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-rw-r--r--pod/perldiag.pod51
1 files changed, 28 insertions, 23 deletions
diff --git a/pod/perldiag.pod b/pod/perldiag.pod
index 603dfc8812..e2728d1798 100644
--- a/pod/perldiag.pod
+++ b/pod/perldiag.pod
@@ -4350,26 +4350,35 @@ instance. This is almost always a typographical error. Note that the
earlier variable will still exist until the end of the scope or until
all closure referents to it are destroyed.
-=item Variable "%s" may be unavailable
+=item Variable "%s" is not available
-(W closure) An inner (nested) I<anonymous> subroutine is inside a
-I<named> subroutine, and outside that is another subroutine; and the
-anonymous (innermost) subroutine is referencing a lexical variable
-defined in the outermost subroutine. For example:
+(W closure) During compilation, an inner named subroutine or eval is
+attempting to capture an outer lexical that is not currently available.
+This can be happen for one of two reasons. First, the outer lexical may be
+declared in an outer anonymous subroutine that has not yet been created.
+(Remember that named subs are created at compile time, while anonymous
+subs are created at run-time. For example,
- sub outermost { my $a; sub middle { sub { $a } } }
+ sub { my $a; sub f { $a } }
-If the anonymous subroutine is called or referenced (directly or
-indirectly) from the outermost subroutine, it will share the variable as
-you would expect. But if the anonymous subroutine is called or
-referenced when the outermost subroutine is not active, it will see the
-value of the shared variable as it was before and during the *first*
-call to the outermost subroutine, which is probably not what you want.
+At the time that f is created, it can't capture the current value of $a,
+since the anonymous subroutine hasn't been created yet. Conversely,
+the following won't give a warning since the anonymous subroutine has by
+now been created and is live:
-In these circumstances, it is usually best to make the middle subroutine
-anonymous, using the C<sub {}> syntax. Perl has specific support for
-shared variables in nested anonymous subroutines; a named subroutine in
-between interferes with this feature.
+ sub { my $a; eval 'sub f { $a }' }->();
+
+The second situation is caused by an eval accessing a variable that has
+gone out of scope, for example,
+
+ sub f {
+ my $a;
+ sub { eval '$a' }
+ }
+ f()->();
+
+Here, when the '$a' in the eval is being compiled, f() is not currently being
+executed, so its $a is not available for capture.
=item Variable syntax
@@ -4380,22 +4389,18 @@ Perl yourself.
=item Variable "%s" will not stay shared
(W closure) An inner (nested) I<named> subroutine is referencing a
-lexical variable defined in an outer subroutine.
+lexical variable defined in an outer named subroutine.
-When the inner subroutine is called, it will probably see the value of
+When the inner subroutine is called, it will see the value of
the outer subroutine's variable as it was before and during the *first*
call to the outer subroutine; in this case, after the first call to the
outer subroutine is complete, the inner and outer subroutines will no
longer share a common value for the variable. In other words, the
variable will no longer be shared.
-Furthermore, if the outer subroutine is anonymous and references a
-lexical variable outside itself, then the outer and inner subroutines
-will I<never> share the given variable.
-
This problem can usually be solved by making the inner subroutine
anonymous, using the C<sub {}> syntax. When inner anonymous subs that
-reference variables in outer subroutines are called or referenced, they
+reference variables in outer subroutines are created, they
are automatically rebound to the current values of such variables.
=item Version number must be a constant number