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Man Page for PERLFORM
NAME
perlform - 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. The keywords used are borrowed
from FORTRAN: format() to declare and write() to execute;
see their entries in the manfunc 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.
Since use of 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 ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< . 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 expression (for
at fields, not caret fields), you can farm out any 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 = 0;
. '^' . '<' x $cols . "0;
. '$entry' . "0;
. "^" . "<" x ($cols-8) . "~~0;
. '$entry' . "0;
. ".0;
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/0s*/ /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!0;
Or to make an swrite() subroutine which is to write() what
sprintf() is to printf(), do this:
use English;
use Carp;
sub swrite {
croak "usage: swrite PICTURE ARGS" unless @ARG;
local($ACCUMULATOR);
formline(@ARG);
return $ACCUMULATOR;
}
$string = swrite(<<'END', 1, 2, 3);
Check me out
@<<<@||| @>>>
END
print $string;
WARNING
During the execution of a format, only global variables are
visible, or dynamically-scoped ones declared with local().
Lexically scoped variables declared with my() are NOT
available, as they are not considered to reside in the same
lexical scope as the format.
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