Skunkware 5/Skunkware 5.iso/man/cat.1/perlform.1

Text File | 1995-07-26 | 18.3 KB | 463 lines

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     NNNNAAAAMMMMEEEE
          perlform - Perl formats

     DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
          Perl has a mechanism to help you generate simple reports and
          charts.  To facilitate this, Perl helps you lay out your
          output page in your code in a fashion that's close to how it
          will look when it's printed.  It can keep track of things
          like how many lines on a page, what page you're, when to
          print page headers, etc.  The keywords used are borrowed
          from FORTRAN: _f_o_r_m_a_t() to declare and _w_r_i_t_e() to execute;
          see their entries in the _m_a_n_f_u_n_c manpage.  Fortunately, the
          layout is much more legible, more like BASIC's PRINT USING
          statement.  Think of it as a poor man's _n_r_o_f_f(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 name "STDOUT", and the default format for filehandle TEMP
          is name "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 of 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



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          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,
          non-truncated 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.

          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 _w_r_i_t_e()
          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.

          Since use of 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!)




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          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 <perlfunc/_w_r_i_t_e()>.

          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 _p_r_i_n_t()s with _w_r_i_t_e()s on the
          same output channel, but you'll have to handle $-
          ($FORMAT_LINES_LEFT) yourself.




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          FFFFoooorrrrmmmmaaaatttt VVVVaaaarrrriiiiaaaabbbblllleeeessss

          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 _s_e_l_e_c_t() 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 _s_e_l_e_c_t()s.  So just use the
          FileHandle module.  Now, you can access these special
          variables using lower-case method names instead:

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

          Much better!

     NNNNOOOOTTTTEEEESSSS
          Since the values line may contain arbitrary expression (for
          at fields, not caret fields), you can farm out any more



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          sophisticated processing to other functions, like _s_p_r_i_n_t_f()
          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 _e_v_a_l() 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 _f_m_t(1):

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



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           .

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


          FFFFooooooootttteeeerrrrssss

          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
          _w_r_i_t_e() and print the footer yourself if necessary.

          Here's another strategy; open a pipe to yourself, using
          open(MESELF, "|-") (see the open() entry in the _p_e_r_l_f_u_n_c
          manpage) and always _w_r_i_t_e() to MESELF instead of STDOUT.
          Have your child process postprocesses its STDIN to rearrange
          headers and footers however you like.  Not very convenient,
          but doable.

          AAAAcccccccceeeessssssssiiiinnnngggg FFFFoooorrrrmmmmaaaattttttttiiiinnnngggg IIIInnnntttteeeerrrrnnnnaaaallllssss

          For low-level access to the formatting mechanism.  you may
          use _f_o_r_m_l_i_n_e() 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 _s_w_r_i_t_e() subroutine which is to _w_r_i_t_e() what
          _s_p_r_i_n_t_f() is to _p_r_i_n_t_f(), do this:











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              use English;
              use Carp;
              sub swrite {
                  croak "usage: swrite PICTURE ARGS" unless @ARG;
                  local($ACCUMULATOR);
                  formline(@ARG);
                  return $ACCUMULATOR;
              }

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


     WWWWAAAARRRRNNNNIIIINNNNGGGG
          During the execution of a format, only global variables are
          visible, or dynamically-scoped ones declared with _l_o_c_a_l().
          Lexically scoped variables declared with _m_y() are _N_O_T
          available, as they are not considered to reside in the same
          lexical scope as the format.

































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