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diff --git a/Master/tlpkg/tlperl/lib/pods/perlfaq7.pod b/Master/tlpkg/tlperl/lib/pods/perlfaq7.pod
index fcf270d9ce8..35c9330f2dc 100644
--- a/Master/tlpkg/tlperl/lib/pods/perlfaq7.pod
+++ b/Master/tlpkg/tlperl/lib/pods/perlfaq7.pod
@@ -10,7 +10,7 @@ clearly fit into any of the other sections.
=head2 Can I get a BNF/yacc/RE for the Perl language?
There is no BNF, but you can paw your way through the yacc grammar in
-perly.y in the source distribution if you're particularly brave. The
+perly.y in the source distribution if you're particularly brave. The
grammar relies on very smart tokenizing code, so be prepared to
venture into toke.c as well.
@@ -22,201 +22,193 @@ and mirrors."
They are type specifiers, as detailed in L<perldata>:
- $ for scalar values (number, string or reference)
- @ for arrays
- % for hashes (associative arrays)
- & for subroutines (aka functions, procedures, methods)
- * for all types of that symbol name. In version 4 you used them like
- pointers, but in modern perls you can just use references.
+ $ for scalar values (number, string or reference)
+ @ for arrays
+ % for hashes (associative arrays)
+ & for subroutines (aka functions, procedures, methods)
+ * for all types of that symbol name. In version 4 you used them like
+ pointers, but in modern perls you can just use references.
There are a couple of other symbols that
you're likely to encounter that aren't
really type specifiers:
- <> are used for inputting a record from a filehandle.
- \ takes a reference to something.
+ <> are used for inputting a record from a filehandle.
+ \ takes a reference to something.
Note that <FILE> is I<neither> the type specifier for files
-nor the name of the handle. It is the C<< <> >> operator applied
-to the handle FILE. It reads one line (well, record--see
+nor the name of the handle. It is the C<< <> >> operator applied
+to the handle FILE. It reads one line (well, record--see
L<perlvar/$E<sol>>) from the handle FILE in scalar context, or I<all> lines
-in list context. When performing open, close, or any other operation
+in list context. When performing open, close, or any other operation
besides C<< <> >> on files, or even when talking about the handle, do
-I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0,
+I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0,
2)> and "copying from STDIN to FILE".
=head2 Do I always/never have to quote my strings or use semicolons and commas?
Normally, a bareword doesn't need to be quoted, but in most cases
-probably should be (and must be under C<use strict>). But a hash key
+probably should be (and must be under C<use strict>). But a hash key
consisting of a simple word and the left-hand
operand to the C<< => >> operator both
count as though they were quoted:
- This is like this
- ------------ ---------------
- $foo{line} $foo{'line'}
- bar => stuff 'bar' => stuff
+ This is like this
+ ------------ ---------------
+ $foo{line} $foo{'line'}
+ bar => stuff 'bar' => stuff
The final semicolon in a block is optional, as is the final comma in a
-list. Good style (see L<perlstyle>) says to put them in except for
+list. Good style (see L<perlstyle>) says to put them in except for
one-liners:
- if ($whoops) { exit 1 }
- @nums = (1, 2, 3);
+ if ($whoops) { exit 1 }
+ my @nums = (1, 2, 3);
- if ($whoops) {
- exit 1;
- }
+ if ($whoops) {
+ exit 1;
+ }
- @lines = (
- "There Beren came from mountains cold",
- "And lost he wandered under leaves",
- );
+ my @lines = (
+ "There Beren came from mountains cold",
+ "And lost he wandered under leaves",
+ );
=head2 How do I skip some return values?
One way is to treat the return values as a list and index into it:
- $dir = (getpwnam($user))[7];
+ $dir = (getpwnam($user))[7];
Another way is to use undef as an element on the left-hand-side:
- ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+ ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
You can also use a list slice to select only the elements that
you need:
- ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];
+ ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];
=head2 How do I temporarily block warnings?
If you are running Perl 5.6.0 or better, the C<use warnings> pragma
-allows fine control of what warning are produced.
+allows fine control of what warnings are produced.
See L<perllexwarn> for more details.
- {
- no warnings; # temporarily turn off warnings
- $a = $b + $c; # I know these might be undef
- }
+ {
+ no warnings; # temporarily turn off warnings
+ $x = $y + $z; # I know these might be undef
+ }
Additionally, you can enable and disable categories of warnings.
You turn off the categories you want to ignore and you can still
-get other categories of warnings. See L<perllexwarn> for the
+get other categories of warnings. See L<perllexwarn> for the
complete details, including the category names and hierarchy.
- {
- no warnings 'uninitialized';
- $a = $b + $c;
- }
+ {
+ no warnings 'uninitialized';
+ $x = $y + $z;
+ }
If you have an older version of Perl, the C<$^W> variable (documented
in L<perlvar>) controls runtime warnings for a block:
- {
- local $^W = 0; # temporarily turn off warnings
- $a = $b + $c; # I know these might be undef
- }
+ {
+ local $^W = 0; # temporarily turn off warnings
+ $x = $y + $z; # I know these might be undef
+ }
Note that like all the punctuation variables, you cannot currently
use my() on C<$^W>, only local().
=head2 What's an extension?
-An extension is a way of calling compiled C code from Perl. Reading
+An extension is a way of calling compiled C code from Perl. Reading
L<perlxstut> is a good place to learn more about extensions.
=head2 Why do Perl operators have different precedence than C operators?
-Actually, they don't. All C operators that Perl copies have the same
-precedence in Perl as they do in C. The problem is with operators that C
+Actually, they don't. All C operators that Perl copies have the same
+precedence in Perl as they do in C. The problem is with operators that C
doesn't have, especially functions that give a list context to everything
-on their right, eg. print, chmod, exec, and so on. Such functions are
+on their right, eg. print, chmod, exec, and so on. Such functions are
called "list operators" and appear as such in the precedence table in
L<perlop>.
A common mistake is to write:
- unlink $file || die "snafu";
+ unlink $file || die "snafu";
This gets interpreted as:
- unlink ($file || die "snafu");
+ unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or use the
super low precedence C<or> operator:
- (unlink $file) || die "snafu";
- unlink $file or die "snafu";
+ (unlink $file) || die "snafu";
+ unlink $file or die "snafu";
The "English" operators (C<and>, C<or>, C<xor>, and C<not>)
deliberately have precedence lower than that of list operators for
just such situations as the one above.
-Another operator with surprising precedence is exponentiation. It
+Another operator with surprising precedence is exponentiation. It
binds more tightly even than unary minus, making C<-2**2> produce a
-negative not a positive four. It is also right-associating, meaning
+negative four and not a positive one. It is also right-associating, meaning
that C<2**3**2> is two raised to the ninth power, not eight squared.
Although it has the same precedence as in C, Perl's C<?:> operator
-produces an lvalue. This assigns $x to either $a or $b, depending
+produces an lvalue. This assigns $x to either $if_true or $if_false, depending
on the trueness of $maybe:
- ($maybe ? $a : $b) = $x;
+ ($maybe ? $if_true : $if_false) = $x;
=head2 How do I declare/create a structure?
-In general, you don't "declare" a structure. Just use a (probably
-anonymous) hash reference. See L<perlref> and L<perldsc> for details.
+In general, you don't "declare" a structure. Just use a (probably
+anonymous) hash reference. See L<perlref> and L<perldsc> for details.
Here's an example:
- $person = {}; # new anonymous hash
- $person->{AGE} = 24; # set field AGE to 24
- $person->{NAME} = "Nat"; # set field NAME to "Nat"
+ $person = {}; # new anonymous hash
+ $person->{AGE} = 24; # set field AGE to 24
+ $person->{NAME} = "Nat"; # set field NAME to "Nat"
If you're looking for something a bit more rigorous, try L<perltoot>.
=head2 How do I create a module?
-(contributed by brian d foy)
-
-L<perlmod>, L<perlmodlib>, L<perlmodstyle> explain modules
-in all the gory details. L<perlnewmod> gives a brief
-overview of the process along with a couple of suggestions
-about style.
+L<perlnewmod> is a good place to start, ignore the bits
+about uploading to CPAN if you don't want to make your
+module publicly available.
-If you need to include C code or C library interfaces in
-your module, you'll need h2xs. h2xs will create the module
-distribution structure and the initial interface files
-you'll need. L<perlxs> and L<perlxstut> explain the details.
+L<ExtUtils::ModuleMaker> and L<Module::Starter> are also
+good places to start. Many CPAN authors now use L<Dist::Zilla>
+to automate as much as possible.
-If you don't need to use C code, other tools such as
-ExtUtils::ModuleMaker and Module::Starter, can help you
-create a skeleton module distribution.
+Detailed documentation about modules can be found at:
+L<perlmod>, L<perlmodlib>, L<perlmodstyle>.
-You may also want to see Sam Tregar's "Writing Perl Modules
-for CPAN" ( http://apress.com/book/bookDisplay.html?bID=14 )
-which is the best hands-on guide to creating module
-distributions.
+If you need to include C code or C library interfaces
+use h2xs. h2xs will create the module distribution structure
+and the initial interface files.
+L<perlxs> and L<perlxstut> explain the details.
=head2 How do I adopt or take over a module already on CPAN?
-(contributed by brian d foy)
-
-The easiest way to take over a module is to have the current
-module maintainer either make you a co-maintainer or transfer
-the module to you.
+Ask the current maintainer to make you a co-maintainer or
+transfer the module to you.
-If you can't reach the author for some reason (e.g. email bounces),
-the PAUSE admins at modules@perl.org can help. The PAUSE admins
-treat each case individually.
+If you can not reach the author for some reason contact
+the PAUSE admins at modules@perl.org who may be able to help,
+but each case it treated seperatly.
=over 4
=item *
Get a login for the Perl Authors Upload Server (PAUSE) if you don't
-already have one: http://pause.perl.org
+already have one: L<http://pause.perl.org>
=item *
@@ -274,47 +266,47 @@ Closures are most often used in programming languages where you can
have the return value of a function be itself a function, as you can
in Perl. Note that some languages provide anonymous functions but are
not capable of providing proper closures: the Python language, for
-example. For more information on closures, check out any textbook on
-functional programming. Scheme is a language that not only supports
+example. For more information on closures, check out any textbook on
+functional programming. Scheme is a language that not only supports
but encourages closures.
Here's a classic non-closure function-generating function:
- sub add_function_generator {
- return sub { shift() + shift() };
- }
+ sub add_function_generator {
+ return sub { shift() + shift() };
+ }
- $add_sub = add_function_generator();
- $sum = $add_sub->(4,5); # $sum is 9 now.
+ my $add_sub = add_function_generator();
+ my $sum = $add_sub->(4,5); # $sum is 9 now.
The anonymous subroutine returned by add_function_generator() isn't
technically a closure because it refers to no lexicals outside its own
-scope. Using a closure gives you a I<function template> with some
+scope. Using a closure gives you a I<function template> with some
customization slots left out to be filled later.
Contrast this with the following make_adder() function, in which the
returned anonymous function contains a reference to a lexical variable
-outside the scope of that function itself. Such a reference requires
+outside the scope of that function itself. Such a reference requires
that Perl return a proper closure, thus locking in for all time the
value that the lexical had when the function was created.
- sub make_adder {
- my $addpiece = shift;
- return sub { shift() + $addpiece };
- }
+ sub make_adder {
+ my $addpiece = shift;
+ return sub { shift() + $addpiece };
+ }
- $f1 = make_adder(20);
- $f2 = make_adder(555);
+ my $f1 = make_adder(20);
+ my $f2 = make_adder(555);
-Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas
-C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece
+Now C<< $f1->($n) >> is always 20 plus whatever $n you pass in, whereas
+C<< $f2->($n) >> is always 555 plus whatever $n you pass in. The $addpiece
in the closure sticks around.
-Closures are often used for less esoteric purposes. For example, when
+Closures are often used for less esoteric purposes. For example, when
you want to pass in a bit of code into a function:
- my $line;
- timeout( 30, sub { $line = <STDIN> } );
+ my $line;
+ timeout( 30, sub { $line = <STDIN> } );
If the code to execute had been passed in as a string,
C<< '$line = <STDIN>' >>, there would have been no way for the
@@ -328,10 +320,10 @@ This is sometimes used with a BEGIN block in package files to make
sure a variable doesn't get meddled with during the lifetime of the
package:
- BEGIN {
- my $id = 0;
- sub next_id { ++$id }
- }
+ BEGIN {
+ my $id = 0;
+ sub next_id { ++$id }
+ }
This is discussed in more detail in L<perlsub>; see the entry on
I<Persistent Private Variables>.
@@ -342,41 +334,41 @@ This problem was fixed in perl 5.004_05, so preventing it means upgrading
your version of perl. ;)
Variable suicide is when you (temporarily or permanently) lose the value
-of a variable. It is caused by scoping through my() and local()
+of a variable. It is caused by scoping through my() and local()
interacting with either closures or aliased foreach() iterator variables
-and subroutine arguments. It used to be easy to inadvertently lose a
-variable's value this way, but now it's much harder. Take this code:
+and subroutine arguments. It used to be easy to inadvertently lose a
+variable's value this way, but now it's much harder. Take this code:
- my $f = 'foo';
- sub T {
- while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
- }
+ my $f = 'foo';
+ sub T {
+ while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
+ }
- T;
- print "Finally $f\n";
+ T;
+ print "Finally $f\n";
If you are experiencing variable suicide, that C<my $f> in the subroutine
-doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output
-shows that inside the subroutine the value of C<$f> leaks through when it
-shouldn't, as in this output:
+doesn't pick up a fresh copy of the C<$f> whose value is C<'foo'>. The
+output shows that inside the subroutine the value of C<$f> leaks through
+when it shouldn't, as in this output:
- foobar
- foobarbar
- foobarbarbar
- Finally foo
+ foobar
+ foobarbar
+ foobarbarbar
+ Finally foo
The $f that has "bar" added to it three times should be a new C<$f>
C<my $f> should create a new lexical variable each time through the loop.
The expected output is:
- foobar
- foobar
- foobar
- Finally foo
+ foobar
+ foobar
+ foobar
+ Finally foo
=head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
-You need to pass references to these objects. See L<perlsub/"Pass by
+You need to pass references to these objects. See L<perlsub/"Pass by
Reference"> for this particular question, and L<perlref> for
information on references.
@@ -387,30 +379,30 @@ information on references.
Regular variables and functions are quite easy to pass: just pass in a
reference to an existing or anonymous variable or function:
- func( \$some_scalar );
+ func( \$some_scalar );
- func( \@some_array );
- func( [ 1 .. 10 ] );
+ func( \@some_array );
+ func( [ 1 .. 10 ] );
- func( \%some_hash );
- func( { this => 10, that => 20 } );
+ func( \%some_hash );
+ func( { this => 10, that => 20 } );
- func( \&some_func );
- func( sub { $_[0] ** $_[1] } );
+ func( \&some_func );
+ func( sub { $_[0] ** $_[1] } );
=item Passing Filehandles
As of Perl 5.6, you can represent filehandles with scalar variables
which you treat as any other scalar.
- open my $fh, $filename or die "Cannot open $filename! $!";
- func( $fh );
+ open my $fh, $filename or die "Cannot open $filename! $!";
+ func( $fh );
- sub func {
- my $passed_fh = shift;
+ sub func {
+ my $passed_fh = shift;
- my $line = <$passed_fh>;
- }
+ my $line = <$passed_fh>;
+ }
Before Perl 5.6, you had to use the C<*FH> or C<\*FH> notations.
These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles">
@@ -422,34 +414,34 @@ Here's an example of how to pass in a string and a regular expression
for it to match against. You construct the pattern with the C<qr//>
operator:
- sub compare($$) {
- my ($val1, $regex) = @_;
- my $retval = $val1 =~ /$regex/;
- return $retval;
- }
- $match = compare("old McDonald", qr/d.*D/i);
+ sub compare($$) {
+ my ($val1, $regex) = @_;
+ my $retval = $val1 =~ /$regex/;
+ return $retval;
+ }
+ $match = compare("old McDonald", qr/d.*D/i);
=item Passing Methods
To pass an object method into a subroutine, you can do this:
- call_a_lot(10, $some_obj, "methname")
- sub call_a_lot {
- my ($count, $widget, $trick) = @_;
- for (my $i = 0; $i < $count; $i++) {
- $widget->$trick();
- }
- }
+ call_a_lot(10, $some_obj, "methname")
+ sub call_a_lot {
+ my ($count, $widget, $trick) = @_;
+ for (my $i = 0; $i < $count; $i++) {
+ $widget->$trick();
+ }
+ }
Or, you can use a closure to bundle up the object, its
method call, and arguments:
- my $whatnot = sub { $some_obj->obfuscate(@args) };
- func($whatnot);
- sub func {
- my $code = shift;
- &$code();
- }
+ my $whatnot = sub { $some_obj->obfuscate(@args) };
+ func($whatnot);
+ sub func {
+ my $code = shift;
+ &$code();
+ }
You could also investigate the can() method in the UNIVERSAL class
(part of the standard perl distribution).
@@ -464,7 +456,7 @@ In Perl 5.10, declare the variable with C<state>. The C<state>
declaration creates the lexical variable that persists between calls
to the subroutine:
- sub counter { state $count = 1; $counter++ }
+ sub counter { state $count = 1; $count++ }
You can fake a static variable by using a lexical variable which goes
out of scope. In this example, you define the subroutine C<counter>, and
@@ -480,16 +472,16 @@ can access the value (and each time you do, you increment the value).
The data in chunk of memory defined by C<$count> is private to
C<counter>.
- BEGIN {
- my $count = 1;
- sub counter { $count++ }
- }
+ BEGIN {
+ my $count = 1;
+ sub counter { $count++ }
+ }
- my $start = counter();
+ my $start = counter();
- .... # code that calls counter();
+ .... # code that calls counter();
- my $end = counter();
+ my $end = counter();
In the previous example, you created a function-private variable
because only one function remembered its reference. You could define
@@ -502,11 +494,11 @@ function adds to the value and the other simply returns the value.
They can both access C<$count>, and since it has gone out of scope,
there is no other way to access it.
- BEGIN {
- my $count = 1;
- sub increment_count { $count++ }
- sub return_count { $count }
- }
+ BEGIN {
+ my $count = 1;
+ sub increment_count { $count++ }
+ sub return_count { $count }
+ }
To declare a file-private variable, you still use a lexical variable.
A file is also a scope, so a lexical variable defined in the file
@@ -517,47 +509,47 @@ The discussion of closures in L<perlref> may help you even though we
did not use anonymous subroutines in this answer. See
L<perlsub/"Persistent Private Variables"> for details.
-=head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
+=head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
C<local($x)> saves away the old value of the global variable C<$x>
and assigns a new value for the duration of the subroutine I<which is
-visible in other functions called from that subroutine>. This is done
-at run-time, so is called dynamic scoping. local() always affects global
+visible in other functions called from that subroutine>. This is done
+at run-time, so is called dynamic scoping. local() always affects global
variables, also called package variables or dynamic variables.
C<my($x)> creates a new variable that is only visible in the current
-subroutine. This is done at compile-time, so it is called lexical or
-static scoping. my() always affects private variables, also called
+subroutine. This is done at compile-time, so it is called lexical or
+static scoping. my() always affects private variables, also called
lexical variables or (improperly) static(ly scoped) variables.
For instance:
- sub visible {
- print "var has value $var\n";
- }
+ sub visible {
+ print "var has value $var\n";
+ }
- sub dynamic {
- local $var = 'local'; # new temporary value for the still-global
- visible(); # variable called $var
- }
+ sub dynamic {
+ local $var = 'local'; # new temporary value for the still-global
+ visible(); # variable called $var
+ }
- sub lexical {
- my $var = 'private'; # new private variable, $var
- visible(); # (invisible outside of sub scope)
- }
+ sub lexical {
+ my $var = 'private'; # new private variable, $var
+ visible(); # (invisible outside of sub scope)
+ }
- $var = 'global';
+ $var = 'global';
- visible(); # prints global
- dynamic(); # prints local
- lexical(); # prints global
+ visible(); # prints global
+ dynamic(); # prints local
+ lexical(); # prints global
-Notice how at no point does the value "private" get printed. That's
+Notice how at no point does the value "private" get printed. That's
because $var only has that value within the block of the lexical()
function, and it is hidden from the called subroutine.
In summary, local() doesn't make what you think of as private, local
-variables. It gives a global variable a temporary value. my() is
+variables. It gives a global variable a temporary value. my() is
what you're looking for if you want private variables.
See L<perlsub/"Private Variables via my()"> and
@@ -570,46 +562,46 @@ $Some_Pack::var. Note that the notation $::var is B<not> the dynamic $var
in the current package, but rather the one in the "main" package, as
though you had written $main::var.
- use vars '$var';
- local $var = "global";
- my $var = "lexical";
+ use vars '$var';
+ local $var = "global";
+ my $var = "lexical";
- print "lexical is $var\n";
- print "global is $main::var\n";
+ print "lexical is $var\n";
+ print "global is $main::var\n";
Alternatively you can use the compiler directive our() to bring a
dynamic variable into the current lexical scope.
- require 5.006; # our() did not exist before 5.6
- use vars '$var';
+ require 5.006; # our() did not exist before 5.6
+ use vars '$var';
- local $var = "global";
- my $var = "lexical";
+ local $var = "global";
+ my $var = "lexical";
- print "lexical is $var\n";
+ print "lexical is $var\n";
- {
- our $var;
- print "global is $var\n";
- }
+ {
+ our $var;
+ print "global is $var\n";
+ }
=head2 What's the difference between deep and shallow binding?
In deep binding, lexical variables mentioned in anonymous subroutines
are the same ones that were in scope when the subroutine was created.
In shallow binding, they are whichever variables with the same names
-happen to be in scope when the subroutine is called. Perl always uses
+happen to be in scope when the subroutine is called. Perl always uses
deep binding of lexical variables (i.e., those created with my()).
However, dynamic variables (aka global, local, or package variables)
-are effectively shallowly bound. Consider this just one more reason
-not to use them. See the answer to L<"What's a closure?">.
+are effectively shallowly bound. Consider this just one more reason
+not to use them. See the answer to L<"What's a closure?">.
-=head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
+=head2 Why doesn't "my($foo) = E<lt>$fhE<gt>;" work right?
C<my()> and C<local()> give list context to the right hand side
-of C<=>. The <FH> read operation, like so many of Perl's
+of C<=>. The <$fh> read operation, like so many of Perl's
functions and operators, can tell which context it was called in and
-behaves appropriately. In general, the scalar() function can help.
+behaves appropriately. In general, the scalar() function can help.
This function does nothing to the data itself (contrary to popular myth)
but rather tells its argument to behave in whatever its scalar fashion is.
If that function doesn't have a defined scalar behavior, this of course
@@ -618,15 +610,15 @@ doesn't help you (such as with sort()).
To enforce scalar context in this particular case, however, you need
merely omit the parentheses:
- local($foo) = <FILE>; # WRONG
- local($foo) = scalar(<FILE>); # ok
- local $foo = <FILE>; # right
+ local($foo) = <$fh>; # WRONG
+ local($foo) = scalar(<$fh>); # ok
+ local $foo = <$fh>; # right
You should probably be using lexical variables anyway, although the
issue is the same here:
- my($foo) = <FILE>; # WRONG
- my $foo = <FILE>; # right
+ my($foo) = <$fh>; # WRONG
+ my $foo = <$fh>; # right
=head2 How do I redefine a builtin function, operator, or method?
@@ -634,7 +626,7 @@ Why do you want to do that? :-)
If you want to override a predefined function, such as open(),
then you'll have to import the new definition from a different
-module. See L<perlsub/"Overriding Built-in Functions">.
+module. See L<perlsub/"Overriding Built-in Functions">.
If you want to overload a Perl operator, such as C<+> or C<**>,
then you'll want to use the C<use overload> pragma, documented
@@ -652,29 +644,29 @@ the prototype of C<foo> and passes it the current value of the argument
list, C<@_>. Here's an example; the C<bar> subroutine calls C<&foo>,
which prints its arguments list:
- sub bar { &foo }
+ sub bar { &foo }
- sub foo { print "Args in foo are: @_\n" }
+ sub foo { print "Args in foo are: @_\n" }
- bar( qw( a b c ) );
+ bar( qw( a b c ) );
When you call C<bar> with arguments, you see that C<foo> got the same C<@_>:
- Args in foo are: a b c
+ Args in foo are: a b c
Calling the subroutine with trailing parentheses, with or without arguments,
does not use the current C<@_> and respects the subroutine prototype. Changing
the example to put parentheses after the call to C<foo> changes the program:
- sub bar { &foo() }
+ sub bar { &foo() }
- sub foo { print "Args in foo are: @_\n" }
+ sub foo { print "Args in foo are: @_\n" }
- bar( qw( a b c ) );
+ bar( qw( a b c ) );
Now the output shows that C<foo> doesn't get the C<@_> from its caller.
- Args in foo are:
+ Args in foo are:
The main use of the C<@_> pass-through feature is to write subroutines
whose main job it is to call other subroutines for you. For further
@@ -684,24 +676,24 @@ details, see L<perlsub>.
In Perl 5.10, use the C<given-when> construct described in L<perlsyn>:
- use 5.010;
+ use 5.010;
- given ( $string ) {
- when( 'Fred' ) { say "I found Fred!" }
- when( 'Barney' ) { say "I found Barney!" }
- when( /Bamm-?Bamm/ ) { say "I found Bamm-Bamm!" }
- default { say "I don't recognize the name!" }
- };
+ given ( $string ) {
+ when( 'Fred' ) { say "I found Fred!" }
+ when( 'Barney' ) { say "I found Barney!" }
+ when( /Bamm-?Bamm/ ) { say "I found Bamm-Bamm!" }
+ default { say "I don't recognize the name!" }
+ };
If one wants to use pure Perl and to be compatible with Perl versions
prior to 5.10, the general answer is to use C<if-elsif-else>:
- for ($variable_to_test) {
- if (/pat1/) { } # do something
- elsif (/pat2/) { } # do something else
- elsif (/pat3/) { } # do something else
- else { } # default
- }
+ for ($variable_to_test) {
+ if (/pat1/) { } # do something
+ elsif (/pat2/) { } # do something else
+ elsif (/pat3/) { } # do something else
+ else { } # default
+ }
Here's a simple example of a switch based on pattern matching,
lined up in a way to make it look more like a switch statement.
@@ -710,31 +702,31 @@ in $whatchamacallit:
SWITCH: for (ref $whatchamacallit) {
- /^$/ && die "not a reference";
+ /^$/ && die "not a reference";
- /SCALAR/ && do {
- print_scalar($$ref);
- last SWITCH;
- };
+ /SCALAR/ && do {
+ print_scalar($$ref);
+ last SWITCH;
+ };
- /ARRAY/ && do {
- print_array(@$ref);
- last SWITCH;
- };
+ /ARRAY/ && do {
+ print_array(@$ref);
+ last SWITCH;
+ };
- /HASH/ && do {
- print_hash(%$ref);
- last SWITCH;
- };
+ /HASH/ && do {
+ print_hash(%$ref);
+ last SWITCH;
+ };
- /CODE/ && do {
- warn "can't print function ref";
- last SWITCH;
- };
+ /CODE/ && do {
+ warn "can't print function ref";
+ last SWITCH;
+ };
- # DEFAULT
+ # DEFAULT
- warn "User defined type skipped";
+ warn "User defined type skipped";
}
@@ -748,29 +740,29 @@ different characters or if you want to arrange the matches so that
one takes precedence over another, as C<"SEND"> has precedence over
C<"STOP"> here:
- chomp($answer = <>);
- if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
- elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
- elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
- elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
- elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
+ chomp($answer = <>);
+ if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
+ elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
+ elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
+ elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
+ elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
A totally different approach is to create a hash of function references.
- my %commands = (
- "happy" => \&joy,
- "sad", => \&sullen,
- "done" => sub { die "See ya!" },
- "mad" => \&angry,
- );
-
- print "How are you? ";
- chomp($string = <STDIN>);
- if ($commands{$string}) {
- $commands{$string}->();
- } else {
- print "No such command: $string\n";
- }
+ my %commands = (
+ "happy" => \&joy,
+ "sad", => \&sullen,
+ "done" => sub { die "See ya!" },
+ "mad" => \&angry,
+ );
+
+ print "How are you? ";
+ chomp($string = <STDIN>);
+ if ($commands{$string}) {
+ $commands{$string}->();
+ } else {
+ print "No such command: $string\n";
+ }
Starting from Perl 5.8, a source filter module, C<Switch>, can also be
used to get switch and case. Its use is now discouraged, because it's
@@ -787,23 +779,23 @@ undefined functions and methods.
When it comes to undefined variables that would trigger a warning
under C<use warnings>, you can promote the warning to an error.
- use warnings FATAL => qw(uninitialized);
+ use warnings FATAL => qw(uninitialized);
=head2 Why can't a method included in this same file be found?
Some possible reasons: your inheritance is getting confused, you've
-misspelled the method name, or the object is of the wrong type. Check
-out L<perltoot> for details about any of the above cases. You may
+misspelled the method name, or the object is of the wrong type. Check
+out L<perltoot> for details about any of the above cases. You may
also use C<print ref($object)> to find out the class C<$object> was
blessed into.
Another possible reason for problems is that you've used the
indirect object syntax (eg, C<find Guru "Samy">) on a class name
-before Perl has seen that such a package exists. It's wisest to make
+before Perl has seen that such a package exists. It's wisest to make
sure your packages are all defined before you start using them, which
will be taken care of if you use the C<use> statement instead of
-C<require>. If not, make sure to use arrow notation (eg.,
-C<< Guru->find("Samy") >>) instead. Object notation is explained in
+C<require>. If not, make sure to use arrow notation (eg.,
+C<< Guru->find("Samy") >>) instead. Object notation is explained in
L<perlobj>.
Make sure to read about creating modules in L<perlmod> and
@@ -818,39 +810,39 @@ C<__PACKAGE__>, as documented in L<perldata>. You can only use the
special literals as separate tokens, so you can't interpolate them
into strings like you can with variables:
- my $current_package = __PACKAGE__;
- print "I am in package $current_package\n";
+ my $current_package = __PACKAGE__;
+ print "I am in package $current_package\n";
If you want to find the package calling your code, perhaps to give better
-diagnostics as C<Carp> does, use the C<caller> built-in:
+diagnostics as L<Carp> does, use the C<caller> built-in:
- sub foo {
- my @args = ...;
- my( $package, $filename, $line ) = caller;
+ sub foo {
+ my @args = ...;
+ my( $package, $filename, $line ) = caller;
- print "I was called from package $package\n";
- );
+ print "I was called from package $package\n";
+ );
By default, your program starts in package C<main>, so you will
always be in some package.
This is different from finding out the package an object is blessed
into, which might not be the current package. For that, use C<blessed>
-from C<Scalar::Util>, part of the Standard Library since Perl 5.8:
+from L<Scalar::Util>, part of the Standard Library since Perl 5.8:
- use Scalar::Util qw(blessed);
- my $object_package = blessed( $object );
+ use Scalar::Util qw(blessed);
+ my $object_package = blessed( $object );
Most of the time, you shouldn't care what package an object is blessed
into, however, as long as it claims to inherit from that class:
- my $is_right_class = eval { $object->isa( $package ) }; # true or false
+ my $is_right_class = eval { $object->isa( $package ) }; # true or false
And, with Perl 5.10 and later, you don't have to check for an
inheritance to see if the object can handle a role. For that, you can
use C<DOES>, which comes from C<UNIVERSAL>:
- my $class_does_it = eval { $object->DOES( $role ) }; # true or false
+ my $class_does_it = eval { $object->DOES( $role ) }; # true or false
You can safely replace C<isa> with C<DOES> (although the converse is not true).
@@ -861,18 +853,18 @@ You can safely replace C<isa> with C<DOES> (although the converse is not true).
The quick-and-dirty way to comment out more than one line of Perl is
to surround those lines with Pod directives. You have to put these
directives at the beginning of the line and somewhere where Perl
-expects a new statement (so not in the middle of statements like the #
+expects a new statement (so not in the middle of statements like the C<#>
comments). You end the comment with C<=cut>, ending the Pod section:
- =pod
+ =pod
- my $object = NotGonnaHappen->new();
+ my $object = NotGonnaHappen->new();
- ignored_sub();
+ ignored_sub();
- $wont_be_assigned = 37;
+ $wont_be_assigned = 37;
- =cut
+ =cut
The quick-and-dirty method only works well when you don't plan to
leave the commented code in the source. If a Pod parser comes along,
@@ -885,17 +877,17 @@ the comments with C<comment>. End the comment using C<=end> with the
same label. You still need the C<=cut> to go back to Perl code from
the Pod comment:
- =begin comment
+ =begin comment
- my $object = NotGonnaHappen->new();
+ my $object = NotGonnaHappen->new();
- ignored_sub();
+ ignored_sub();
- $wont_be_assigned = 37;
+ $wont_be_assigned = 37;
- =end comment
+ =end comment
- =cut
+ =cut
For more information on Pod, check out L<perlpod> and L<perlpodspec>.
@@ -903,23 +895,23 @@ For more information on Pod, check out L<perlpod> and L<perlpodspec>.
Use this code, provided by Mark-Jason Dominus:
- sub scrub_package {
- no strict 'refs';
- my $pack = shift;
- die "Shouldn't delete main package"
- if $pack eq "" || $pack eq "main";
- my $stash = *{$pack . '::'}{HASH};
- my $name;
- foreach $name (keys %$stash) {
- my $fullname = $pack . '::' . $name;
- # Get rid of everything with that name.
- undef $$fullname;
- undef @$fullname;
- undef %$fullname;
- undef &$fullname;
- undef *$fullname;
- }
- }
+ sub scrub_package {
+ no strict 'refs';
+ my $pack = shift;
+ die "Shouldn't delete main package"
+ if $pack eq "" || $pack eq "main";
+ my $stash = *{$pack . '::'}{HASH};
+ my $name;
+ foreach $name (keys %$stash) {
+ my $fullname = $pack . '::' . $name;
+ # Get rid of everything with that name.
+ undef $$fullname;
+ undef @$fullname;
+ undef %$fullname;
+ undef &$fullname;
+ undef *$fullname;
+ }
+ }
Or, if you're using a recent release of Perl, you can
just use the Symbol::delete_package() function instead.
@@ -929,17 +921,17 @@ just use the Symbol::delete_package() function instead.
Beginners often think they want to have a variable contain the name
of a variable.
- $fred = 23;
- $varname = "fred";
- ++$$varname; # $fred now 24
+ $fred = 23;
+ $varname = "fred";
+ ++$$varname; # $fred now 24
This works I<sometimes>, but it is a very bad idea for two reasons.
The first reason is that this technique I<only works on global
-variables>. That means that if $fred is a lexical variable created
+variables>. That means that if $fred is a lexical variable created
with my() in the above example, the code wouldn't work at all: you'd
accidentally access the global and skip right over the private lexical
-altogether. Global variables are bad because they can easily collide
+altogether. Global variables are bad because they can easily collide
accidentally and in general make for non-scalable and confusing code.
Symbolic references are forbidden under the C<use strict> pragma.
@@ -948,78 +940,78 @@ or garbage-collected.
The other reason why using a variable to hold the name of another
variable is a bad idea is that the question often stems from a lack of
-understanding of Perl data structures, particularly hashes. By using
+understanding of Perl data structures, particularly hashes. By using
symbolic references, you are just using the package's symbol-table hash
-(like C<%main::>) instead of a user-defined hash. The solution is to
+(like C<%main::>) instead of a user-defined hash. The solution is to
use your own hash or a real reference instead.
- $USER_VARS{"fred"} = 23;
- $varname = "fred";
- $USER_VARS{$varname}++; # not $$varname++
+ $USER_VARS{"fred"} = 23;
+ my $varname = "fred";
+ $USER_VARS{$varname}++; # not $$varname++
There we're using the %USER_VARS hash instead of symbolic references.
Sometimes this comes up in reading strings from the user with variable
references and wanting to expand them to the values of your perl
-program's variables. This is also a bad idea because it conflates the
-program-addressable namespace and the user-addressable one. Instead of
+program's variables. This is also a bad idea because it conflates the
+program-addressable namespace and the user-addressable one. Instead of
reading a string and expanding it to the actual contents of your program's
own variables:
- $str = 'this has a $fred and $barney in it';
- $str =~ s/(\$\w+)/$1/eeg; # need double eval
+ $str = 'this has a $fred and $barney in it';
+ $str =~ s/(\$\w+)/$1/eeg; # need double eval
it would be better to keep a hash around like %USER_VARS and have
variable references actually refer to entries in that hash:
- $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
+ $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
-That's faster, cleaner, and safer than the previous approach. Of course,
-you don't need to use a dollar sign. You could use your own scheme to
+That's faster, cleaner, and safer than the previous approach. Of course,
+you don't need to use a dollar sign. You could use your own scheme to
make it less confusing, like bracketed percent symbols, etc.
- $str = 'this has a %fred% and %barney% in it';
- $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
+ $str = 'this has a %fred% and %barney% in it';
+ $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
Another reason that folks sometimes think they want a variable to
contain the name of a variable is that they don't know how to build
-proper data structures using hashes. For example, let's say they
+proper data structures using hashes. For example, let's say they
wanted two hashes in their program: %fred and %barney, and that they
wanted to use another scalar variable to refer to those by name.
- $name = "fred";
- $$name{WIFE} = "wilma"; # set %fred
+ $name = "fred";
+ $$name{WIFE} = "wilma"; # set %fred
- $name = "barney";
- $$name{WIFE} = "betty"; # set %barney
+ $name = "barney";
+ $$name{WIFE} = "betty"; # set %barney
This is still a symbolic reference, and is still saddled with the
-problems enumerated above. It would be far better to write:
+problems enumerated above. It would be far better to write:
- $folks{"fred"}{WIFE} = "wilma";
- $folks{"barney"}{WIFE} = "betty";
+ $folks{"fred"}{WIFE} = "wilma";
+ $folks{"barney"}{WIFE} = "betty";
And just use a multilevel hash to start with.
The only times that you absolutely I<must> use symbolic references are
-when you really must refer to the symbol table. This may be because it's
+when you really must refer to the symbol table. This may be because it's
something that one can't take a real reference to, such as a format name.
Doing so may also be important for method calls, since these always go
through the symbol table for resolution.
In those cases, you would turn off C<strict 'refs'> temporarily so you
-can play around with the symbol table. For example:
+can play around with the symbol table. For example:
- @colors = qw(red blue green yellow orange purple violet);
- for my $name (@colors) {
- no strict 'refs'; # renege for the block
- *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
- }
+ @colors = qw(red blue green yellow orange purple violet);
+ for my $name (@colors) {
+ no strict 'refs'; # renege for the block
+ *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
+ }
All those functions (red(), blue(), green(), etc.) appear to be separate,
but the real code in the closure actually was compiled only once.
So, sometimes you might want to use symbolic references to manipulate
-the symbol table directly. This doesn't matter for formats, handles, and
+the symbol table directly. This doesn't matter for formats, handles, and
subroutines, because they are always global--you can't use my() on them.
For scalars, arrays, and hashes, though--and usually for subroutines--
you probably only want to use hard references.
@@ -1028,7 +1020,7 @@ you probably only want to use hard references.
(contributed by brian d foy)
-The "bad interpreter" message comes from the shell, not perl. The
+The "bad interpreter" message comes from the shell, not perl. The
actual message may vary depending on your platform, shell, and locale
settings.
@@ -1048,7 +1040,7 @@ script executable.
In either case, you should still be able to run the scripts with perl
explicitly:
- % perl script.pl
+ % perl script.pl
If you get a message like "perl: command not found", perl is not in
your PATH, which might also mean that the location of perl is not
@@ -1063,8 +1055,7 @@ This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file
-are hereby placed into the public domain. You are permitted and
+are hereby placed into the public domain. You are permitted and
encouraged to use this code in your own programs for fun
-or for profit as you see fit. A simple comment in the code giving
+or for profit as you see fit. A simple comment in the code giving
credit would be courteous but is not required.
-