perlsyn - Perl syntax

- NAME
- DESCRIPTION
  - Declarations
  - Simple statements
  - Compound statements
  - Loop Control
  - For Loops
  - Foreach Loops
  - Basic BLOCKs and Switch Statements
  - Goto
  - PODs: Embedded Documentation
  - Plain Old Comments (Not!)

# NAME

perlsyn - Perl syntax

# DESCRIPTION

A Perl script consists of a sequence of declarations and statements. The only things that need to be declared in Perl are report formats and subroutines. See the sections below for more information on those declarations. All uninitialized user-created objects are assumed to start with a null or 0 value until they are defined by some explicit operation such as assignment. (Though you can get warnings about the use of undefined values if you like.) The sequence of statements is executed just once, unlike in sed and awk scripts, where the sequence of statements is executed for each input line. While this means that you must explicitly loop over the lines of your input file (or files), it also means you have much more control over which files and which lines you look at. (Actually, I'm lying--it is possible to do an implicit loop with either the -n or -p switch. It's just not the mandatory default like it is in sed and awk.)

## Declarations

Perl is, for the most part, a free-form language. (The only exception to this is format declarations, for obvious reasons.) Comments are indicated by the ``#'' character, and extend to the end of the line. If you attempt to use `/* */` C-style comments, it will be interpreted either as division or pattern matching, depending on the context, and C++ `//` comments just look like a null regular expression, so don't do 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 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 without defining it by saying `sub name`, thus:

        sub myname;
        $me = myname $0             or die "can't get myname";

Note that it functions as a list operator, not as a unary operator; so be careful to use `or` instead of `||` in this case. However, if you were to declare the subroutine as `sub myname ($)`, then `myname` would functonion as a unary operator, so either `or` or `||` would work.

Subroutines declarations can also be loaded up with the require statement or both loaded and imported into your namespace with a use statement. See the perlmod manpage 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.

## Simple statements

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 there if the block takes up more than one line, because you may eventually add another line.) Note that there are some operators like eval {} and 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.

Any simple statement may optionally be followed by a SINGLE modifier, just before the terminating semicolon (or block ending). The possible modifiers are:

        if EXPR
        unless EXPR
        while EXPR
        until EXPR

The `if` and `unless` modifiers have the expected semantics, presuming you're a speaker of English. The `while` and `until` modifiers also have the usual ``while loop'' semantics (conditional evaluated first), except when applied to a do-BLOCK (or to the now-deprecated 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 do. Note also that the loop control statements described later will NOT work in this construct, because modifiers don't take loop labels. Sorry. You can always wrap another block around it to do that sort of thing.

## Compound statements

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 for (EXPR; EXPR; EXPR) BLOCK
        LABEL foreach VAR (LIST) 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 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 `if` statement is straightforward. Because BLOCKs are always bounded by curly brackets, there is never any ambiguity about which `if` an `else` goes with. If you use `unless` in place of `if`, the sense of the test is reversed.

The `while` statement executes the block as long as the expression is true (does not evaluate to the null string or 0 or ``0''). 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 next, last, and 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 -w flag.

If there is a continue BLOCK, it is always executed just before the conditional is about to be evaluated again, just like the third part of a `for` loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the next statement (which is similar to the C continue statement).

## Loop Control

The next command is like the continue statement in C; it starts the next iteration of the loop:

        LINE: while (<STDIN>) {
            next LINE if /^#/;      # discard comments
            ...
        }

The last command is like the `break` statement in C (as used in loops); it immediately exits the loop in question. The continue block, if any, is not executed:

        LINE: while (<STDIN>) {
            last LINE if /^$/;      # exit when done with header
            ...
        }

The redo command restarts the loop block without evaluating the conditional again. The continue block, if any, is 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 /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
        }

Or here's a simpleminded Pascal comment stripper (warning: assumes no { or } in strings).

        LINE: while (<STDIN>) {
            while (s|({.*}.*){.*}|$1 |) {}
            s|{.*}| |;
            if (s|{.*| |) {
                $front = $_;
                while (<STDIN>) {
                    if (/}/) {      # end of comment?
                        s|^|$front{|;
                        redo LINE;
                    }
                }
            }
            print;
        }

Note that if there were a continue block on the above code, it would get executed even on discarded lines.

If the word `while` is replaced by the word `until`, the sense of the test is reversed, but the conditional is still tested before the first iteration.

The form `while/if BLOCK BLOCK`, available in Perl 4, is no longer available. Replace any occurrence of `if BLOCK` by `if (do BLOCK)`.

## For Loops

Perl's C-style `for` loop works exactly like the corresponding `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: The first form implies a lexical scope for variables declared with my in the initialization expression.)

Besides the normal array index looping, `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.

        $on_a_tty = -t STDIN && -t STDOUT;
        sub prompt { print "yes? " if $on_a_tty }
        for ( prompt(); <STDIN>; prompt() ) {
            # do something
        }

## Foreach Loops

The `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 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 my, it uses that variable instead of the global one, but it's still localized to the loop. (Note that a lexically scoped variable can cause problems with you have subroutine or format declarations.)

The `foreach` keyword is actually a synonym for the `for` keyword, so you can use `foreach` for readability or `for` for brevity. If VAR is omitted, `$_` is set to each value. If LIST is an actual array (as opposed to an expression returning a list value), you can modify each element of the array by modifying VAR inside the loop. That's because the `foreach` loop index variable is an implicit alias for each item in the list that you're looping over.

Examples:

        for (@ary) { s/foo/bar/ }

        foreach 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: 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 between the inner and outer loops later on, the new code won't be accidentally executed. The next explicitly iterates the other loop rather than merely terminating the inner one. And it's faster because Perl executes a `foreach` statement more rapidly than it would the equivalent `for` loop.

## Basic BLOCKs and Switch Statements

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 NOT true in eval{}, sub{}, or contrary to popular belief do{} blocks, which do NOT count as loops.) The continue block is optional.

The BLOCK construct is particularly nice for doing case structures.

        SWITCH: {
            if (/^abc/) { $abc = 1; last SWITCH; }
            if (/^def/) { $def = 1; last SWITCH; }
            if (/^xyz/) { $xyz = 1; last SWITCH; }
            $nothing = 1;
        }

There is no official switch statement in Perl, because there are already several ways to write the equivalent. In addition to the above, you could write

        SWITCH: {
            $abc = 1, last SWITCH  if /^abc/;
            $def = 1, last SWITCH  if /^def/;
            $xyz = 1, last SWITCH  if /^xyz/;
            $nothing = 1;
        }

(That's actually not as strange as it looks once you realize that you can use loop control ``operators'' within an expression, That's just the normal C comma operator.)

or

        SWITCH: {
            /^abc/ && do { $abc = 1; last SWITCH; };
            /^def/ && do { $def = 1; last SWITCH; };
            /^xyz/ && do { $xyz = 1; last SWITCH; };
            $nothing = 1;
        }

or formatted so it stands out more as a ``proper'' switch statement:

        SWITCH: {
            /^abc/      && do {
                                $abc = 1;
                                last SWITCH;
                           };

            /^def/      && do {
                                $def = 1;
                                last SWITCH;
                           };

            /^xyz/      && do {
                                $xyz = 1;
                                last SWITCH;
                            };
            $nothing = 1;
        }

or

        SWITCH: {
            /^abc/ and $abc = 1, last SWITCH;
            /^def/ and $def = 1, last SWITCH;
            /^xyz/ and $xyz = 1, last SWITCH;
            $nothing = 1;
        }

or even, horrors,

        if (/^abc/)
            { $abc = 1 }
        elsif (/^def/)
            { $def = 1 }
        elsif (/^xyz/)
            { $xyz = 1 }
        else
            { $nothing = 1 }

A common idiom for a switch statement is to use `foreach`'s aliasing to make a temporary assignment to `$_` for convenient matching:

        SWITCH: for ($where) {
                    /In Card Names/     && do { push @flags, '-e'; last; };
                    /Anywhere/          && do { push @flags, '-h'; last; };
                    /In Rulings/        && do {                    last; };
                    die "unknown value for form variable where: `$where'";
                }

Another interesting approach to a switch statement is arrange for a do block to return the proper value:

        $amode = do {
            if     ($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+" }
            }
        };

## Goto

Although not for the faint of heart, Perl does support a goto statement. A loop's LABEL is not actually a valid target for a goto; it's just the name of the loop. There are three forms: goto-LABEL, goto-EXPR, and goto-&NAME.

The 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 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 last or die. The author of Perl has never felt the need to use this form of goto (in Perl, that is--C is another matter).

The goto-EXPR form expects a label name, whose scope will be resolved dynamically. This allows for computed gotos per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:

        goto ("FOO", "BAR", "GLARCH")[$i];

The goto-&NAME form is highly magical, and substitutes a call to the named subroutine for the currently running subroutine. This is used by `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 `@_` in the current subroutine are propagated to the other subroutine.) After the goto, not even `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 next, last, or redo instead of resorting to a goto. For certain applications, the catch and throw pair of eval{} and `die()` for exception processing can also be a prudent approach.

## PODs: Embedded 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 `=cut` will be ignored. The format of the intervening text is described in the perlpod manpage.

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 `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.

## Plain Old Comments (Not!)

Much like the C preprocessor, perl can process line directives. 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 `eval()).` The syntax for this mechanism is the same as for most C preprocessors: it matches the regular expression `/^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/` with `$1` being the line number for the next line, and `$2` being the optional filename (specified within quotes).

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.