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perlform ()
  • >> perlform (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perlform (1) ( Разные man: Команды и прикладные программы пользовательского уровня )


         perlform - Perl formats


         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
         Output record formats are declared as follows:
             format NAME =
         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
         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.
          # 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: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
          Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                 $index,                       $description
          Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                    $priority,        $date,   $description
          From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                $from,                         $description
          Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                       $programmer,            $description
          ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
          ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
          ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
          ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
          ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<...
         It is possible to intermix print()s with write()s on the
         same output channel, but you'll have to handle `$-'
         (`$FORMAT_LINES_LEFT') yourself.
         Format Variables
         The current format name is stored in the variable `$~'
         (`$FORMAT_NAME'), and the current top of form format name is
         in `$^' (`$FORMAT_TOP_NAME').  The current output page
         number is stored in `$%' (`$FORMAT_PAGE_NUMBER'), and the
         number of lines on the page is in `$='
         (`$FORMAT_LINES_PER_PAGE').  Whether to autoflush output on
         this handle is stored in `$|' (`$OUTPUT_AUTOFLUSH').  The
         string output before each top of page (except the first) is
         stored in `$^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:
                     $~ = "My_Other_Format",
                     $^ = "My_Top_Format"
         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";
         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";
         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!


         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 =
         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 =
         Here's a little program that's somewhat like fmt(1):
          format =
          ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
          $/ = '';
          while (<>) {
              s/\s*\n\s*/ /g;
         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)
         For example:
             $str = formline <<'END', 1,2,3;
             @<<<  @|||  @>>>
             print "Wow, I just stored `$^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 = "";
                 return $^A;
             $string = swrite(<<'END', 1, 2, 3);
          Check me out
          @<<<  @|||  @>>>
             print $string;


         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.

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