perlform.pod

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=head1 NAME
X<format> X<report> X<chart>

perlform - Perl formats

=head1 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 L<perlfunc>.  Fortunately, the layout is
much more legible, more like BASIC's PRINT USING statement.  Think of it
as a poor man's nroff(1).
X<nroff>

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 the name is omitted, format "STDOUT" is defined. A single "." in 
column 1 is used to terminate a format.  FORMLIST consists of a sequence 
of lines, each of which may be one of three types:

=over 4

=item 1.

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

=item 2.

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

=item 3.

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

=back

Picture lines contain output field definitions, intermingled with
literal text. These lines do not undergo any kind of variable interpolation.
Field definitions are made up from a set of characters, for starting and
extending a field to its desired width. This is the complete set of
characters for field definitions:
X<format, picture line>
X<@> X<^> X<< < >> X<< | >> X<< > >> X<#> X<0> X<.> X<...>
X<@*> X<^*> X<~> X<~~>
  
   @    start of regular field
   ^    start of special field
   <    pad character for left adjustification
   |    pad character for centering
   >    pad character for right adjustificat
   #    pad character for a right justified numeric field
   0    instead of first #: pad number with leading zeroes
   .    decimal point within a numeric field
   ...  terminate a text field, show "..." as truncation evidence
   @*   variable width field for a multi-line value
   ^*   variable width field for next line of a multi-line value
   ~    suppress line with all fields empty
   ~~   repeat line until all fields are exhausted

Each field in a picture line starts with either "@" (at) or "^" (caret),
indicating what we'll call, respectively, a "regular" or "special" field.
The choice of pad characters determines whether a field is textual or
numeric. The tilde operators are not part of a field.  Let's look at
the various possibilities in detail.


=head2 Text Fields
X<format, text field>

The length of the field is supplied by padding out the field with multiple 
"E<lt>", "E<gt>", or "|" characters to specify a non-numeric field with,
respectively, left justification, right justification, or centering. 
For a regular field, the value (up to the first newline) is taken and
printed according to the selected justification, truncating excess characters.
If you terminate a text field with "...", three dots will be shown if
the value is truncated. A special text field may be used to do rudimentary 
multi-line text block filling; see L</Using Fill Mode> for details.

   Example:
      format STDOUT =
      @<<<<<<   @||||||   @>>>>>>
      "left",   "middle", "right"
      .
   Output:
      left      middle    right


=head2 Numeric Fields
X<#> X<format, numeric field>

Using "#" as a padding character specifies a numeric field, with
right justification. An optional "." defines the position of the
decimal point. With a "0" (zero) instead of the first "#", the
formatted number will be padded with leading zeroes if necessary.
A special numeric field is blanked out if the value is undefined.
If the resulting value would exceed the width specified the field is
filled with "#" as overflow evidence.

   Example:
      format STDOUT =
      @###   @.###   @##.###  @###   @###   ^####
       42,   3.1415,  undef,    0, 10000,   undef
      .
   Output:
        42   3.142     0.000     0   ####


=head2 The Field @* for Variable Width Multi-Line Text
X<@*>

The field "@*" can be used for printing multi-line, nontruncated
values; it should (but need not) appear by itself on a line. A final
line feed is chomped off, but all other characters are emitted verbatim.


=head2 The Field ^* for Variable Width One-line-at-a-time Text
X<^*>

Like "@*", this is a variable width field. The value supplied must be a 
scalar variable. Perl puts the first line (up to the first "\n") of the 
text 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. 
The variable will I<not> be restored.

   Example:
      $text = "line 1\nline 2\nline 3";
      format STDOUT =
      Text: ^*
            $text
      ~~    ^*
            $text
      .
   Output:
      Text: line 1
            line 2
            line 3


=head2 Specifying Values
X<format, specifying values>

The values are specified on the following format line in the same order as
the picture fields.  The expressions providing the values must be
separated by commas.  They 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 B<without> an embedded "."), the character used for the decimal
point is B<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
L<perllocale> and L<"WARNINGS"> for more information.


=head2 Using Fill Mode
X<format, fill mode>

On text fields the caret enables a kind of fill mode.  Instead of an
arbitrary expression, the value supplied must be a scalar variable
that contains a text string.  Perl puts the next portion of the text 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 restored.)  The next portion of text is determined by
a crude line breaking algorithm. You may use the carriage return character
(C<\r>) to force a line break. You can change which characters are legal 
to break on by changing the variable C<$:> (that's 
$FORMAT_LINE_BREAK_CHARACTERS if you're using the English module) to a 
list of the desired characters.

Normally you would use a sequence of fields in a vertical stack associated 
with the same scalar variable 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.  


=head2 Suppressing Lines Where All Fields Are Void
X<format, suppressing lines>

Using caret fields can produce lines where all fields are blank. You can
suppress such lines by putting a "~" (tilde) character anywhere in the
line.  The tilde will be translated to a space upon output.


=head2 Repeating Format Lines
X<format, repeating lines>

If you put two contiguous tilde characters "~~" anywhere into a line,
the line will be repeated until all the fields on the line are exhausted,
i.e. undefined. For special (caret) text fields this will occur sooner or
later, but if you use a text field of the at variety, the  expression you
supply had better not give the same value every time forever! (C<shift(@f)>
is a simple example that would work.)  Don't use a regular (at) numeric 
field in such lines, because it will never go blank.


=head2 Top of Form Processing
X<format, top of form> X<top> X<header>

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 L<perlfunc/write>.

Examples:

 # a report on the /etc/passwd file
 format STDOUT_TOP =
                         Passwd File
 Name                Login    Office   Uid   Gid Home
 ------------------------------------------------------------------