PERLFAQ7
NAME DESCRIPTION AUTHOR AND COPYRIGHT
perlfaq7 - Perl Language Issues ($Revision: 1.28 $, $Date: 1999/05/23 20:36:18 $)
This section deals with general Perl language issues that don't clearly fit into any of the other sections.
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 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.''
What are all these $@%
They are type specifiers, as detailed in
@ for arrays % for hashes (associative arrays) There are couple of other symbols that you're likely to encounter that aren't really type specifiers:
Note that FILE neither the type specifier for files nor the name of the handle. It is the operator applied to the handle FILE . It reads one line (well, record--see ``$/ in perlvar) from the handle FILE in scalar context, or all lines in list context. When performing open, close, or any other operation besides on files, or even when talking about the handle, do not use the brackets. These are correct: eof(FH), seek(FH, 0, 2) and ``copying from STDIN to FILE ''.
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 use strict). But a hash key consisting of a simple word (that isn't the name of a defined subroutine) and the left-hand operand to the = operator both count
------------ --------------- $foo{line} $foo{
@nums = (1, 2, 3);
if ($whoops) {
exit 1; } @lines = (
How do I skip some return values?
One way is to treat the return values as a list and index
$dir = (getpwnam($user))[7?;
How do I temporarily block warnings?
If you are running Perl 5.6.0 or better, the use warnings pragma allows fine control of what warning are produced. See perllexwarn for more details.
{
no warnings; # temporarily turn off warnings $a = $b + $c; # I know these might be undef }
local $^W = 0; # temporarily turn off warnings $a = $b + $c; # I know these might be undef } Note that like all the punctuation variables, you cannot currently use my() on $^W, only local().
What's an extension?
An extension is a way of calling compiled C code from Perl. Reading perlxstut is a good place to learn more about extensions.
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 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.
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 product a negative not a positive four. 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 $a or $b, depending
How do I declare/create a structure?
In general, you don't ``declare'' a structure. Just use a (probably anonymous) hash reference. See perlref and perldsc
$person- If you're looking for something a bit more rigorous, try perltoot.
How do I create a module?
A module is a package that lives in a file of the same name. For example, the Hello::There module would live in Hello/There.pm. For details, read perlmod. You'll also find Exporter helpful. If you're writing a C or mixed-language module with both C and Perl, then you should study perlxstut.
Here's a convenient template you might wish you use when starting your own module. Make sure to change the names appropriately.
package Some::Module; # assumes Some/Module.pm use strict;
use warnings;
BEGIN {
use Exporter (); our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
- set the version for version checking; uncomment to use
$VERSION = 1.00;
- if using RCS/CVS, this next line may be preferred,
$VERSION = do{my@r=q$Revision: 1.28 $=/d+/g;sprintf '%d.'.'%02d'x$#r,@r};
@ISA = qw(Exporter);
@EXPORT = qw(
- your exported package globals go here,
@EXPORT_OK = qw($Var1 %Hashit); } our @EXPORT_OK;
our $Var1; our %Hashit;
our @more; our $stuff;
$Var1 = ''; %Hashit = ();
$stuff = ''; @more = ();
the functions below that use them.
my $priv_var = ''; my %secret_hash = ();
callable as
sub func1 {} # no prototype sub func2() {} # proto'd void sub func3($$) {} # proto'd to 2 scalars
sub func4(%) {} # proto'd to 1 hash ref
END { } # module clean-up code here (global destructor) 1; # modules must return true
How do I create a class?
See perltoot for an introduction to classes and objects, as well as perlobj and perlbot.
How can I tell if a variable is tainted?
See ``Laundering and Detecting Tainted Data in perlsec. Here's an example (which doesn't use any system calls, because the kill()'' is given no processes to
return ! eval { join('',@_), kill 0; 1; }; } This is not -w clean, however. There is no -w clean way to detect taintedness--take this as a hint that you should untaint all possibly-tainted data.
What's a closure?
Closures are documented in perlref.
Closure is a computer science term with a precise but hard-to-explain meaning. 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 make sense in any programming language 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.
return sub { shift + shift }; }
$add_sub = add_function_generator();
$sum = $add_sub- The closure works as a function template with some customization slots left out to be filled later. The anonymous subroutine returned by add_function_generator() isn't technically a closure because it refers to no lexicals outside its own scope.
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 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 }; }
$f1 = make_adder(20);
$f2 = make_adder(555); Now is always 20 plus whatever $n you pass in, whereas 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
timeout( 30, sub { $line = If the code to execute had been passed in as a string, '$line = , there would have been no way for the hypothetical timeout() function to access the lexical variable $line back in its caller's scope.
What is variable suicide and how can I prevent it?
Variable suicide is when you (temporarily or permanently) lose the value 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
The $f that has ``bar'' added to it three times should be a new $f (my $f should create a new local variable each time through the loop). It isn't, however. This was a bug, now fixed in the latest releases (tested against 5.004_05, 5.005_03, and 5.005_56).
How can I pass/return a {Function, !FileHandle?, Array, Hash, Method, Regex}?
With the exception of regexes, you need to pass references to these objects. See ``Pass by Reference'' in perlsub for this particular question, and perlref for information on references.
See ``Passing Regexes'', below, for information on passing regular expressions.
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
func( [ 1 .. 10? );
func( %some_hash );
func( { this =
func(
Passing Filehandles
To pass filehandles to subroutines, use the *FH or
``Typeglobs and Filehandles in perldata and especially ``Pass by Reference in perlsub for more information.
Here's an excerpt:
If you're passing around filehandles, you could usually just use the bare typeglob, like *STDOUT, but typeglobs references would be better because they'll still work properly under use strict 'refs'. For
sub splutter { my $fh = shift; print $fh
$rec = get_rec(*STDIN);
sub get_rec { my $fh = shift; return scalar
my $path = shift; local *FH; return open (FH, $path) ? *FH : undef; } $fh = openit('
Passing Regexes
To pass regexes around, you'll need to be using a release of Perl sufficiently recent as to support the qr// construct, pass around strings and use an exception-trapping eval, or else be very, very clever.
Here's an example of how to pass in a string to be regex
my ($val1, $regex) = @_; my $retval = $val1 = /$regex/; return $retval; } $match = compare(
my ($val1, $regex) = @_; my $retval = eval { $val1 = /$regex/ }; die if $@; return $retval; }
$match = compare(
Those preferring to be very, very clever might see the O'Reilly book, Mastering Regular Expressions, by Jeffrey Friedl. Page 273's Build_!MatchMany?_Function() is particularly interesting. A complete citation of this book is given in perlfaq2.
Passing Methods
To pass an object method into a subroutine, you can do
You could also investigate the can() method in the UNIVERSAL class (part of the standard perl distribution).
How do I create a static variable?
As with most things in Perl, TMTOWTDI . What is a ``static variable'' in other languages could be either a function-private variable (visible only within a single function, retaining its value between calls to that function), or a file-private variable (visible only to functions within the file it was declared in) in Perl.
Here's code to implement a function-private
my $counter = 42; sub prev_counter { return --$counter } sub next_counter { return $counter++ } } Now prev_counter() and next_counter() share a private variable $counter that was initialized at compile time.
To declare a file-private variable, you'll still use a my(), putting the declaration at the outer scope level at the top of the file. Assume this is in file
my $started = scalar(localtime(time()));
sub begun { return $started }
When use Pax or require Pax loads this module, the variable will be initialized. It won't get garbage-collected the way most variables going out of scope do, because the begun() function cares about it, but no one else can get it. It is not called $Pax::started because its scope is unrelated to the package. It's scoped to the file. You could conceivably have several packages in that same file all accessing the same private variable, but another file with the same package couldn't get to it.
See ``Persistent Private Variables'' in perlsub for details.
What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
local($x) saves away the old value of the global variable $x and assigns a new value for the duration of the subroutine which is visible in other functions called from that subroutine. This is done at run-time, so is called dynamic scoping. local() always affects global variables, also called package variables or dynamic variables.
my($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. my() always affects private variables, also called lexical variables or (improperly) static(ly scoped) variables.
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 lexical()'' function, and it is hidden from 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 what you're looking for if you want private variables.
See ``Private Variables via my() in perlsub and ``Temporary Values via local() in perlsub for excruciating details.
How can I access a dynamic variable while a similarly named lexical is in scope?
You can do this via symbolic references, provided you haven't set use strict . So instead of $var, use ${'var'}.
local $var = print no strict 'refs';
print If you know your package, you can just mention it explicitly, as in $Some_Pack::var. Note that the notation $::var is not the dynamic $var in the current package, but rather the one in the main package, as though you had written $main::var. Specifying the package directly makes you hard-code its name, but it executes faster and avoids running afoul of use strict .
What's the difference between deep and shallow binding?
In deep binding, lexical variables mentioned in anonymous subroutines are the same ones that were in scope when the subroutine was created. In shallow binding, they are whichever variables with the same names happen to be in scope when the subroutine is called. Perl always uses deep binding of lexical variables (i.e., those created with my()). However, dynamic variables (aka global, local, or package variables) are effectively shallowly bound. Consider this just one more reason not to use them. See the answer to ``What's a closure?''.
Why doesn't ``my($foo) = FILE
my() and local() give list context to the right hand side of =. The FH 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 doesn't help you (such as with sort()'').
To enforce scalar context in this particular case, however,
How do I redefine a builtin function, operator, or method?
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 ``Overriding Built-in Functions in perlsub. There's also an example in ``Class::Template in perltoot.
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 ``Overridden Methods'' in perltoot.
What's the difference between calling a function as foo()?
When you call a function as , you allow that function access to your current @_ values, and you bypass prototypes. The function doesn't get an empty @_--it gets yours! While not strictly speaking a bug (it's documented that way in perlsub), it would be hard to consider this a feature in most cases.
When you call your function as , then you do get a new @_, but prototyping is still circumvented.
Normally, you want to call a function using foo(). You may only omit the parentheses if the function is already known to the compiler because it already saw the definition (use but not require), or via a forward reference or use subs declaration. Even in this case, you get a clean @_ without any of the old values leaking through where they don't belong.
How do I create a switch or case statement?
This is explained in more depth in the perlsyn. Briefly, there's no official case statement, because of the variety of tests possible in Perl (numeric comparison, string comparison, glob comparison, regex matching, overloaded comparisons, ...). Larry couldn't decide how best to do this, so he left it out, even though it's been on the wish list since perl1.
The general answer is to write a construct like
if (/pat1/) { } # do something elsif (/pat2/) { } # do something else elsif (/pat3/) { } # do something else else { } # default }
SWITCH: for (ref $whatchamacallit) {
/^$/ /SCALAR/ /ARRAY/ /HASH/ /CODE/
- DEFAULT
warn
}
See perlsyn/ for many 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
has precedence over
- here
- chomp($answer =
A totally different approach is to create a hash of function references.
my %commands = ( print
How can I catch accesses to undefined variables/functions/methods?
The AUTOLOAD method, discussed in ``Autoloading in perlsub and `` AUTOLOAD: Proxy Methods in perltoot, lets you capture calls to undefined functions and methods.
When it comes to undefined variables that would trigger a warning under -w, you can use a handler to trap the
$SIG{WARN} = sub {
for ( $_[0? ) { # voici un switch statement
/Use of uninitialized value/
- other warning cases to catch could go here;
warn $_;
}
};
Why can't a method included in this same file be found?
Some possible reasons: your inheritance is getting confused, you've misspelled the method name, or the object is of the wrong type. Check out perltoot for details about any of the above cases. You may also use print ref($object) to find out the class $object was blessed into.
Another possible reason for problems is because you've used the indirect object syntax (eg, find Guru ) 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-) instead. Object notation is explained in perlobj.
Make sure to read about creating modules in perlmod and the perils of indirect objects in `` WARNING '' in perlobj.
How can I find out my current package?
If you're just a random program, you can do this to find out
my $self = shift; my $class = ref($self) $self; warn
How can I comment out a large block of perl code?
=for nobody
This paragraph is commented out
=begin comment text all of this stuff here will be ignored
by everyone
=end comment text =cut
This can't go just anywhere. You have to put a pod directive where the parser is expecting a new statement, not just in the middle of an expression or some other arbitrary yacc grammar production.
How do I clear a package?
no strict 'refs'; my $pack = shift; die Or, if you're using a recent release of Perl, you can just use the Symbol::delete_package() function instead.
How can I use a variable as a variable name?
Beginners often think they want to have a variable contain the name of a variable.
$fred = 23;
$varname = This works sometimes, but it is a very bad idea for two reasons.
The first reason is that this technique only works on global 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 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.
$fred = 23;
$varname =
$str = s/($w+)/$1/eeg; # need double eval
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 because 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 = $name =
And just use a multilevel hash to start with.
The only times that you absolutely must use symbolic references are when you really must refer to the symbol table. This may be because it's something that 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 my $name (@colors) { no strict 'refs'; # renege for the block
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 directly manipulate the symbol table. 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.
Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. All rights reserved.
When included as part of the Standard Version of Perl, or as part of its complete documentation whether printed or otherwise, this work may be distributed only under the terms of Perl's Artistic License. Any distribution of this file or derivatives thereof outside of that package require that special arrangements be made with copyright holder.
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.
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