PERLFAQ7(1) Perl Programmers Reference Guide PERLFAQ7(1) #
PERLFAQ7(1) Perl Programmers Reference Guide PERLFAQ7(1)
NNAAMMEE #
perlfaq7 - General Perl Language Issues
VVEERRSSIIOONN #
version 5.20210520
DDEESSCCRRIIPPTTIIOONN #
This section deals with general Perl language issues that don't clearly
fit into any of the other sections.
CCaann II ggeett aa BBNNFF//yyaacccc//RREE ffoorr tthhee PPeerrll llaanngguuaaggee?? 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 grammar relies on very smart tokenizing code, so be prepared to venture into toke.c as well.
In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF.
The work of parsing perl is distributed between yacc, the lexer, smoke
and mirrors."
WWhhaatt aarree aallll tthheessee $$@@%%&& ppuunnccttuuaattiioonn ssiiggnnss,, aanndd hhooww ddoo II kknnooww wwhheenn ttoo uussee tthheemm?? They are type specifiers, as detailed in 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.
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.
Note that <FILE> is _n_e_i_t_h_e_r the type specifier for files nor the name of
the handle. It is the "<>" operator applied to the handle FILE. It reads
one line (well, record--see "$/" in perlvar) from the handle FILE in
scalar context, or _a_l_l lines in list context. When performing open,
close, or any other operation besides "<>" on files, or even when talking
about the handle, do _n_o_t use the brackets. These are correct: "eof(FH)",
"seek(FH, 0, 2)" and "copying from STDIN to FILE".
DDoo II aallwwaayyss//nneevveerr hhaavvee ttoo qquuoottee mmyy ssttrriinnggss oorr uussee sseemmiiccoolloonnss aanndd ccoommmmaass?? Normally, a bareword doesn’t need to be quoted, but in most cases probably should be (and must be under “use strict”). But a hash key consisting of a simple word and the left-hand operand to the “=>” operator both count as though they were quoted:
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 perlstyle) says to put them in except for one-
liners:
if ($whoops) { exit 1 }
my @nums = (1, 2, 3);
if ($whoops) {
exit 1;
}
my @lines = (
"There Beren came from mountains cold",
"And lost he wandered under leaves",
);
HHooww ddoo II sskkiipp ssoommee rreettuurrnn vvaalluueess?? One way is to treat the return values as a list and index into it:
$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);
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];
HHooww ddoo II tteemmppoorraarriillyy bblloocckk wwaarrnniinnggss?? If you are running Perl 5.6.0 or better, the “use warnings” pragma allows fine control of what warnings are produced. See perllexwarn for more details.
{
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 perllexwarn for the complete details,
including the category names and hierarchy.
{
no warnings 'uninitialized';
$x = $y + $z;
}
If you have an older version of Perl, the $^W variable (documented in
perlvar) controls runtime warnings for a block:
{
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
mmyy(()) on $^W, only llooccaall(()).
WWhhaatt’’ss aann eexxtteennssiioonn?? An extension is a way of calling compiled C code from Perl. Reading perlxstut is a good place to learn more about extensions.
WWhhyy ddoo PPeerrll ooppeerraattoorrss hhaavvee ddiiffffeerreenntt pprreecceeddeennccee tthhaann CC ooppeerraattoorrss?? 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 called “list operators” and appear as such in the precedence table in perlop.
A common mistake is to write:
unlink $file || die "snafu";
This gets interpreted as:
unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or use the super
low precedence "or" operator:
(unlink $file) || die "snafu";
unlink $file or die "snafu";
The "English" operators ("and", "or", "xor", and "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 binds
more tightly even than unary minus, making "-2**2" produce a negative
four and not a positive one. It is also right-associating, meaning that
"2**3**2" is two raised to the ninth power, not eight squared.
Although it has the same precedence as in C, Perl's "?:" operator
produces an lvalue. This assigns $x to either $if_true or $if_false,
depending on the trueness of $maybe:
($maybe ? $if_true : $if_false) = $x;
HHooww ddoo II ddeeccllaarree//ccrreeaattee aa ssttrruuccttuurree?? In general, you don’t “declare” a structure. Just use a (probably anonymous) hash reference. See perlref and 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"
If you're looking for something a bit more rigorous, try perlootut.
HHooww ddoo II ccrreeaattee aa mmoodduullee?? 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.
ExtUtils::ModuleMaker and Module::Starter are also good places to start.
Many CPAN authors now use Dist::Zilla to automate as much as possible.
Detailed documentation about modules can be found at: perlmod,
perlmodlib, perlmodstyle.
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.
perlxs and perlxstut explain the details.
HHooww ddoo II aaddoopptt oorr ttaakkee oovveerr aa mmoodduullee aallrreeaaddyy oonn CCPPAANN?? Ask the current maintainer to make you a co-maintainer or transfer the module to you.
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 is treated
separately.
• Get a login for the Perl Authors Upload Server (PAUSE) if you don't
already have one: <http://pause.perl.org>
• Write to modules@perl.org explaining what you did to contact the
current maintainer. The PAUSE admins will also try to reach the
maintainer.
• Post a public message in a heavily trafficked site announcing your
intention to take over the module.
• Wait a bit. The PAUSE admins don't want to act too quickly in case
the current maintainer is on holiday. If there's no response to
private communication or the public post, a PAUSE admin can transfer
it to you.
HHooww ddoo II ccrreeaattee aa ccllaassss?? (contributed by brian d foy)
In Perl, a class is just a package, and methods are just subroutines.
Perl doesn't get more formal than that and lets you set up the package
just the way that you like it (that is, it doesn't set up anything for
you).
See also perlootut, a tutorial that covers class creation, and perlobj.
HHooww ccaann II tteellll iiff aa vvaarriiaabbllee iiss ttaaiinntteedd?? You can use the ttaaiinntteedd(()) function of the Scalar::Util module, available from CPAN (or included with Perl since release 5.8.0). See also “Laundering and Detecting Tainted Data” in perlsec.
WWhhaatt’’ss aa cclloossuurree?? Closures are documented in perlref.
_C_l_o_s_u_r_e is a computer science term with a precise but hard-to-explain
meaning. Usually, closures are implemented in Perl as anonymous
subroutines with lasting references to lexical variables outside their
own scopes. These lexicals magically refer to the variables that were
around when the subroutine was defined (deep binding).
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 but encourages
closures.
Here's a classic non-closure function-generating function:
sub add_function_generator {
return sub { shift() + shift() };
}
my $add_sub = add_function_generator();
my $sum = $add_sub->(4,5); # $sum is 9 now.
The anonymous subroutine returned by aadddd__ffuunnccttiioonn__ggeenneerraattoorr(()) isn't
technically a closure because it refers to no lexicals outside its own
scope. Using a closure gives you a _f_u_n_c_t_i_o_n _t_e_m_p_l_a_t_e with some
customization slots left out to be filled later.
Contrast this with the following mmaakkee__aaddddeerr(()) 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 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 };
}
my $f1 = make_adder(20);
my $f2 = make_adder(555);
Now "$f1->($n)" is always 20 plus whatever $n you pass in, whereas
"$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 you
want to pass in a bit of code into a function:
my $line;
timeout( 30, sub { $line = <STDIN> } );
If the code to execute had been passed in as a string, '$line = <STDIN>',
there would have been no way for the hypothetical ttiimmeeoouutt(()) function to
access the lexical variable $line back in its caller's scope.
Another use for a closure is to make a variable _p_r_i_v_a_t_e to a named
subroutine, e.g. a counter that gets initialized at creation time of the
sub and can only be modified from within the sub. 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 }
}
This is discussed in more detail in perlsub; see the entry on _P_e_r_s_i_s_t_e_n_t
_P_r_i_v_a_t_e _V_a_r_i_a_b_l_e_s.
WWhhaatt iiss vvaarriiaabbllee ssuuiicciiddee aanndd hhooww ccaann II pprreevveenntt iitt?? 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 mmyy(()) and llooccaall(())
interacting with either closures or aliased ffoorreeaacchh(()) 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:
my $f = 'foo';
sub T {
while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
}
T; #
print "Finally $f\n";
If you are experiencing variable suicide, that "my $f" in the subroutine
doesn't pick up a fresh copy of the $f whose value is 'foo'. The output
shows that inside the subroutine the value of $f leaks through when it
shouldn't, as in this output:
foobar
foobarbar
foobarbarbar
Finally foo
The $f that has "bar" added to it three times should be a new $f "my $f"
should create a new lexical variable each time through the loop. The
expected output is:
foobar
foobar
foobar
Finally foo
HHooww ccaann II ppaassss//rreettuurrnn aa {{FFuunnccttiioonn,, FFiilleeHHaannddllee,, AArrrraayy,, HHaasshh,, MMeetthhoodd,, RReeggeexx}}?? You need to pass references to these objects. See “Pass by Reference” in perlsub for this particular question, and perlref for information on references.
Passing Variables and Functions
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_array );
func( [ 1 .. 10 ] );
func( \%some_hash );
func( { this => 10, that => 20 } );
func( \&some_func );
func( sub { $_[0] ** $_[1] } );
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 );
sub func {
my $passed_fh = shift;
my $line = <$passed_fh>;
}
Before Perl 5.6, you had to use the *FH or "\*FH" notations. These
are "typeglobs"--see "Typeglobs and Filehandles" in perldata and
especially "Pass by Reference" in perlsub for more information.
Passing Regexes
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 "qr//"
operator:
sub compare {
my ($val1, $regex) = @_;
my $retval = $val1 =~ /$regex/;
return $retval;
}
$match = compare("old McDonald", qr/d.*D/i);
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();
}
}
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();
}
You could also investigate the ccaann(()) method in the UNIVERSAL class
(part of the standard perl distribution).
HHooww ddoo II ccrreeaattee aa ssttaattiicc vvaarriiaabbllee?? (contributed by brian d foy)
In Perl 5.10, declare the variable with "state". The "state" declaration
creates the lexical variable that persists between calls to the
subroutine:
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 "counter", and it
uses the lexical variable $count. Since you wrap this in a BEGIN block,
$count is defined at compile-time, but also goes out of scope at the end
of the BEGIN block. The BEGIN block also ensures that the subroutine and
the value it uses is defined at compile-time so the subroutine is ready
to use just like any other subroutine, and you can put this code in the
same place as other subroutines in the program text (i.e. at the end of
the code, typically). The subroutine "counter" still has a reference to
the data, and is the only way you can access the value (and each time you
do, you increment the value). The data in chunk of memory defined by
$count is private to "counter".
BEGIN { #
my $count = 1;
sub counter { $count++ }
}
my $start = counter();
.... # code that calls counter();
my $end = counter();
In the previous example, you created a function-private variable because
only one function remembered its reference. You could define multiple
functions while the variable is in scope, and each function can share the
"private" variable. It's not really "static" because you can access it
outside the function while the lexical variable is in scope, and even
create references to it. In this example, "increment_count" and
"return_count" share the variable. One function adds to the value and the
other simply returns the value. They can both access $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 }
}
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 cannot be
seen from any other file.
See "Persistent Private Variables" in perlsub for more information. The
discussion of closures in perlref may help you even though we did not use
anonymous subroutines in this answer. See "Persistent Private Variables"
in perlsub for details.
WWhhaatt’’ss tthhee ddiiffffeerreennccee bbeettwweeeenn ddyynnaammiicc aanndd lleexxiiccaall ((ssttaattiicc)) ssccooppiinngg?? BBeettwweeeenn llooccaall(()) aanndd mmyy(())?? “local($x)” saves away the old value of the global variable $x and assigns a new value for the duration of the subroutine _w_h_i_c_h _i_s _v_i_s_i_b_l_e _i_n _o_t_h_e_r _f_u_n_c_t_i_o_n_s _c_a_l_l_e_d _f_r_o_m _t_h_a_t _s_u_b_r_o_u_t_i_n_e. This is done at run-time, so is called dynamic scoping. llooccaall(()) always affects global variables, also called package variables or dynamic variables.
"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. mmyy(()) 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 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)
}
$var = 'global';
visible(); # prints global
dynamic(); # prints local
lexical(); # prints global
Notice how at no point does the value "private" get printed. That's
because $var only has that value within the block of the lleexxiiccaall(())
function, and it is hidden from the called subroutine.
In summary, llooccaall(()) doesn't make what you think of as private, local
variables. It gives a global variable a temporary value. mmyy(()) is what
you're looking for if you want private variables.
See "Private Variables via mmyy(())" in perlsub and "Temporary Values via
llooccaall(())" in perlsub for excruciating details.
HHooww ccaann II aacccceessss aa ddyynnaammiicc vvaarriiaabbllee wwhhiillee aa ssiimmiillaarrllyy nnaammeedd lleexxiiccaall iiss iinn ssccooppee?? If you know your package, you can just mention it explicitly, as in $Some_Pack::var. Note that the notation $::var is nnoott 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";
print "lexical is $var\n";
print "global is $main::var\n";
Alternatively you can use the compiler directive oouurr(()) to bring a dynamic
variable into the current lexical scope.
require 5.006; # our() did not exist before 5.6
use vars '$var';
local $var = "global";
my $var = "lexical";
print "lexical is $var\n";
{
our $var;
print "global is $var\n";
}
WWhhaatt’’ss tthhee ddiiffffeerreennccee bbeettwweeeenn ddeeeepp aanndd sshhaallllooww bbiinnddiinngg?? 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 deep binding of lexical variables (i.e., those created with mmyy(())). 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 “What’s a closure?”.
WWhhyy ddooeessnn’’tt “"mmyy(($$ffoooo)) == <<$$ffhh>>;;“” wwoorrkk rriigghhtt?? “my()” and “local()” give list context to the right hand side of “=”. 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 ssccaallaarr(()) 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 doesn’t help you (such as with ssoorrtt(())).
To enforce scalar context in this particular case, however, you need
merely omit the parentheses:
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) = <$fh>; # WRONG
my $foo = <$fh>; # right
HHooww ddoo II rreeddeeffiinnee aa bbuuiillttiinn ffuunnccttiioonn,, ooppeerraattoorr,, oorr mmeetthhoodd?? Why do you want to do that? :-)
If you want to override a predefined function, such as ooppeenn(()), then
you'll have to import the new definition from a different module. See
"Overriding Built-in Functions" in perlsub.
If you want to overload a Perl operator, such as "+" or "**", then you'll
want to use the "use overload" pragma, documented in overload.
If you're talking about obscuring method calls in parent classes, see
"Overriding methods and method resolution" in perlootut.
WWhhaatt’’ss tthhee ddiiffffeerreennccee bbeettwweeeenn ccaalllliinngg aa ffuunnccttiioonn aass &&ffoooo aanndd ffoooo(())?? (contributed by brian d foy)
Calling a subroutine as &foo with no trailing parentheses ignores the
prototype of "foo" and passes it the current value of the argument list,
@_. Here's an example; the "bar" subroutine calls &foo, which prints its
arguments list:
sub foo { print "Args in foo are: @_\n"; }
sub bar { &foo; }
bar( "a", "b", "c" );
When you call "bar" with arguments, you see that "foo" got the same @_:
Args in foo are: a b c
Calling the subroutine with trailing parentheses, with or without
arguments, does not use the current @_. Changing the example to put
parentheses after the call to "foo" changes the program:
sub foo { print "Args in foo are: @_\n"; }
sub bar { &foo(); }
bar( "a", "b", "c" );
Now the output shows that "foo" doesn't get the @_ from its caller.
Args in foo are:
However, using "&" in the call still overrides the prototype of "foo" if
present:
sub foo ($$$) { print "Args infoo are: @_\n"; }
sub bar_1 { &foo; }
sub bar_2 { &foo(); }
sub bar_3 { foo( $_[0], $_[1], $_[2] ); }
# sub bar_4 { foo(); }
# bar_4 doesn't compile: "Not enough arguments for main::foo at ..."
bar_1( "a", "b", "c" );
# Args in foo are: a b c
bar_2( "a", "b", "c" );
# Args in foo are:
bar_3( "a", "b", "c" );
# Args in foo are: a b c
The main use of the @_ pass-through feature is to write subroutines whose
main job it is to call other subroutines for you. For further details,
see perlsub.
HHooww ddoo II ccrreeaattee aa sswwiittcchh oorr ccaassee ssttaatteemmeenntt?? There is a given/when statement in Perl, but it is experimental and likely to change in future. See perlsyn for more details.
The general answer is to use a CPAN module such as Switch::Plain:
use Switch::Plain;
sswitch($variable_holding_a_string) {
case 'first': { }
case 'second': { }
default: { }
}
or for more complicated comparisons, "if-elsif-else":
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. We'll do a
multiway conditional based on the type of reference stored in
$whatchamacallit:
SWITCH: for (ref $whatchamacallit) {
/^$/ && die "not a reference";
/SCALAR/ && do {
print_scalar($$ref);
last SWITCH;
};
/ARRAY/ && do {
print_array(@$ref);
last SWITCH;
};
/HASH/ && do {
print_hash(%$ref);
last SWITCH;
};
/CODE/ && do {
warn "can't print function ref";
last SWITCH;
};
# DEFAULT #
warn "User defined type skipped";
}
See perlsyn for other examples in this style.
Sometimes you should change the positions of the constant and the
variable. For example, let's say you wanted to test which of many
answers you were given, but in a case-insensitive way that also allows
abbreviations. You can use the following technique if the strings all
start with different characters or if you want to arrange the matches so
that one takes precedence over another, as "SEND" has precedence over
"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" }
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";
}
Starting from Perl 5.8, a source filter module, "Switch", can also be
used to get switch and case. Its use is now discouraged, because it's not
fully compatible with the native switch of Perl 5.10, and because, as
it's implemented as a source filter, it doesn't always work as intended
when complex syntax is involved.
HHooww ccaann II ccaattcchh aacccceesssseess ttoo uunnddeeffiinneedd vvaarriiaabblleess,, ffuunnccttiioonnss,, oorr mmeetthhooddss?? The AUTOLOAD method, discussed in “Autoloading” in perlsub lets you capture calls to undefined functions and methods.
When it comes to undefined variables that would trigger a warning under
"use warnings", you can promote the warning to an error.
use warnings FATAL => qw(uninitialized);
WWhhyy ccaann’’tt aa mmeetthhoodd iinncclluuddeedd iinn tthhiiss ssaammee ffiillee bbee ffoouunndd?? Some possible reasons: your inheritance is getting confused, you’ve misspelled the method name, or the object is of the wrong type. Check out perlootut for details about any of the above cases. You may also use “print ref($object)” to find out the class $object was blessed into.
Another possible reason for problems is that you've used the indirect
object syntax (eg, "find Guru "Samy"") on a class name 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 "use" statement instead of "require". If not, make sure to
use arrow notation (eg., "Guru->find("Samy")") instead. Object notation
is explained in perlobj.
Make sure to read about creating modules in perlmod and the perils of
indirect objects in "Method Invocation" in perlobj.
HHooww ccaann II ffiinndd oouutt mmyy ccuurrrreenntt oorr ccaalllliinngg ppaacckkaaggee?? (contributed by brian d foy)
To find the package you are currently in, use the special literal
"__PACKAGE__", as documented in 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";
If you want to find the package calling your code, perhaps to give better
diagnostics as Carp does, use the "caller" built-in:
sub foo {
my @args = ...;
my( $package, $filename, $line ) = caller;
print "I was called from package $package\n";
);
By default, your program starts in package "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 "blessed" from
Scalar::Util, part of the Standard Library since Perl 5.8:
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
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 "DOES",
which comes from "UNIVERSAL":
my $class_does_it = eval { $object->DOES( $role ) }; # true or false
You can safely replace "isa" with "DOES" (although the converse is not
true).
HHooww ccaann II ccoommmmeenntt oouutt aa llaarrggee bblloocckk ooff PPeerrll ccooddee?? (contributed by brian d foy)
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 "#"
comments). You end the comment with "=cut", ending the Pod section:
=pod
my $object = NotGonnaHappen->new();
ignored_sub();
$wont_be_assigned = 37;
=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, your
multiline comment is going to show up in the Pod translation. A better
way hides it from Pod parsers as well.
The "=begin" directive can mark a section for a particular purpose. If
the Pod parser doesn't want to handle it, it just ignores it. Label the
comments with "comment". End the comment using "=end" with the same
label. You still need the "=cut" to go back to Perl code from the Pod
comment:
=begin comment
my $object = NotGonnaHappen->new();
ignored_sub();
$wont_be_assigned = 37;
=end comment
=cut
For more information on Pod, check out perlpod and perlpodspec.
HHooww ddoo II cclleeaarr aa ppaacckkaaggee?? 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;
}
}
Or, if you're using a recent release of Perl, you can just use the
SSyymmbbooll::::ddeelleettee__ppaacckkaaggee(()) function instead.
HHooww ccaann II uussee aa vvaarriiaabbllee aass aa vvaarriiaabbllee nnaammee?? Beginners often think they want to have a variable contain the name of a variable.
$fred = 23;
$varname = "fred";
++$$varname; # $fred now 24
This works _s_o_m_e_t_i_m_e_s, but it is a very bad idea for two reasons.
The first reason is that this technique _o_n_l_y _w_o_r_k_s _o_n _g_l_o_b_a_l _v_a_r_i_a_b_l_e_s.
That means that if $fred is a lexical variable created with mmyy(()) in the
above example, the code wouldn't work at all: you'd accidentally access
the global and skip right over the private lexical altogether. Global
variables are bad because they can easily collide accidentally and in
general make for non-scalable and confusing code.
Symbolic references are forbidden under the "use strict" pragma. They
are not true references and consequently are not reference-counted or
garbage-collected.
The other reason why using a variable to hold the name of another
variable is a bad idea is that the question often stems from a lack of
understanding of Perl data structures, particularly hashes. By using
symbolic references, you are just using the package's symbol-table hash
(like %main::) instead of a user-defined hash. The solution is to use
your own hash or a real reference instead.
$USER_VARS{"fred"} = 23;
my $varname = "fred";
$USER_VARS{$varname}++; # not $$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
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
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
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
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 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";
$$name{WIFE} = "wilma"; # set %fred
$name = "barney";
$$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:
$folks{"fred"}{WIFE} = "wilma";
$folks{"barney"}{WIFE} = "betty";
And just use a multilevel hash to start with.
The only times that you absolutely _m_u_s_t use symbolic references are 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 "strict 'refs'" temporarily so you 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>" };
}
All those functions (rreedd(()), bblluuee(()), ggrreeeenn(()), 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
subroutines, because they are always global--you can't use mmyy(()) on them.
For scalars, arrays, and hashes, though--and usually for subroutines--
you probably only want to use hard references.
WWhhaatt ddooeess “"bbaadd iinntteerrpprreetteerr"” mmeeaann?? (contributed by brian d foy)
The "bad interpreter" message comes from the shell, not perl. The actual
message may vary depending on your platform, shell, and locale settings.
If you see "bad interpreter - no such file or directory", the first line
in your perl script (the "shebang" line) does not contain the right path
to perl (or any other program capable of running scripts). Sometimes
this happens when you move the script from one machine to another and
each machine has a different path to perl--/usr/bin/perl versus
/usr/local/bin/perl for instance. It may also indicate that the source
machine has CRLF line terminators and the destination machine has LF
only: the shell tries to find /usr/bin/perl<CR>, but can't.
If you see "bad interpreter: Permission denied", you need to make your
script executable.
In either case, you should still be able to run the scripts with perl
explicitly:
% 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 where you
expect it so you need to adjust your shebang line.
DDoo II nneeeedd ttoo rreeccoommppiillee XXSS mmoodduulleess wwhheenn tthheerree iiss aa cchhaannggee iinn tthhee CC lliibbrraarryy?? (contributed by Alex Beamish)
If the new version of the C library is ABI-compatible (that's Application
Binary Interface compatible) with the version you're upgrading from, and
if the shared library version didn't change, no re-compilation should be
necessary.
AAUUTTHHOORR AANNDD CCOOPPYYRRIIGGHHTT #
Copyright (c) 1997-2013 Tom Christiansen, Nathan Torkington, and other
authors as noted. All rights reserved.
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 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 credit would be courteous but is not
required.
perl v5.36.3 2023-02-15 PERLFAQ7(1)