PERLFORM(1)PERLFORM(1)NAMEperlform - Perl formats
DESCRIPTION
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. 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 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
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 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.
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 <perlfunc/write()>.
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 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 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 lower-case
method names instead:
use FileHandle;
format_name OUTF "My_Other_Format";
format_top_name OUTF "My_Top_Format";
Much better!
NOTES
Since 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"
.
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(MESELF,
"⎪-") (see the open() entry in the perlfunc manpage) and always write()
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.
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;
WARNING
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 visiblie at all before version 5.001.)
3rd Berkeley DistributionPERLFORM(1)
