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   1  =head1 NAME
   2  X<format> X<report> X<chart>
   3  
   4  perlform - Perl formats
   5  
   6  =head1 DESCRIPTION
   7  
   8  Perl has a mechanism to help you generate simple reports and charts.  To
   9  facilitate this, Perl helps you code up your output page close to how it
  10  will look when it's printed.  It can keep track of things like how many
  11  lines are on a page, what page you're on, when to print page headers,
  12  etc.  Keywords are borrowed from FORTRAN: format() to declare and write()
  13  to execute; see their entries in L<perlfunc>.  Fortunately, the layout is
  14  much more legible, more like BASIC's PRINT USING statement.  Think of it
  15  as a poor man's nroff(1).
  16  X<nroff>
  17  
  18  Formats, like packages and subroutines, are declared rather than
  19  executed, so they may occur at any point in your program.  (Usually it's
  20  best to keep them all together though.) They have their own namespace
  21  apart from all the other "types" in Perl.  This means that if you have a
  22  function named "Foo", it is not the same thing as having a format named
  23  "Foo".  However, the default name for the format associated with a given
  24  filehandle is the same as the name of the filehandle.  Thus, the default
  25  format for STDOUT is named "STDOUT", and the default format for filehandle
  26  TEMP is named "TEMP".  They just look the same.  They aren't.
  27  
  28  Output record formats are declared as follows:
  29  
  30      format NAME =
  31      FORMLIST
  32      .
  33  
  34  If the name is omitted, format "STDOUT" is defined. A single "." in 
  35  column 1 is used to terminate a format.  FORMLIST consists of a sequence 
  36  of lines, each of which may be one of three types:
  37  
  38  =over 4
  39  
  40  =item 1.
  41  
  42  A comment, indicated by putting a '#' in the first column.
  43  
  44  =item 2.
  45  
  46  A "picture" line giving the format for one output line.
  47  
  48  =item 3.
  49  
  50  An argument line supplying values to plug into the previous picture line.
  51  
  52  =back
  53  
  54  Picture lines contain output field definitions, intermingled with
  55  literal text. These lines do not undergo any kind of variable interpolation.
  56  Field definitions are made up from a set of characters, for starting and
  57  extending a field to its desired width. This is the complete set of
  58  characters for field definitions:
  59  X<format, picture line>
  60  X<@> X<^> X<< < >> X<< | >> X<< > >> X<#> X<0> X<.> X<...>
  61  X<@*> X<^*> X<~> X<~~>
  62    
  63     @    start of regular field
  64     ^    start of special field
  65     <    pad character for left adjustification
  66     |    pad character for centering
  67     >    pad character for right adjustificat
  68     #    pad character for a right justified numeric field
  69     0    instead of first #: pad number with leading zeroes
  70     .    decimal point within a numeric field
  71     ...  terminate a text field, show "..." as truncation evidence
  72     @*   variable width field for a multi-line value
  73     ^*   variable width field for next line of a multi-line value
  74     ~    suppress line with all fields empty
  75     ~~   repeat line until all fields are exhausted
  76  
  77  Each field in a picture line starts with either "@" (at) or "^" (caret),
  78  indicating what we'll call, respectively, a "regular" or "special" field.
  79  The choice of pad characters determines whether a field is textual or
  80  numeric. The tilde operators are not part of a field.  Let's look at
  81  the various possibilities in detail.
  82  
  83  
  84  =head2 Text Fields
  85  X<format, text field>
  86  
  87  The length of the field is supplied by padding out the field with multiple 
  88  "E<lt>", "E<gt>", or "|" characters to specify a non-numeric field with,
  89  respectively, left justification, right justification, or centering. 
  90  For a regular field, the value (up to the first newline) is taken and
  91  printed according to the selected justification, truncating excess characters.
  92  If you terminate a text field with "...", three dots will be shown if
  93  the value is truncated. A special text field may be used to do rudimentary 
  94  multi-line text block filling; see L</Using Fill Mode> for details.
  95  
  96     Example:
  97        format STDOUT =
  98        @<<<<<<   @||||||   @>>>>>>
  99        "left",   "middle", "right"
 100        .
 101     Output:
 102        left      middle    right
 103  
 104  
 105  =head2 Numeric Fields
 106  X<#> X<format, numeric field>
 107  
 108  Using "#" as a padding character specifies a numeric field, with
 109  right justification. An optional "." defines the position of the
 110  decimal point. With a "0" (zero) instead of the first "#", the
 111  formatted number will be padded with leading zeroes if necessary.
 112  A special numeric field is blanked out if the value is undefined.
 113  If the resulting value would exceed the width specified the field is
 114  filled with "#" as overflow evidence.
 115  
 116     Example:
 117        format STDOUT =
 118        @###   @.###   @##.###  @###   @###   ^####
 119         42,   3.1415,  undef,    0, 10000,   undef
 120        .
 121     Output:
 122          42   3.142     0.000     0   ####
 123  
 124  
 125  =head2 The Field @* for Variable Width Multi-Line Text
 126  X<@*>
 127  
 128  The field "@*" can be used for printing multi-line, nontruncated
 129  values; it should (but need not) appear by itself on a line. A final
 130  line feed is chomped off, but all other characters are emitted verbatim.
 131  
 132  
 133  =head2 The Field ^* for Variable Width One-line-at-a-time Text
 134  X<^*>
 135  
 136  Like "@*", this is a variable width field. The value supplied must be a 
 137  scalar variable. Perl puts the first line (up to the first "\n") of the 
 138  text into the field, and then chops off the front of the string so that 
 139  the next time the variable is referenced, more of the text can be printed. 
 140  The variable will I<not> be restored.
 141  
 142     Example:
 143        $text = "line 1\nline 2\nline 3";
 144        format STDOUT =
 145        Text: ^*
 146              $text
 147        ~~    ^*
 148              $text
 149        .
 150     Output:
 151        Text: line 1
 152              line 2
 153              line 3
 154  
 155  
 156  =head2 Specifying Values
 157  X<format, specifying values>
 158  
 159  The values are specified on the following format line in the same order as
 160  the picture fields.  The expressions providing the values must be
 161  separated by commas.  They are all evaluated in a list context
 162  before the line is processed, so a single list expression could produce
 163  multiple list elements.  The expressions may be spread out to more than
 164  one line if enclosed in braces.  If so, the opening brace must be the first
 165  token on the first line.  If an expression evaluates to a number with a
 166  decimal part, and if the corresponding picture specifies that the decimal
 167  part should appear in the output (that is, any picture except multiple "#"
 168  characters B<without> an embedded "."), the character used for the decimal
 169  point is B<always> determined by the current LC_NUMERIC locale.  This
 170  means that, if, for example, the run-time environment happens to specify a
 171  German locale, "," will be used instead of the default ".".  See
 172  L<perllocale> and L<"WARNINGS"> for more information.
 173  
 174  
 175  =head2 Using Fill Mode
 176  X<format, fill mode>
 177  
 178  On text fields the caret enables a kind of fill mode.  Instead of an
 179  arbitrary expression, the value supplied must be a scalar variable
 180  that contains a text string.  Perl puts the next portion of the text into
 181  the field, and then chops off the front of the string so that the next time
 182  the variable is referenced, more of the text can be printed.  (Yes, this
 183  means that the variable itself is altered during execution of the write()
 184  call, and is not restored.)  The next portion of text is determined by
 185  a crude line breaking algorithm. You may use the carriage return character
 186  (C<\r>) to force a line break. You can change which characters are legal 
 187  to break on by changing the variable C<$:> (that's 
 188  $FORMAT_LINE_BREAK_CHARACTERS if you're using the English module) to a 
 189  list of the desired characters.
 190  
 191  Normally you would use a sequence of fields in a vertical stack associated 
 192  with the same scalar variable to print out a block of text. You might wish 
 193  to end the final field with the text "...", which will appear in the output 
 194  if the text was too long to appear in its entirety.  
 195  
 196  
 197  =head2 Suppressing Lines Where All Fields Are Void
 198  X<format, suppressing lines>
 199  
 200  Using caret fields can produce lines where all fields are blank. You can
 201  suppress such lines by putting a "~" (tilde) character anywhere in the
 202  line.  The tilde will be translated to a space upon output.
 203  
 204  
 205  =head2 Repeating Format Lines
 206  X<format, repeating lines>
 207  
 208  If you put two contiguous tilde characters "~~" anywhere into a line,
 209  the line will be repeated until all the fields on the line are exhausted,
 210  i.e. undefined. For special (caret) text fields this will occur sooner or
 211  later, but if you use a text field of the at variety, the  expression you
 212  supply had better not give the same value every time forever! (C<shift(@f)>
 213  is a simple example that would work.)  Don't use a regular (at) numeric 
 214  field in such lines, because it will never go blank.
 215  
 216  
 217  =head2 Top of Form Processing
 218  X<format, top of form> X<top> X<header>
 219  
 220  Top-of-form processing is by default handled by a format with the
 221  same name as the current filehandle with "_TOP" concatenated to it.
 222  It's triggered at the top of each page.  See L<perlfunc/write>.
 223  
 224  Examples:
 225  
 226   # a report on the /etc/passwd file
 227   format STDOUT_TOP =
 228                           Passwd File
 229   Name                Login    Office   Uid   Gid Home
 230   ------------------------------------------------------------------
 231   .
 232   format STDOUT =
 233   @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
 234   $name,              $login,  $office,$uid,$gid, $home
 235   .
 236  
 237  
 238   # a report from a bug report form
 239   format STDOUT_TOP =
 240                           Bug Reports
 241   @<<<<<<<<<<<<<<<<<<<<<<<     @|||         @>>>>>>>>>>>>>>>>>>>>>>>
 242   $system,                      $%,         $date
 243   ------------------------------------------------------------------
 244   .
 245   format STDOUT =
 246   Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 247            $subject
 248   Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 249          $index,                       $description
 250   Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 251             $priority,        $date,   $description
 252   From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 253         $from,                         $description
 254   Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 255                $programmer,            $description
 256   ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 257                                        $description
 258   ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 259                                        $description
 260   ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 261                                        $description
 262   ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 263                                        $description
 264   ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<...
 265                                        $description
 266   .
 267  
 268  It is possible to intermix print()s with write()s on the same output
 269  channel, but you'll have to handle C<$-> (C<$FORMAT_LINES_LEFT>)
 270  yourself.
 271  
 272  =head2 Format Variables
 273  X<format variables>
 274  X<format, variables>
 275  
 276  The current format name is stored in the variable C<$~> (C<$FORMAT_NAME>),
 277  and the current top of form format name is in C<$^> (C<$FORMAT_TOP_NAME>).
 278  The current output page number is stored in C<$%> (C<$FORMAT_PAGE_NUMBER>),
 279  and the number of lines on the page is in C<$=> (C<$FORMAT_LINES_PER_PAGE>).
 280  Whether to autoflush output on this handle is stored in C<$|>
 281  (C<$OUTPUT_AUTOFLUSH>).  The string output before each top of page (except
 282  the first) is stored in C<$^L> (C<$FORMAT_FORMFEED>).  These variables are
 283  set on a per-filehandle basis, so you'll need to select() into a different
 284  one to affect them:
 285  
 286      select((select(OUTF),
 287          $~ = "My_Other_Format",
 288          $^ = "My_Top_Format"
 289         )[0]);
 290  
 291  Pretty ugly, eh?  It's a common idiom though, so don't be too surprised
 292  when you see it.  You can at least use a temporary variable to hold
 293  the previous filehandle: (this is a much better approach in general,
 294  because not only does legibility improve, you now have intermediary
 295  stage in the expression to single-step the debugger through):
 296  
 297      $ofh = select(OUTF);
 298      $~ = "My_Other_Format";
 299      $^ = "My_Top_Format";
 300      select($ofh);
 301  
 302  If you use the English module, you can even read the variable names:
 303  
 304      use English '-no_match_vars';
 305      $ofh = select(OUTF);
 306      $FORMAT_NAME     = "My_Other_Format";
 307      $FORMAT_TOP_NAME = "My_Top_Format";
 308      select($ofh);
 309  
 310  But you still have those funny select()s.  So just use the FileHandle
 311  module.  Now, you can access these special variables using lowercase
 312  method names instead:
 313  
 314      use FileHandle;
 315      format_name     OUTF "My_Other_Format";
 316      format_top_name OUTF "My_Top_Format";
 317  
 318  Much better!
 319  
 320  =head1 NOTES
 321  
 322  Because the values line may contain arbitrary expressions (for at fields,
 323  not caret fields), you can farm out more sophisticated processing
 324  to other functions, like sprintf() or one of your own.  For example:
 325  
 326      format Ident =
 327      @<<<<<<<<<<<<<<<
 328      &commify($n)
 329      .
 330  
 331  To get a real at or caret into the field, do this:
 332  
 333      format Ident =
 334      I have an @ here.
 335          "@"
 336      .
 337  
 338  To center a whole line of text, do something like this:
 339  
 340      format Ident =
 341      @|||||||||||||||||||||||||||||||||||||||||||||||
 342          "Some text line"
 343      .
 344  
 345  There is no builtin way to say "float this to the right hand side
 346  of the page, however wide it is."  You have to specify where it goes.
 347  The truly desperate can generate their own format on the fly, based
 348  on the current number of columns, and then eval() it:
 349  
 350      $format  = "format STDOUT = \n"
 351               . '^' . '<' x $cols . "\n"
 352               . '$entry' . "\n"
 353               . "\t^" . "<" x ($cols-8) . "~~\n"
 354               . '$entry' . "\n"
 355               . ".\n";
 356      print $format if $Debugging;
 357      eval $format;
 358      die $@ if $@;
 359  
 360  Which would generate a format looking something like this:
 361  
 362   format STDOUT =
 363   ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 364   $entry
 365           ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~
 366   $entry
 367   .
 368  
 369  Here's a little program that's somewhat like fmt(1):
 370  
 371   format =
 372   ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
 373   $_
 374  
 375   .
 376  
 377   $/ = '';
 378   while (<>) {
 379       s/\s*\n\s*/ /g;
 380       write;
 381   }
 382  
 383  =head2 Footers
 384  X<format, footer> X<footer>
 385  
 386  While $FORMAT_TOP_NAME contains the name of the current header format,
 387  there is no corresponding mechanism to automatically do the same thing
 388  for a footer.  Not knowing how big a format is going to be until you
 389  evaluate it is one of the major problems.  It's on the TODO list.
 390  
 391  Here's one strategy:  If you have a fixed-size footer, you can get footers
 392  by checking $FORMAT_LINES_LEFT before each write() and print the footer
 393  yourself if necessary.
 394  
 395  Here's another strategy: Open a pipe to yourself, using C<open(MYSELF, "|-")>
 396  (see L<perlfunc/open()>) and always write() to MYSELF instead of STDOUT.
 397  Have your child process massage its STDIN to rearrange headers and footers
 398  however you like.  Not very convenient, but doable.
 399  
 400  =head2 Accessing Formatting Internals
 401  X<format, internals>
 402  
 403  For low-level access to the formatting mechanism.  you may use formline()
 404  and access C<$^A> (the $ACCUMULATOR variable) directly.
 405  
 406  For example:
 407  
 408      $str = formline <<'END', 1,2,3;
 409      @<<<  @|||  @>>>
 410      END
 411  
 412      print "Wow, I just stored `$^A' in the accumulator!\n";
 413  
 414  Or to make an swrite() subroutine, which is to write() what sprintf()
 415  is to printf(), do this:
 416  
 417      use Carp;
 418      sub swrite {
 419      croak "usage: swrite PICTURE ARGS" unless @_;
 420      my $format = shift;
 421      $^A = "";
 422      formline($format,@_);
 423      return $^A;
 424      }
 425  
 426      $string = swrite(<<'END', 1, 2, 3);
 427   Check me out
 428   @<<<  @|||  @>>>
 429   END
 430      print $string;
 431  
 432  =head1 WARNINGS
 433  
 434  The lone dot that ends a format can also prematurely end a mail
 435  message passing through a misconfigured Internet mailer (and based on
 436  experience, such misconfiguration is the rule, not the exception).  So
 437  when sending format code through mail, you should indent it so that
 438  the format-ending dot is not on the left margin; this will prevent
 439  SMTP cutoff.
 440  
 441  Lexical variables (declared with "my") are not visible within a
 442  format unless the format is declared within the scope of the lexical
 443  variable.  (They weren't visible at all before version 5.001.)
 444  
 445  Formats are the only part of Perl that unconditionally use information
 446  from a program's locale; if a program's environment specifies an
 447  LC_NUMERIC locale, it is always used to specify the decimal point
 448  character in formatted output.  Perl ignores all other aspects of locale
 449  handling unless the C<use locale> pragma is in effect.  Formatted output
 450  cannot be controlled by C<use locale> because the pragma is tied to the
 451  block structure of the program, and, for historical reasons, formats
 452  exist outside that block structure.  See L<perllocale> for further
 453  discussion of locale handling.
 454  
 455  Within strings that are to be displayed in a fixed length text field,
 456  each control character is substituted by a space. (But remember the
 457  special meaning of C<\r> when using fill mode.) This is done to avoid
 458  misalignment when control characters "disappear" on some output media.
 459