[Chapter 2] 2.8 Formats

2.8 Formats

Perl has a mechanism to help you generate simple, formatted reports and charts. It can keep track of things like how many lines on a page, what page you're on, when to print page headers, and so on. Keywords are borrowed from FORTRAN: format to declare and write to execute; see the relevant entries in Chapter 3, Functions. Fortunately, the layout is much more legible, more like the PRINT USING statement of BASIC. Think of it as a poor man's nroff (1). (If you know nroff, that may not sound like a recommendation.)

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.) 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 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 really 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:

A comment, indicated by putting a # in the first column.
A "picture" line giving the format for one output line.
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.[48] Each substitution field in a picture line starts with either @ (at) or ^ (caret). These lines do not undergo any kind of variable interpolation. The @ field (not to be confused with the array marker @) is the normal kind of field; the other kind, the ^ field, is used to do rudimentary multiline text-block filling. The length of the field is indicated 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.

[48] Even those fields maintain the integrity of the columns you put them in, however. There is nothing in a picture line that can cause fields to grow or shrink or shift back and forth. The columns you see are sacred in a WYSIWYG sense.

As an alternate form of right justification, you may also use # characters (after an initial @ or ^, and 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 generally appear on a picture line by itself.

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 write call, and is not preserved. Use a scratch variable if you want to preserve the original value.) 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 (or after) 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 ^ fields can produce variable-length records. If the text to be formatted is short, just repeat the format line with the ^ field in it a few times. If you just do this for short data you'd end up getting a few blank lines. To suppress lines that would end up blank, put a ~ (tilde) character anywhere in the line. (The tilde itself 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 text in the fields on that line have been printed. (This works because the ^ fields chew up the strings they print. But if you use a field of the @ variety in conjunction with two tildes, the expression you supply had better not give the same value every time forever! Use a shift, or some other operator with a side effect that exhausts the set of values.)

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 write in Chapter 3, Functions.

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 prints with writes on the same output channel, but you'll have to handle the $- special variable ($FORMAT_LINES_LEFT if you're using the English module) 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 to be 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 the filehandle associated with a format in order to affect its format variables.

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, but you now have an intermediary statement in the code to stop on when you're single-stepping the debugger):

$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 calls to select. So just use the FileHandle module. Now you can access these special variables using lowercase method names instead:

use FileHandle;
OUTF->format_name("My_Other_Format");
OUTF->format_top_name("My_Top_Format");

Much better!

Since the values line following your picture line may contain arbitrary expressions (for @ fields, not ^ fields), you can farm out more sophisticated processing to other functions, like sprintf or one of your own. For example, to insert commas into a number:

format Ident = 
    @<<<<<<<<<<<<<<<
    commify($n)
.

To get a real @, ~, or ^ 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"
.

The > field-length indicator ensures that the text will be right-justified within the field, but the field as a whole occurs exactly where you show it occurring. There is no built-in way to say "float this field to the right-hand side of the page, however wide it is." You have to specify where it goes relative to the left margin. 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 $@;

The most important line there is probably the print. What the print would print out looks 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 then print the footer yourself if necessary.

Here's another strategy; open a pipe to yourself, using open(MESELF, `|-`) (see the open entry in Chapter 3, Functions) and always write to MESELF instead of STDOUT. Have your child process postprocess 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) directly. (Formats essentially compile into a sequence of calls to formline.) 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 as 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;

Lexical variables (declared with my) are not visible within a format unless the format is declared within the scope of the lexical variable.