = do{ no strict 'refs'; \&$s }` If you are downvoting this for other than lack of interest, thanks for sending me a message or reply to let me know why so I can do better (or just stay quiet) next time.	[reply] [d/l] [select]
Re: How to convert a symbolic reference to a hard reference by Roger (Parson) on Nov 06, 2003 at 00:28 UTC
Perhaps the simplest fix would be to change the & to . `M::x = eval "&$s";` [download] change to `M::x = eval "$s";` [download]	[reply] [d/l] [select]
Re: How to convert a symbolic reference to a hard reference by Anonymous Monk on Nov 06, 2003 at 14:28 UTC
This should work just fine. `HandleError => \&$symbolic_ref` [download]	[reply] [d/l]