Acorn User 10/AU_CD10.iso/Updates/Perl/Non-RPC/Docs/perlsyn

Text File | 1999-04-17 | 23KB | 678 lines

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.)
    Text from a `"#"' character until the end of the line is a
    comment, and is ignored. 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 function 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
        foreach EXPR


    The `if' and `unless' modifiers have the expected semantics,
    presuming you're a speaker of English. The `foreach' modifier is
    an iterator: For each value in EXPR, it aliases `$_' to the
    value and executes the statement. The `while' and `until'
    modifiers 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 the "do" entry in the perlfunc manpage. 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 put another block inside of it (for `next') or around
    it (for `last') to do that sort of thing. For `next', just
    double the braces:

        do {{
        next if $x == $y;
        # do something here
        }} until $x++ > $z;


    For `last', you have to be more elaborate:

        LOOP: {
            do {
            last if $x = $y**2;
            # do something here
            } while $x++ <= $z;
        }


  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
        }


    Note that if there were a `continue' block on the above code, it
    would get executed even on discarded lines. This is often used
    to reset line counters or `?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 `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 loop control statements don't work in an `if' or `unless',
    since they aren't loops. You can double the braces to make them
    such, though.

        if (/pattern/) {{
        next if /fred/;
        next if /barney/;
        # so something here
        }}


    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 if you have
    subroutine or format declarations within the loop which refer to
    it.)

    The `foreach' keyword is actually a synonym for the `for'
    keyword, so you can use `foreach' for readability or `for' for
    brevity. (Or because the Bourne shell is more familiar to you
    than *csh*, so writing `for' comes more naturally.) If VAR is
    omitted, `$_' is set to each value. If any element of LIST is an
    lvalue, you can modify it 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.

    If any part of LIST is an array, `foreach' will get very
    confused if you add or remove elements within the loop body, for
    example with `splice'. So don't do that.

    `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/ }

        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" }    # XXX: isn't this 0?
        elsif  ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
        elsif  ($flag & O_RDWR)   {
            if ($flag & O_CREAT)  { "w+" }
            else                  { ($flag & O_APPEND) ? "a+" : "r+" }
        }
        };


    Or

            print do {
                ($flags & O_WRONLY) ? "write-only"          :
                ($flags & O_RDWR)   ? "read-write"          :
                                      "read-only";
            };


    Or if you are certainly that all the `&&' clauses are true, you
    can use something like this, which "switches" on the value of
    the `HTTP_USER_AGENT' envariable.

        #!/usr/bin/perl
        # pick out jargon file page based on browser
        $dir = 'http://www.wins.uva.nl/~mes/jargon';
        for ($ENV{HTTP_USER_AGENT}) {
        $page  =    /Mac/            && 'm/Macintrash.html'
             || /Win(dows )?NT/  && 'e/evilandrude.html'
             || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
             || /Linux/          && 'l/Linux.html'
             || /HP-UX/          && 'h/HP-SUX.html'
             || /SunOS/          && 's/ScumOS.html'
             ||                     'a/AppendixB.html';
        }
        print "Location: $dir/$page\015\012\015\012";


    That kind of switch statement only works when you know the `&&'
    clauses will be true. If you don't, the previous `?:' example
    should be used.

    You might also consider writing a hash instead of synthesizing a
    `switch' statement.

  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 `goto's 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.