PERLFORM(1) Perl Programmers Reference Guide PERLFORM(1)NAMEperlform - Perl formats
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 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 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 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
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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.
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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.
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
.
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# 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 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:
select((select(OUTF),
$~ = "My_Other_Format",
$^ = "My_Top_Format"
)[0]);
Pretty ugly, eh? It's a common idiom though, so don't be
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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 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!
NOTES
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 =
@<<<<<<<<<<<<<<<
&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"
.
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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 =
^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$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 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
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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)
directly.
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()
what 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;
WARNINGS
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
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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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