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diff --git a/Master/tlpkg/tlperl/lib/pods/perlsyn.pod b/Master/tlpkg/tlperl/lib/pods/perlsyn.pod new file mode 100755 index 00000000000..cd9501211c3 --- /dev/null +++ b/Master/tlpkg/tlperl/lib/pods/perlsyn.pod @@ -0,0 +1,900 @@ +=head1 NAME +X<syntax> + +perlsyn - Perl syntax + +=head1 DESCRIPTION + +A Perl program consists of a sequence of declarations and statements +which run from the top to the bottom. Loops, subroutines and other +control structures allow you to jump around within the code. + +Perl is a B<free-form> language, you can format and indent it however +you like. Whitespace mostly serves to separate tokens, unlike +languages like Python where it is an important part of the syntax. + +Many of Perl's syntactic elements are B<optional>. Rather than +requiring you to put parentheses around every function call and +declare every variable, you can often leave such explicit elements off +and Perl will figure out what you meant. This is known as B<Do What I +Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to +code in a style with which they are comfortable. + +Perl B<borrows syntax> and concepts from many languages: awk, sed, C, +Bourne Shell, Smalltalk, Lisp and even English. Other +languages have borrowed syntax from Perl, particularly its regular +expression extensions. So if you have programmed in another language +you will see familiar pieces in Perl. They often work the same, but +see L<perltrap> for information about how they differ. + +=head2 Declarations +X<declaration> X<undef> X<undefined> X<uninitialized> + +The only things you need to declare in Perl are report formats and +subroutines (and sometimes not even subroutines). A variable holds +the undefined value (C<undef>) until it has been assigned a defined +value, which is anything other than C<undef>. When used as a number, +C<undef> is treated as C<0>; when used as a string, it is treated as +the empty string, C<"">; and when used as a reference that isn't being +assigned to, it is treated as an error. If you enable warnings, +you'll be notified of an uninitialized value whenever you treat +C<undef> as a string or a number. Well, usually. Boolean contexts, +such as: + + my $a; + if ($a) {} + +are exempt from warnings (because they care about truth rather than +definedness). Operators such as C<++>, C<-->, C<+=>, +C<-=>, and C<.=>, that operate on undefined left values such as: + + my $a; + $a++; + +are also always exempt from such warnings. + +A declaration can be put anywhere a statement can, but has no effect on +the execution of the primary sequence of statements--declarations all +take effect at compile time. Typically all the declarations are put at +the beginning or the end of the script. However, if you're using +lexically-scoped private variables created with C<my()>, you'll +have to make sure +your format or subroutine definition is within the same block scope +as the my if you expect to be able to access those private variables. + +Declaring a subroutine allows a subroutine name to be used as if it were a +list operator from that point forward in the program. You can declare a +subroutine without defining it by saying C<sub name>, thus: +X<subroutine, declaration> + + sub myname; + $me = myname $0 or die "can't get myname"; + +Note that myname() functions as a list operator, not as a unary operator; +so be careful to use C<or> instead of C<||> in this case. However, if +you were to declare the subroutine as C<sub myname ($)>, then +C<myname> would function as a unary operator, so either C<or> or +C<||> would work. + +Subroutines declarations can also be loaded up with the C<require> statement +or both loaded and imported into your namespace with a C<use> statement. +See L<perlmod> for details on this. + +A statement sequence may contain declarations of lexically-scoped +variables, but apart from declaring a variable name, the declaration acts +like an ordinary statement, and is elaborated within the sequence of +statements as if it were an ordinary statement. That means it actually +has both compile-time and run-time effects. + +=head2 Comments +X<comment> X<#> + +Text from a C<"#"> character until the end of the line is a comment, +and is ignored. Exceptions include C<"#"> inside a string or regular +expression. + +=head2 Simple Statements +X<statement> X<semicolon> X<expression> X<;> + +The only kind of simple statement is an expression evaluated for its +side effects. Every simple statement must be terminated with a +semicolon, unless it is the final statement in a block, in which case +the semicolon is optional. (A semicolon is still encouraged if the +block takes up more than one line, because you may eventually add +another line.) Note that there are some operators like C<eval {}> and +C<do {}> that look like compound statements, but aren't (they're just +TERMs in an expression), and thus need an explicit termination if used +as the last item in a statement. + +=head2 Truth and Falsehood +X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0> + +The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and +C<undef> are all false in a boolean context. All other values are true. +Negation of a true value by C<!> or C<not> returns a special false value. +When evaluated as a string it is treated as C<''>, but as a number, it +is treated as 0. + +=head2 Statement Modifiers +X<statement modifier> X<modifier> X<if> X<unless> X<while> +X<until> X<foreach> X<for> + +Any simple statement may optionally be followed by a I<SINGLE> modifier, +just before the terminating semicolon (or block ending). The possible +modifiers are: + + if EXPR + unless EXPR + while EXPR + until EXPR + foreach LIST + +The C<EXPR> following the modifier is referred to as the "condition". +Its truth or falsehood determines how the modifier will behave. + +C<if> executes the statement once I<if> and only if the condition is +true. C<unless> is the opposite, it executes the statement I<unless> +the condition is true (i.e., if the condition is false). + + print "Basset hounds got long ears" if length $ear >= 10; + go_outside() and play() unless $is_raining; + +The C<foreach> modifier is an iterator: it executes the statement once +for each item in the LIST (with C<$_> aliased to each item in turn). + + print "Hello $_!\n" foreach qw(world Dolly nurse); + +C<while> repeats the statement I<while> the condition is true. +C<until> does the opposite, it repeats the statement I<until> the +condition is true (or while the condition is false): + + # Both of these count from 0 to 10. + print $i++ while $i <= 10; + print $j++ until $j > 10; + +The C<while> and C<until> modifiers have the usual "C<while> loop" +semantics (conditional evaluated first), except when applied to a +C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in +which case the block executes once before the conditional is +evaluated. This is so that you can write loops like: + + do { + $line = <STDIN>; + ... + } until $line eq ".\n"; + +See L<perlfunc/do>. Note also that the loop control statements described +later will I<NOT> work in this construct, because modifiers don't take +loop labels. Sorry. You can always put another block inside of it +(for C<next>) or around it (for C<last>) to do that sort of thing. +For C<next>, just double the braces: +X<next> X<last> X<redo> + + do {{ + next if $x == $y; + # do something here + }} until $x++ > $z; + +For C<last>, you have to be more elaborate: +X<last> + + LOOP: { + do { + last if $x = $y**2; + # do something here + } while $x++ <= $z; + } + +B<NOTE:> The behaviour of a C<my> statement modified with a statement +modifier conditional or loop construct (e.g. C<my $x if ...>) is +B<undefined>. The value of the C<my> variable may be C<undef>, any +previously assigned value, or possibly anything else. Don't rely on +it. Future versions of perl might do something different from the +version of perl you try it out on. Here be dragons. +X<my> + +=head2 Compound Statements +X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace> +X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue> + +In Perl, a sequence of statements that defines a scope is called a block. +Sometimes a block is delimited by the file containing it (in the case +of a required file, or the program as a whole), and sometimes a block +is delimited by the extent of a string (in the case of an eval). + +But generally, a block is delimited by curly brackets, also known as braces. +We will call this syntactic construct a BLOCK. + +The following compound statements may be used to control flow: + + if (EXPR) BLOCK + if (EXPR) BLOCK else BLOCK + if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK + LABEL while (EXPR) BLOCK + LABEL while (EXPR) BLOCK continue BLOCK + LABEL until (EXPR) BLOCK + LABEL until (EXPR) BLOCK continue BLOCK + LABEL for (EXPR; EXPR; EXPR) BLOCK + LABEL foreach VAR (LIST) BLOCK + LABEL foreach VAR (LIST) BLOCK continue BLOCK + LABEL BLOCK continue BLOCK + +Note that, unlike C and Pascal, these are defined in terms of BLOCKs, +not statements. This means that the curly brackets are I<required>--no +dangling statements allowed. If you want to write conditionals without +curly brackets there are several other ways to do it. The following +all do the same thing: + + if (!open(FOO)) { die "Can't open $FOO: $!"; } + die "Can't open $FOO: $!" unless open(FOO); + open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! + open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; + # a bit exotic, that last one + +The C<if> statement is straightforward. Because BLOCKs are always +bounded by curly brackets, there is never any ambiguity about which +C<if> an C<else> goes with. If you use C<unless> in place of C<if>, +the sense of the test is reversed. + +The C<while> statement executes the block as long as the expression is +L<true|/"Truth and Falsehood">. +The C<until> statement executes the block as long as the expression is +false. +The LABEL is optional, and if present, consists of an identifier followed +by a colon. The LABEL identifies the loop for the loop control +statements C<next>, C<last>, and C<redo>. +If the LABEL is omitted, the loop control statement +refers to the innermost enclosing loop. This may include dynamically +looking back your call-stack at run time to find the LABEL. Such +desperate behavior triggers a warning if you use the C<use warnings> +pragma or the B<-w> flag. + +If there is a C<continue> BLOCK, it is always executed just before the +conditional is about to be evaluated again. Thus it can be used to +increment a loop variable, even when the loop has been continued via +the C<next> statement. + +=head2 Loop Control +X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue> + +The C<next> command starts the next iteration of the loop: + + LINE: while (<STDIN>) { + next LINE if /^#/; # discard comments + ... + } + +The C<last> command immediately exits the loop in question. The +C<continue> block, if any, is not executed: + + LINE: while (<STDIN>) { + last LINE if /^$/; # exit when done with header + ... + } + +The C<redo> command restarts the loop block without evaluating the +conditional again. The C<continue> block, if any, is I<not> executed. +This command is normally used by programs that want to lie to themselves +about what was just input. + +For example, when processing a file like F</etc/termcap>. +If your input lines might end in backslashes to indicate continuation, you +want to skip ahead and get the next record. + + while (<>) { + chomp; + if (s/\\$//) { + $_ .= <>; + redo unless eof(); + } + # now process $_ + } + +which is Perl short-hand for the more explicitly written version: + + LINE: while (defined($line = <ARGV>)) { + chomp($line); + if ($line =~ s/\\$//) { + $line .= <ARGV>; + redo LINE unless eof(); # not eof(ARGV)! + } + # now process $line + } + +Note that if there were a C<continue> block on the above code, it would +get executed only on lines discarded by the regex (since redo skips the +continue block). A continue block is often used to reset line counters +or C<?pat?> one-time matches: + + # inspired by :1,$g/fred/s//WILMA/ + while (<>) { + ?(fred)? && s//WILMA $1 WILMA/; + ?(barney)? && s//BETTY $1 BETTY/; + ?(homer)? && s//MARGE $1 MARGE/; + } continue { + print "$ARGV $.: $_"; + close ARGV if eof(); # reset $. + reset if eof(); # reset ?pat? + } + +If the word C<while> is replaced by the word C<until>, the sense of the +test is reversed, but the conditional is still tested before the first +iteration. + +The loop control statements don't work in an C<if> or C<unless>, since +they aren't loops. You can double the braces to make them such, though. + + if (/pattern/) {{ + last if /fred/; + next if /barney/; # same effect as "last", but doesn't document as well + # do something here + }} + +This is caused by the fact that a block by itself acts as a loop that +executes once, see L<"Basic BLOCKs">. + +The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer +available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. + +=head2 For Loops +X<for> X<foreach> + +Perl's C-style C<for> loop works like the corresponding C<while> loop; +that means that this: + + for ($i = 1; $i < 10; $i++) { + ... + } + +is the same as this: + + $i = 1; + while ($i < 10) { + ... + } continue { + $i++; + } + +There is one minor difference: if variables are declared with C<my> +in the initialization section of the C<for>, the lexical scope of +those variables is exactly the C<for> loop (the body of the loop +and the control sections). +X<my> + +Besides the normal array index looping, C<for> can lend itself +to many other interesting applications. Here's one that avoids the +problem you get into if you explicitly test for end-of-file on +an interactive file descriptor causing your program to appear to +hang. +X<eof> X<end-of-file> X<end of file> + + $on_a_tty = -t STDIN && -t STDOUT; + sub prompt { print "yes? " if $on_a_tty } + for ( prompt(); <STDIN>; prompt() ) { + # do something + } + +Using C<readline> (or the operator form, C<< <EXPR> >>) as the +conditional of a C<for> loop is shorthand for the following. This +behaviour is the same as a C<while> loop conditional. +X<readline> X<< <> >> + + for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { + # do something + } + +=head2 Foreach Loops +X<for> X<foreach> + +The C<foreach> loop iterates over a normal list value and sets the +variable VAR to be each element of the list in turn. If the variable +is preceded with the keyword C<my>, then it is lexically scoped, and +is therefore visible only within the loop. Otherwise, the variable is +implicitly local to the loop and regains its former value upon exiting +the loop. If the variable was previously declared with C<my>, it uses +that variable instead of the global one, but it's still localized to +the loop. This implicit localisation occurs I<only> in a C<foreach> +loop. +X<my> X<local> + +The C<foreach> keyword is actually a synonym for the C<for> keyword, so +you can use C<foreach> for readability or C<for> for brevity. (Or because +the Bourne shell is more familiar to you than I<csh>, so writing C<for> +comes more naturally.) If VAR is omitted, C<$_> is set to each value. +X<$_> + +If any element of LIST is an lvalue, you can modify it by modifying +VAR inside the loop. Conversely, if any element of LIST is NOT an +lvalue, any attempt to modify that element will fail. In other words, +the C<foreach> loop index variable is an implicit alias for each item +in the list that you're looping over. +X<alias> + +If any part of LIST is an array, C<foreach> will get very confused if +you add or remove elements within the loop body, for example with +C<splice>. So don't do that. +X<splice> + +C<foreach> probably won't do what you expect if VAR is a tied or other +special variable. Don't do that either. + +Examples: + + for (@ary) { s/foo/bar/ } + + for my $elem (@elements) { + $elem *= 2; + } + + for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { + print $count, "\n"; sleep(1); + } + + for (1..15) { print "Merry Christmas\n"; } + + foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { + print "Item: $item\n"; + } + +Here's how a C programmer might code up a particular algorithm in Perl: + + for (my $i = 0; $i < @ary1; $i++) { + for (my $j = 0; $j < @ary2; $j++) { + if ($ary1[$i] > $ary2[$j]) { + last; # can't go to outer :-( + } + $ary1[$i] += $ary2[$j]; + } + # this is where that last takes me + } + +Whereas here's how a Perl programmer more comfortable with the idiom might +do it: + + OUTER: for my $wid (@ary1) { + INNER: for my $jet (@ary2) { + next OUTER if $wid > $jet; + $wid += $jet; + } + } + +See how much easier this is? It's cleaner, safer, and faster. It's +cleaner because it's less noisy. It's safer because if code gets added +between the inner and outer loops later on, the new code won't be +accidentally executed. The C<next> explicitly iterates the other loop +rather than merely terminating the inner one. And it's faster because +Perl executes a C<foreach> statement more rapidly than it would the +equivalent C<for> loop. + +=head2 Basic BLOCKs +X<block> + +A BLOCK by itself (labeled or not) is semantically equivalent to a +loop that executes once. Thus you can use any of the loop control +statements in it to leave or restart the block. (Note that this is +I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief +C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> +block is optional. + +The BLOCK construct can be used to emulate case structures. + + SWITCH: { + if (/^abc/) { $abc = 1; last SWITCH; } + if (/^def/) { $def = 1; last SWITCH; } + if (/^xyz/) { $xyz = 1; last SWITCH; } + $nothing = 1; + } + +Such constructs are quite frequently used, because older versions +of Perl had no official C<switch> statement. + +=head2 Switch statements +X<switch> X<case> X<given> X<when> X<default> + +Starting from Perl 5.10, you can say + + use feature "switch"; + +which enables a switch feature that is closely based on the +Perl 6 proposal. + +The keywords C<given> and C<when> are analogous +to C<switch> and C<case> in other languages, so the code +above could be written as + + given($_) { + when (/^abc/) { $abc = 1; } + when (/^def/) { $def = 1; } + when (/^xyz/) { $xyz = 1; } + default { $nothing = 1; } + } + +This construct is very flexible and powerful. For example: + + use feature ":5.10"; + given($foo) { + when (undef) { + say '$foo is undefined'; + } + when ("foo") { + say '$foo is the string "foo"'; + } + when ([1,3,5,7,9]) { + say '$foo is an odd digit'; + continue; # Fall through + } + when ($_ < 100) { + say '$foo is numerically less than 100'; + } + when (\&complicated_check) { + say 'a complicated check for $foo is true'; + } + default { + die q(I don't know what to do with $foo); + } + } + +C<given(EXPR)> will assign the value of EXPR to C<$_> +within the lexical scope of the block, so it's similar to + + do { my $_ = EXPR; ... } + +except that the block is automatically broken out of by a +successful C<when> or an explicit C<break>. + +Most of the power comes from implicit smart matching: + + when($foo) + +is exactly equivalent to + + when($_ ~~ $foo) + +Most of the time, C<when(EXPR)> is treated as an implicit smart match of +C<$_>, i.e. C<$_ ~~ EXPR>. (See L</"Smart matching in detail"> for more +information on smart matching.) But when EXPR is one of the below +exceptional cases, it is used directly as a boolean: + +=over 4 + +=item * + +a subroutine or method call + +=item * + +a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>, +or a negated regular expression match (C<!/REGEX/> or C<$foo !~ /REGEX/>). + +=item * + +a comparison such as C<$_ E<lt> 10> or C<$x eq "abc"> +(or of course C<$_ ~~ $c>) + +=item * + +C<defined(...)>, C<exists(...)>, or C<eof(...)> + +=item * + +a negated expression C<!(...)> or C<not (...)>, or a logical +exclusive-or C<(...) xor (...)>. + +=item * + +a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>, +that return numerical values, not boolean ones. + +=item * + +the C<..> and C<...> flip-flop operators. + +=back + +In those cases the value of EXPR is used directly as a boolean. + +Furthermore: + +=over 4 + +=item * + +If EXPR is C<... && ...> or C<... and ...>, the test +is applied recursively to both arguments. If I<both> +arguments pass the test, then the argument is treated +as boolean. + +=item * + +If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test +is applied recursively to the first argument. + +=back + +These rules look complicated, but usually they will do what +you want. For example you could write: + + when (/^\d+$/ && $_ < 75) { ... } + +Another useful shortcut is that, if you use a literal array +or hash as the argument to C<given>, it is turned into a +reference. So C<given(@foo)> is the same as C<given(\@foo)>, +for example. + +C<default> behaves exactly like C<when(1 == 1)>, which is +to say that it always matches. + +=head3 Breaking out + +You can use the C<break> keyword to break out of the enclosing +C<given> block. Every C<when> block is implicitly ended with +a C<break>. + +=head3 Fall-through + +You can use the C<continue> keyword to fall through from one +case to the next: + + given($foo) { + when (/x/) { say '$foo contains an x'; continue } + when (/y/) { say '$foo contains a y' } + default { say '$foo does not contain a y' } + } + +=head3 Switching in a loop + +Instead of using C<given()>, you can use a C<foreach()> loop. +For example, here's one way to count how many times a particular +string occurs in an array: + + my $count = 0; + for (@array) { + when ("foo") { ++$count } + } + print "\@array contains $count copies of 'foo'\n"; + +On exit from the C<when> block, there is an implicit C<next>. +You can override that with an explicit C<last> if you're only +interested in the first match. + +This doesn't work if you explicitly specify a loop variable, +as in C<for $item (@array)>. You have to use the default +variable C<$_>. (You can use C<for my $_ (@array)>.) + +=head3 Smart matching in detail + +The behaviour of a smart match depends on what type of thing its arguments +are. The behaviour is determined by the following table: the first row +that applies determines the match behaviour (which is thus mostly +determined by the type of the right operand). Note that the smart match +implicitly dereferences any non-blessed hash or array ref, so the "Hash" +and "Array" entries apply in those cases. (For blessed references, the +"Object" entries apply.) + +Note that the "Matching Code" column is not always an exact rendition. For +example, the smart match operator short-circuits whenever possible, but +C<grep> does not. + + $a $b Type of Match Implied Matching Code + ====== ===== ===================== ============= + Any undef undefined !defined $a + + Any Object invokes ~~ overloading on $object, or dies + + Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a + Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a + Any CodeRef scalar sub truth $b->($a) + + Hash Hash hash keys identical (every key is found in both hashes) + Array Hash hash slice existence grep { exists $b->{$_} } @$a + Regex Hash hash key grep grep /$a/, keys %$b + undef Hash always false (undef can't be a key) + Any Hash hash entry existence exists $b->{$a} + + Hash Array hash slice existence grep { exists $a->{$_} } @$b + Array Array arrays are comparable[2] + Regex Array array grep grep /$a/, @$b + undef Array array contains undef grep !defined, @$b + Any Array match against an array element[3] + grep $a ~~ $_, @$b + + Hash Regex hash key grep grep /$b/, keys %$a + Array Regex array grep grep /$b/, @$a + Any Regex pattern match $a =~ /$b/ + + Object Any invokes ~~ overloading on $object, or falls back: + Any Num numeric equality $a == $b + Num numish[4] numeric equality $a == $b + undef Any undefined !defined($b) + Any Any string equality $a eq $b + + 1 - empty hashes or arrays will match. + 2 - that is, each element smart-matches the element of same index in the + other array. [3] + 3 - If a circular reference is found, we fall back to referential equality. + 4 - either a real number, or a string that looks like a number + +=head3 Custom matching via overloading + +You can change the way that an object is matched by overloading +the C<~~> operator. This may alter the usual smart match semantics. + +It should be noted that C<~~> will refuse to work on objects that +don't overload it (in order to avoid relying on the object's +underlying structure). + +Note also that smart match's matching rules take precedence over +overloading, so if C<$obj> has smart match overloading, then + + $obj ~~ X + +will not automatically invoke the overload method with X as an argument; +instead the table above is consulted as normal, and based in the type of X, +overloading may or may not be invoked. + +See L<overload>. + +=head3 Differences from Perl 6 + +The Perl 5 smart match and C<given>/C<when> constructs are not +absolutely identical to their Perl 6 analogues. The most visible +difference is that, in Perl 5, parentheses are required around +the argument to C<given()> and C<when()>. Parentheses in Perl 6 +are always optional in a control construct such as C<if()>, +C<while()>, or C<when()>; they can't be made optional in Perl +5 without a great deal of potential confusion, because Perl 5 +would parse the expression + + given $foo { + ... + } + +as though the argument to C<given> were an element of the hash +C<%foo>, interpreting the braces as hash-element syntax. + +The table of smart matches is not identical to that proposed by the +Perl 6 specification, mainly due to the differences between Perl 6's +and Perl 5's data models. + +In Perl 6, C<when()> will always do an implicit smart match +with its argument, whilst it is convenient in Perl 5 to +suppress this implicit smart match in certain situations, +as documented above. (The difference is largely because Perl 5 +does not, even internally, have a boolean type.) + +=head2 Goto +X<goto> + +Although not for the faint of heart, Perl does support a C<goto> +statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and +C<goto>-&NAME. A loop's LABEL is not actually a valid target for +a C<goto>; it's just the name of the loop. + +The C<goto>-LABEL form finds the statement labeled with LABEL and resumes +execution there. It may not be used to go into any construct that +requires initialization, such as a subroutine or a C<foreach> loop. It +also can't be used to go into a construct that is optimized away. It +can be used to go almost anywhere else within the dynamic scope, +including out of subroutines, but it's usually better to use some other +construct such as C<last> or C<die>. The author of Perl has never felt the +need to use this form of C<goto> (in Perl, that is--C is another matter). + +The C<goto>-EXPR form expects a label name, whose scope will be resolved +dynamically. This allows for computed C<goto>s per FORTRAN, but isn't +necessarily recommended if you're optimizing for maintainability: + + goto(("FOO", "BAR", "GLARCH")[$i]); + +The C<goto>-&NAME form is highly magical, and substitutes a call to the +named subroutine for the currently running subroutine. This is used by +C<AUTOLOAD()> subroutines that wish to load another subroutine and then +pretend that the other subroutine had been called in the first place +(except that any modifications to C<@_> in the current subroutine are +propagated to the other subroutine.) After the C<goto>, not even C<caller()> +will be able to tell that this routine was called first. + +In almost all cases like this, it's usually a far, far better idea to use the +structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of +resorting to a C<goto>. For certain applications, the catch and throw pair of +C<eval{}> and die() for exception processing can also be a prudent approach. + +=head2 PODs: Embedded Documentation +X<POD> X<documentation> + +Perl has a mechanism for intermixing documentation with source code. +While it's expecting the beginning of a new statement, if the compiler +encounters a line that begins with an equal sign and a word, like this + + =head1 Here There Be Pods! + +Then that text and all remaining text up through and including a line +beginning with C<=cut> will be ignored. The format of the intervening +text is described in L<perlpod>. + +This allows you to intermix your source code +and your documentation text freely, as in + + =item snazzle($) + + The snazzle() function will behave in the most spectacular + form that you can possibly imagine, not even excepting + cybernetic pyrotechnics. + + =cut back to the compiler, nuff of this pod stuff! + + sub snazzle($) { + my $thingie = shift; + ......... + } + +Note that pod translators should look at only paragraphs beginning +with a pod directive (it makes parsing easier), whereas the compiler +actually knows to look for pod escapes even in the middle of a +paragraph. This means that the following secret stuff will be +ignored by both the compiler and the translators. + + $a=3; + =secret stuff + warn "Neither POD nor CODE!?" + =cut back + print "got $a\n"; + +You probably shouldn't rely upon the C<warn()> being podded out forever. +Not all pod translators are well-behaved in this regard, and perhaps +the compiler will become pickier. + +One may also use pod directives to quickly comment out a section +of code. + +=head2 Plain Old Comments (Not!) +X<comment> X<line> X<#> X<preprocessor> X<eval> + +Perl can process line directives, much like the C preprocessor. Using +this, one can control Perl's idea of filenames and line numbers in +error or warning messages (especially for strings that are processed +with C<eval()>). The syntax for this mechanism is the same as for most +C preprocessors: it matches the regular expression + + # example: '# line 42 "new_filename.plx"' + /^\# \s* + line \s+ (\d+) \s* + (?:\s("?)([^"]+)\2)? \s* + $/x + +with C<$1> being the line number for the next line, and C<$3> being +the optional filename (specified with or without quotes). + +There is a fairly obvious gotcha included with the line directive: +Debuggers and profilers will only show the last source line to appear +at a particular line number in a given file. Care should be taken not +to cause line number collisions in code you'd like to debug later. + +Here are some examples that you should be able to type into your command +shell: + + % perl + # line 200 "bzzzt" + # the `#' on the previous line must be the first char on line + die 'foo'; + __END__ + foo at bzzzt line 201. + + % perl + # line 200 "bzzzt" + eval qq[\n#line 2001 ""\ndie 'foo']; print $@; + __END__ + foo at - line 2001. + + % perl + eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; + __END__ + foo at foo bar line 200. + + % perl + # line 345 "goop" + eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; + print $@; + __END__ + foo at goop line 345. + +=cut |