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

Text File | 1999-04-17 | 15KB | 373 lines

NAME
    perlform - Perl formats

DESCRIPTION
    Perl has a mechanism to help you generate simple reports and
    charts. To facilitate this, Perl helps you code up your output
    page close to how it will look when it's printed. It can keep
    track of things like how many lines are on a page, what page
    you're on, when to print page headers, etc. Keywords are
    borrowed from FORTRAN: format() to declare and write() to
    execute; see their entries in the perlfunc manpage. Fortunately,
    the layout is much more legible, more like BASIC's PRINT USING
    statement. Think of it as a poor man's nroff(1).

    Formats, like packages and subroutines, are declared rather than
    executed, so they may occur at any point in your program.
    (Usually it's best to keep them all together though.) They have
    their own namespace apart from all the other "types" in Perl.
    This means that if you have a function named "Foo", it is not
    the same thing as having a format named "Foo". However, the
    default name for the format associated with a given filehandle
    is the same as the name of the filehandle. Thus, the default
    format for STDOUT is named "STDOUT", and the default format for
    filehandle TEMP is named "TEMP". They just look the same. They
    aren't.

    Output record formats are declared as follows:

        format NAME =
        FORMLIST
        .


    If name is omitted, format "STDOUT" is defined. FORMLIST
    consists of a sequence of lines, each of which may be one of
    three types:

    1.  A comment, indicated by putting a '#' in the first column.

    2.  A "picture" line giving the format for one output line.

    3.  An argument line supplying values to plug into the previous
        picture line.


    Picture lines are printed exactly as they look, except for
    certain fields that substitute values into the line. Each field
    in a picture line starts with either "@" (at) or "^" (caret).
    These lines do not undergo any kind of variable interpolation.
    The at field (not to be confused with the array marker @) is the
    normal kind of field; the other kind, caret fields, are used to
    do rudimentary multi-line text block filling. The length of the
    field is supplied by padding out the field with multiple "<",
    ">", or "|" characters to specify, respectively, left
    justification, right justification, or centering. If the
    variable would exceed the width specified, it is truncated.

    As an alternate form of right justification, you may also use
    "#" characters (with an optional ".") to specify a numeric
    field. This way you can line up the decimal points. If any value
    supplied for these fields contains a newline, only the text up
    to the newline is printed. Finally, the special field "@*" can
    be used for printing multi-line, nontruncated values; it should
    appear by itself on a line.

    The values are specified on the following line in the same order
    as the picture fields. The expressions providing the values
    should be separated by commas. The expressions are all evaluated
    in a list context before the line is processed, so a single list
    expression could produce multiple list elements. The expressions
    may be spread out to more than one line if enclosed in braces.
    If so, the opening brace must be the first token on the first
    line. If an expression evaluates to a number with a decimal
    part, and if the corresponding picture specifies that the
    decimal part should appear in the output (that is, any picture
    except multiple "#" characters without an embedded "."), the
    character used for the decimal point is always determined by the
    current LC_NUMERIC locale. This means that, if, for example, the
    run-time environment happens to specify a German locale, ","
    will be used instead of the default ".". See the perllocale
    manpage and the section on "WARNINGS" for more information.

    Picture fields that begin with ^ rather than @ are treated
    specially. With a # field, the field is blanked out if the value
    is undefined. For other field types, the caret enables a kind of
    fill mode. Instead of an arbitrary expression, the value
    supplied must be a scalar variable name that contains a text
    string. Perl puts as much text as it can into the field, and
    then chops off the front of the string so that the next time the
    variable is referenced, more of the text can be printed. (Yes,
    this means that the variable itself is altered during execution
    of the write() call, and is not returned.) Normally you would
    use a sequence of fields in a vertical stack to print out a
    block of text. You might wish to end the final field with the
    text "...", which will appear in the output if the text was too
    long to appear in its entirety. You can change which characters
    are legal to break on by changing the variable `$:' (that's
    $FORMAT_LINE_BREAK_CHARACTERS if you're using the English
    module) to a list of the desired characters.

    Using caret fields can produce variable length records. If the
    text to be formatted is short, you can suppress blank lines by
    putting a "~" (tilde) character anywhere in the line. The tilde
    will be translated to a space upon output. If you put a second
    tilde contiguous to the first, the line will be repeated until
    all the fields on the line are exhausted. (If you use a field of
    the at variety, the expression you supply had better not give
    the same value every time forever!)

    Top-of-form processing is by default handled by a format with
    the same name as the current filehandle with "_TOP" concatenated
    to it. It's triggered at the top of each page. See the "write"
    entry in the perlfunc manpage.

    Examples:

     # a report on the /etc/passwd file
     format STDOUT_TOP =
                             Passwd File
     Name                Login    Office   Uid   Gid Home
     ------------------------------------------------------------------
     .
     format STDOUT =
     @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
     $name,              $login,  $office,$uid,$gid, $home
     .

     # a report from a bug report form
     format STDOUT_TOP =
                             Bug Reports
     @<<<<<<<<<<<<<<<<<<<<<<<     @|||         @>>>>>>>>>>>>>>>>>>>>>>>
     $system,                      $%,         $date
     ------------------------------------------------------------------
     .
     format STDOUT =
     Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
              $subject
     Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
            $index,                       $description
     Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
               $priority,        $date,   $description
     From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
           $from,                         $description
     Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                  $programmer,            $description
     ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                          $description
     ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                          $description
     ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                          $description
     ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                          $description
     ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<...
                                          $description
     .


    It is possible to intermix print()s with write()s on the same
    output channel, but you'll have to handle `$-'
    (`$FORMAT_LINES_LEFT') yourself.

  Format Variables

    The current format name is stored in the variable `$~'
    (`$FORMAT_NAME'), and the current top of form format name is in
    `$^' (`$FORMAT_TOP_NAME'). The current output page number is
    stored in `$%' (`$FORMAT_PAGE_NUMBER'), and the number of lines
    on the page is in `$=' (`$FORMAT_LINES_PER_PAGE'). Whether to
    autoflush output on this handle is stored in `$|'
    (`$OUTPUT_AUTOFLUSH'). The string output before each top of page
    (except the first) is stored in `$^L' (`$FORMAT_FORMFEED').
    These variables are set on a per-filehandle basis, so you'll
    need to select() into a different one to affect them:

        select((select(OUTF),
            $~ = "My_Other_Format",
            $^ = "My_Top_Format"
           )[0]);


    Pretty ugly, eh? It's a common idiom though, so don't be too
    surprised when you see it. You can at least use a temporary
    variable to hold the previous filehandle: (this is a much better
    approach in general, because not only does legibility improve,
    you now have intermediary stage in the expression to single-step
    the debugger through):

        $ofh = select(OUTF);
        $~ = "My_Other_Format";
        $^ = "My_Top_Format";
        select($ofh);


    If you use the English module, you can even read the variable
    names:

        use English;
        $ofh = select(OUTF);
        $FORMAT_NAME     = "My_Other_Format";
        $FORMAT_TOP_NAME = "My_Top_Format";
        select($ofh);


    But you still have those funny select()s. So just use the
    FileHandle module. Now, you can access these special variables
    using lowercase method names instead:

        use FileHandle;
        format_name     OUTF "My_Other_Format";
        format_top_name OUTF "My_Top_Format";


    Much better!

NOTES
    Because the values line may contain arbitrary expressions (for
    at fields, not caret fields), you can farm out more
    sophisticated processing to other functions, like sprintf() or
    one of your own. For example:

        format Ident =
        @<<<<<<<<<<<<<<<
        &commify($n)
        .


    To get a real at or caret into the field, do this:

        format Ident =
        I have an @ here.
            "@"
        .


    To center a whole line of text, do something like this:

        format Ident =
        @|||||||||||||||||||||||||||||||||||||||||||||||
            "Some text line"
        .


    There is no builtin way to say "float this to the right hand
    side of the page, however wide it is." You have to specify where
    it goes. The truly desperate can generate their own format on
    the fly, based on the current number of columns, and then eval()
    it:

        $format  = "format STDOUT = \n"
                 . '^' . '<' x $cols . "\n"
                 . '$entry' . "\n"
                 . "\t^" . "<" x ($cols-8) . "~~\n"
                 . '$entry' . "\n"
                 . ".\n";
        print $format if $Debugging;
        eval $format;
        die $@ if $@;


    Which would generate a format looking something like this:

     format STDOUT =
     ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
     $entry
             ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~
     $entry
     .


    Here's a little program that's somewhat like fmt(1):

     format =
     ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
     $_

     .

     $/ = '';
     while (<>) {
         s/\s*\n\s*/ /g;
         write;
     }


  Footers

    While $FORMAT_TOP_NAME contains the name of the current header
    format, there is no corresponding mechanism to automatically do
    the same thing for a footer. Not knowing how big a format is
    going to be until you evaluate it is one of the major problems.
    It's on the TODO list.

    Here's one strategy: If you have a fixed-size footer, you can
    get footers by checking $FORMAT_LINES_LEFT before each write()
    and print the footer yourself if necessary.

    Here's another strategy: Open a pipe to yourself, using
    `open(MYSELF, "|-")' (see the "open()" entry in the perlfunc
    manpage) and always write() to MYSELF instead of STDOUT. Have
    your child process massage its STDIN to rearrange headers and
    footers however you like. Not very convenient, but doable.

  Accessing Formatting Internals

    For low-level access to the formatting mechanism. you may use
    formline() and access `$^A' (the $ACCUMULATOR variable)
    directly.

    For example:

        $str = formline <<'END', 1,2,3;
        @<<<  @|||  @>>>
        END

        print "Wow, I just stored `$^A' in the accumulator!\n";


    Or to make an swrite() subroutine, which is to write() what
    sprintf() is to printf(), do this:

        use Carp;
        sub swrite {
        croak "usage: swrite PICTURE ARGS" unless @_;
        my $format = shift;
        $^A = "";
        formline($format,@_);
        return $^A;
        }

        $string = swrite(<<'END', 1, 2, 3);
     Check me out
     @<<<  @|||  @>>>
     END
        print $string;


WARNINGS
    The lone dot that ends a format can also prematurely end a mail
    message passing through a misconfigured Internet mailer (and
    based on experience, such misconfiguration is the rule, not the
    exception). So when sending format code through mail, you should
    indent it so that the format-ending dot is not on the left
    margin; this will prevent SMTP cutoff.

    Lexical variables (declared with "my") are not visible within a
    format unless the format is declared within the scope of the
    lexical variable. (They weren't visible at all before version
    5.001.)

    Formats are the only part of Perl that unconditionally use
    information from a program's locale; if a program's environment
    specifies an LC_NUMERIC locale, it is always used to specify the
    decimal point character in formatted output. Perl ignores all
    other aspects of locale handling unless the `use locale' pragma
    is in effect. Formatted output cannot be controlled by `use
    locale' because the pragma is tied to the block structure of the
    program, and, for historical reasons, formats exist outside that
    block structure. See the perllocale manpage for further
    discussion of locale handling.

    Inside of an expression, the whitespace characters \n, \t and \f
    are considered to be equivalent to a single space. Thus, you
    could think of this filter being applied to each value in the
    format:

     $value =~ tr/\n\t\f/ /;


    The remaining whitespace character, \r, forces the printing of a
    new line if allowed by the picture line.