diff options
Diffstat (limited to 'pod/perlsyn.pod')
| -rw-r--r-- | pod/perlsyn.pod | 84 |
1 files changed, 44 insertions, 40 deletions
diff --git a/pod/perlsyn.pod b/pod/perlsyn.pod index 77dcc597a9..9c3f6617bd 100644 --- a/pod/perlsyn.pod +++ b/pod/perlsyn.pod @@ -32,20 +32,23 @@ that. 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 +the beginning or the end of the script. However, if you're using lexically-scoped private variables created with 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 (prototyped to take one scalar parameter) without defining it by saying just: +subroutine without defining it by saying C<sub name>, thus: - sub myname ($); + sub myname; $me = myname $0 or die "can't get myname"; -Note that it functions as a list operator though, not as a unary -operator, so be careful to use C<or> instead of C<||> there. +Note that it functions as a list operator, not as a unary operator; so +be careful to use C<or> instead of C<||> in this case. However, if +you were to declare the subroutine as C<sub myname ($)>, then +C<myname> would functonion 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. @@ -65,7 +68,7 @@ semicolon, unless it is the final statement in a block, in which case the semicolon is optional. (A semicolon is still encouraged there 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), +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. Any simple statement may optionally be followed by a I<SINGLE> modifier, @@ -178,25 +181,26 @@ want to skip ahead and get the next record. while (<>) { chomp; - if (s/\\$//) { - $_ .= <>; + if (s/\\$//) { + $_ .= <>; redo unless eof(); } # now process $_ - } + } which is Perl short-hand for the more explicitly written version: - LINE: while ($line = <ARGV>) { + LINE: while (defined($line = <ARGV>)) { chomp($line); - if ($line =~ s/\\$//) { - $line .= <ARGV>; + if ($line =~ s/\\$//) { + $line .= <ARGV>; redo LINE unless eof(); # not eof(ARGV)! } # now process $line - } + } -Or here's a simpleminded Pascal comment stripper (warning: assumes no { or } in strings). +Or here's a simpleminded Pascal comment stripper (warning: assumes no +{ or } in strings). LINE: while (<STDIN>) { while (s|({.*}.*){.*}|$1 |) {} @@ -246,15 +250,15 @@ for variables declared with C<my> in the initialization expression.) 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 +problem you get into if you explicitly test for end-of-file on +an interactive file descriptor causing your program to appear to hang. $on_a_tty = -t STDIN && -t STDOUT; sub prompt { print "yes? " if $on_a_tty } for ( prompt(); <STDIN>; prompt() ) { # do something - } + } =head2 Foreach Loops @@ -309,12 +313,12 @@ Here's how a C programmer might code up a particular algorithm in Perl: Whereas here's how a Perl programmer more comfortable with the idiom might do it: - OUTER: foreach my $wid (@ary1) { + OUTER: foreach my $wid (@ary1) { INNER: foreach 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 @@ -370,19 +374,19 @@ or or formatted so it stands out more as a "proper" switch statement: SWITCH: { - /^abc/ && do { - $abc = 1; - last SWITCH; + /^abc/ && do { + $abc = 1; + last SWITCH; }; - /^def/ && do { - $def = 1; - last SWITCH; + /^def/ && do { + $def = 1; + last SWITCH; }; - /^xyz/ && do { - $xyz = 1; - last SWITCH; + /^xyz/ && do { + $xyz = 1; + last SWITCH; }; $nothing = 1; } @@ -416,14 +420,14 @@ a temporary assignment to $_ for convenient matching: /Anywhere/ && do { push @flags, '-h'; last; }; /In Rulings/ && do { last; }; die "unknown value for form variable where: `$where'"; - } + } Another interesting approach to a switch statement is arrange for a C<do> block to return the proper value: $amode = do { - if ($flag & O_RDONLY) { "r" } - elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } + if ($flag & O_RDONLY) { "r" } + elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } elsif ($flag & O_RDWR) { if ($flag & O_CREAT) { "w+" } else { ($flag & O_APPEND) ? "a+" : "r+" } @@ -475,14 +479,14 @@ encounters a line that begins with an equal sign and a word, like this 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>. +text is described in L<perlpod>. This allows you to intermix your source code and your documentation text freely, as in =item snazzle($) - The snazzle() function will behave in the most spectacular + The snazzle() function will behave in the most spectacular form that you can possibly imagine, not even excepting cybernetic pyrotechnics. @@ -491,11 +495,11 @@ and your documentation text freely, as in sub snazzle($) { my $thingie = shift; ......... - } + } -Note that pod translators should look at only paragraphs beginning +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 +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. @@ -532,18 +536,18 @@ shell: 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'"; |