


PERLFORM(1)      Perl Programmers Reference Guide     PERLFORM(1)


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: for-
       mat() 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 for-
       mat 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



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       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 mul-
       tiple 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 refer-
       enced, 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 $FOR-
       MAT_LINE_BREAK_CHARACTERS if you're using the English mod-
       ule) to a list of the desired characters.




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       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
        .






























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        # 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 "$=" ("$FOR-
       MAT_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



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       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 vari-
       able 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"
           .



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       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



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       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 conve-
       nient, but doable.

       Accessing Formatting Internals

       For low-level access to the formatting mechanism.  you may
       use formline() and access "$^A" (the $ACCUMULATOR vari-
       able) 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-end-
       ing 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



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       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 discus-
       sion 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 print-
       ing of a new line if allowed by the picture line.






































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