X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq7.pod;h=9e559f7d0eb994b9fb18c255e08f2d6329386f1f;hb=da58a35d3e499cdd492619302eb044ac1841788f;hp=72f4bb74abb75e2c83be512a73ec075507e7b28f;hpb=77ca0c92d2c0e47301d906d355d9ab3afb6f6bcb;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq7.pod b/pod/perlfaq7.pod index 72f4bb7..9e559f7 100644 --- a/pod/perlfaq7.pod +++ b/pod/perlfaq7.pod @@ -29,18 +29,18 @@ They are type specifiers, as detailed in L: * for all types of that symbol name. In version 4 you used them like pointers, but in modern perls you can just use references. -A couple of others that you're likely to encounter that aren't -really type specifiers are: +There are 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 EFILEE is I the type specifier for files -nor the name of the handle. It is the CE> operator applied -to the handle FILE. It reads one line (well, record - see +Note that is I 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 L) from the handle FILE in scalar context, or I lines in list context. When performing open, close, or any other operation -besides CE> on files, or even talking about the handle, do +besides C<< <> >> on files, or even when talking about the handle, do I use the brackets. These are correct: C, C and "copying from STDIN to FILE". @@ -49,7 +49,7 @@ I use the brackets. These are correct: C, C). 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 C<=E> operator both +subroutine) and the left-hand operand to the C<< => >> operator both count as though they were quoted: This is like this @@ -84,8 +84,17 @@ Another way is to use undef as an element on the left-hand-side: =head2 How do I temporarily block warnings? -The C<$^W> variable (documented in L) controls -runtime warnings for a block: +If you are running Perl 5.6.0 or better, the C pragma +allows fine control of what warning are produced. +See L for more details. + + { + no warnings; # temporarily turn off warnings + $a = $b + $c; # I know these might be undef + } + +If you have an older version of Perl, the C<$^W> variable (documented +in L) controls runtime warnings for a block: { local $^W = 0; # temporarily turn off warnings @@ -95,21 +104,17 @@ runtime warnings for a block: Note that like all the punctuation variables, you cannot currently use my() on C<$^W>, only local(). -A new C pragma is in the works to provide finer control -over all this. The curious should check the perl5-porters mailing list -archives for details. - =head2 What's an extension? -A way of calling compiled C code from Perl. Reading L -is a good place to learn more about extensions. +An extension is a way of calling compiled C code from Perl. Reading +L 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 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. @@ -168,6 +173,7 @@ own module. Make sure to change the names appropriately. package Some::Module; # assumes Some/Module.pm use strict; + use warnings; BEGIN { use Exporter (); @@ -190,6 +196,10 @@ own module. Make sure to change the names appropriately. } our @EXPORT_OK; + # exported package globals go here + our $Var1; + our %Hashit; + # non-exported package globals go here our @more; our $stuff; @@ -248,7 +258,7 @@ is given no processes to signal): } This is not C<-w> clean, however. There is no C<-w> clean way to -detect taintedness - take this as a hint that you should untaint +detect taintedness--take this as a hint that you should untaint all possibly-tainted data. =head2 What's a closure? @@ -264,7 +274,7 @@ 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 +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. @@ -307,10 +317,10 @@ you want to pass in a bit of code into a function: my $line; timeout( 30, sub { $line = } ); -If the code to execute had been passed in as a string, C<'$line = -ESTDINE'>, there would have been no way for the hypothetical -timeout() function to access the lexical variable $line back in its -caller's scope. +If the code to execute had been passed in as a string, +C<< '$line = ' >>, there would have been no way for the +hypothetical timeout() function to access the lexical variable +$line back in its caller's scope. =head2 What is variable suicide and how can I prevent it? @@ -339,11 +349,14 @@ With the exception of regexes, you need to pass references to these objects. See L for this particular question, and L for information on references. +See ``Passing Regexes'', below, for information on passing regular +expressions. + =over 4 =item Passing Variables and Functions -Regular variables and functions are quite easy: just pass in a +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 ); @@ -360,7 +373,7 @@ reference to an existing or anonymous variable or function: =item Passing Filehandles To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations. -These are "typeglobs" - see L +These are "typeglobs"--see L and especially L for more information. Here's an excerpt: @@ -450,8 +463,8 @@ To pass an object method into a subroutine, you can do this: } } -Or you can use a closure to bundle up the object and its method call -and arguments: +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); @@ -485,8 +498,8 @@ 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 -it at the outer scope level at the top of the file. Assume this is in -file Pax.pm: +the declaration at the outer scope level at the top of the file. +Assume this is in file Pax.pm: package Pax; my $started = scalar(localtime(time())); @@ -506,14 +519,14 @@ See L for details. =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? -C saves away the old value of the global variable C<$x>, -and assigns a new value for the duration of the subroutine, I saves away the old value of the global variable C<$x> +and assigns a new value for the duration of the subroutine I. This is done at run-time, so is called dynamic scoping. local() always affects global variables, also called package variables or dynamic variables. C creates a new variable that is only visible in the current -subroutine. This is done at compile-time, so is called lexical or +subroutine. This is done at compile-time, so it is called lexical or static scoping. my() always affects private variables, also called lexical variables or (improperly) static(ly scoped) variables. @@ -547,8 +560,8 @@ In summary, local() doesn't make what you think of as private, local variables. It gives a global variable a temporary value. my() is what you're looking for if you want private variables. -See L and L for excruciating details. +See L and +L for excruciating details. =head2 How can I access a dynamic variable while a similarly named lexical is in scope? @@ -581,10 +594,10 @@ However, dynamic variables (aka global, local, or package variables) are effectively shallowly bound. Consider this just one more reason not to use them. See the answer to L<"What's a closure?">. -=head2 Why doesn't "my($foo) = EFILEE;" work right? +=head2 Why doesn't "my($foo) = ;" work right? C and C give list context to the right hand side -of C<=>. The EFHE read operation, like so many of Perl's +of C<=>. The 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. This function does nothing to the data itself (contrary to popular myth) @@ -611,7 +624,7 @@ 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. There's +module. See L. There's also an example in L. If you want to overload a Perl operator, such as C<+> or C<**>, @@ -624,8 +637,8 @@ see L. =head2 What's the difference between calling a function as &foo and foo()? When you call a function as C<&foo>, you allow that function access to -your current @_ values, and you by-pass prototypes. That means that -the function doesn't get an empty @_, it gets yours! While not +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 L), it would be hard to consider this a feature in most cases. @@ -699,7 +712,7 @@ 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 +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: @@ -757,16 +770,17 @@ C<__WARN__> like this: 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 for details on these. You may also use C to find out the class C<$object> was blessed into. +out L for details about any of the above cases. You may +also use C to find out the class C<$object> was +blessed into. Another possible reason for problems is because you've used the indirect object syntax (eg, C) 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 C statement instead of -C. If not, make sure to use arrow notation (eg, -Cfind("Samy")>) instead. Object notation is explained in +C. If not, make sure to use arrow notation (eg., +C<< Guru->find("Samy") >>) instead. Object notation is explained in L. Make sure to read about creating modules in L and @@ -779,7 +793,7 @@ out what the currently compiled package is: my $packname = __PACKAGE__; -But if you're a method and you want to print an error message +But, if you're a method and you want to print an error message that includes the kind of object you were called on (which is not necessarily the same as the one in which you were compiled): @@ -851,19 +865,19 @@ of a variable. This works I, but it is a very bad idea for two reasons. -The first reason is that they I. -That means above that if $fred is a lexical variable created with my(), -that the code won't work at all: you'll 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. +The first reason is that this technique I. 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 C 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 a bad idea is that the question often stems from a lack of +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 C<%main::>) instead of a user-defined hash. The solution is to @@ -884,7 +898,7 @@ own variables: $str = 'this has a $fred and $barney in it'; $str =~ s/(\$\w+)/$1/eeg; # need double eval -Instead, it would be better to keep a hash around like %USER_VARS and have +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 @@ -896,11 +910,11 @@ 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 to use another scalar -variable to refer to those by name. +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 = "fred"; $$name{WIFE} = "wilma"; # set %fred @@ -936,9 +950,9 @@ 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. -But for scalars, arrays, and hashes -- and usually for subroutines -- -you probably want to use hard references only. +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. =head1 AUTHOR AND COPYRIGHT @@ -957,3 +971,4 @@ 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. +