From: Yuval Kogman <nothingmuch@woobling.org>
Date: Fri, 11 Sep 2009 17:52:41 +0000 (+0900)
Subject: Restore the value of $@ from before the local
X-Git-Tag: Try-Tiny-0.03~12
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=511c05caecd550023e029dcfdeb3995c64def4d9;p=p5sagit%2FTry-Tiny.git

Restore the value of $@ from before the local

This allows exception objects to be constructed with the previous value
of $@, but that behavior would rely on Try::Tiny
---

diff --git a/lib/Try/Tiny.pm b/lib/Try/Tiny.pm
index dd6d466..e3fdf8d 100644
--- a/lib/Try/Tiny.pm
+++ b/lib/Try/Tiny.pm
@@ -23,6 +23,9 @@ sub try (&;$) {
 	# to $failed
 	my $wantarray = wantarray;
 
+	# save the value of $@ so we can set $@ back to it in the begining of the eval
+	my $prev_error = $@;
+
 	my ( @ret, $error, $failed );
 
 	# FIXME consider using local $SIG{__DIE__} to accumilate all errors. It's
@@ -37,6 +40,7 @@ sub try (&;$) {
 		# failed will be true if the eval dies, because 1 will not be returned
 		# from the eval body
 		$failed = not eval {
+			$@ = $prev_error;
 
 			# evaluate the try block in the correct context
 			if ( $wantarray ) {
@@ -180,6 +184,13 @@ not yet handled.
 C<$@> must be properly localized before invoking C<eval> in order to avoid this
 issue.
 
+More specifically, C<$@> is clobbered at the begining of the C<eval>, which
+also makes it impossible to capture the previous error before you die (for
+instance when making exception objects with error stacks).
+
+For this reason C<try> will actually set C<$@> to its previous value (before
+the localization) in the begining of the C<eval> block.
+
 =head2 Localizing $@ silently masks errors
 
 Inside an eval block C<die> behaves sort of like:
diff --git a/t/basic.t b/t/basic.t
index ebc4106..7f1471c 100644
--- a/t/basic.t
+++ b/t/basic.t
@@ -3,7 +3,7 @@
 use strict;
 #use warnings;
 
-use Test::More tests => 19;
+use Test::More tests => 21;
 
 BEGIN { use_ok 'Try::Tiny' };
 
@@ -39,6 +39,8 @@ sub throws_ok (&$$) {
 }
 
 
+my $prev;
+
 lives_ok {
 	try {
 		die "foo";
@@ -125,3 +127,25 @@ sub Evil::new { bless { }, $_[0] }
 	is( $@, "magic", '$@ untouched' );
 	is( $_, "other magic", '$_ untouched' );
 }
+
+{
+	my $caught;
+
+	{
+		local $@;
+
+		eval { die "bar\n" };
+
+		is( $@, "bar\n", 'previous value of $@' );
+
+		try {
+			die {
+				prev => $@,
+			}
+		} catch {
+			$caught = $_;
+		}
+	}
+
+	is_deeply( $caught, { prev => "bar\n" }, 'previous value of $@ available for capture' );
+}