X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlsub.pod;h=ce3b120633e87370984d99a71e034999785b5f78;hb=6f12eb6d2a1dfaf441504d869b27d2e40ef4966a;hp=b929c988d6901d595b7e91e1fdd74deb72f58ccb;hpb=e855abcaff9a6a855a1a816cf0034aa0ebf64476;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlsub.pod b/pod/perlsub.pod index b929c98..ce3b120 100644 --- a/pod/perlsub.pod +++ b/pod/perlsub.pod @@ -169,7 +169,7 @@ Do not, however, be tempted to do this: Like the flattened incoming parameter list, the return list is also flattened on return. So all you have managed to do here is stored -everything in C<@a> and made C<@b> an empty list. See +everything in C<@a> and made C<@b> empty. See L for alternatives. A subroutine may be called using an explicit C<&> prefix. The @@ -220,9 +220,9 @@ Synopsis: my @oof = @bar; # declare @oof lexical, and init it my $x : Foo = $y; # similar, with an attribute applied -B: The use of attribute lists on C declarations is -experimental. This feature should not be relied upon. It may -change or disappear in future releases of Perl. See L. +B: The use of attribute lists on C declarations is still +evolving. The current semantics and interface are subject to change. +See L and L. The C operator declares the listed variables to be lexically confined to the enclosing block, conditional (C), @@ -327,12 +327,12 @@ the scope of $answer extends from its declaration through the rest of that conditional, including any C and C clauses, but not beyond it. -B None of the foregoing text applies to C, -C or C modifiers appended to simple statements. Such -modifiers are not control structures and have no effect on scoping. The -use of such modifiers in conjunction with C is best avoided, as it may -have unexpected effects. A future release of Perl may define precise -semantics for constructs such as C. +B The behaviour of a C statement modified with a statement +modifier conditional or loop construct (e.g. C) is +B. The value of the C variable may be C, any +previously assigned value, or possibly anything else. Don't rely on +it. Future versions of perl might do something different from the +version of perl you try it out on. Here be dragons. The C loop defaults to scoping its index variable dynamically in the manner of C. However, if the index variable is @@ -563,6 +563,13 @@ hash to some other implementation: } [..%ahash back to its initial tied self again..] +B The code example above does not currently work as described. +This will be fixed in a future release of Perl; in the meantime, avoid +code that relies on any particular behaviour of localising tied arrays +or hashes (localising individual elements is still okay). +See L for more +details. + As another example, a custom implementation of C<%ENV> might look like this: @@ -615,14 +622,15 @@ types is subject to change in future. =head2 Lvalue subroutines -B: Lvalue subroutines are still experimental and the implementation -may change in future versions of Perl. +B: Lvalue subroutines are still experimental and the +implementation may change in future versions of Perl. It is possible to return a modifiable value from a subroutine. To do this, you have to declare the subroutine to return an lvalue. my $val; sub canmod : lvalue { + # return $val; this doesn't work, don't say "return" $val; } sub nomod { @@ -648,6 +656,39 @@ and in: all the subroutines are called in a list context. +=over 4 + +=item Lvalue subroutines are EXPERIMENTAL + +They appear to be convenient, but there are several reasons to be +circumspect. + +You can't use the return keyword, you must pass out the value before +falling out of subroutine scope. (see comment in example above). This +is usually not a problem, but it disallows an explicit return out of a +deeply nested loop, which is sometimes a nice way out. + +They violate encapsulation. A normal mutator can check the supplied +argument before setting the attribute it is protecting, an lvalue +subroutine never gets that chance. Consider; + + my $some_array_ref = []; # protected by mutators ?? + + sub set_arr { # normal mutator + my $val = shift; + die("expected array, you supplied ", ref $val) + unless ref $val eq 'ARRAY'; + $some_array_ref = $val; + } + sub set_arr_lv : lvalue { # lvalue mutator + $some_array_ref; + } + + # set_arr_lv cannot stop this ! + set_arr_lv() = { a => 1 }; + +=back + =head2 Passing Symbol Table Entries (typeglobs) B: The mechanism described in this section was originally @@ -727,7 +768,7 @@ table entries: sub ioqueue { local (*READER, *WRITER); # not my! - pipe (READER, WRITER); or die "pipe: $!"; + pipe (READER, WRITER) or die "pipe: $!"; return (*READER, *WRITER); } ($head, $tail) = ioqueue(); @@ -1319,7 +1360,7 @@ parsed and invoked: use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad'; For further details on attribute lists and their manipulation, -see L. +see L and L. =head1 SEE ALSO