PERLFORM(1) PERLFORM(1)
NAME
perlform - 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 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 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.
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PERLFORM(1) PERLFORM(1)
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.
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.
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PERLFORM(1) PERLFORM(1)
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
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PERLFORM(1) PERLFORM(1)
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 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:
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PERLFORM(1) PERLFORM(1)
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;
}
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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 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;
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WARNINGS
The lone dot that ends a format can also prematurely end an email 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 email, you should indent it so that the format-ending
dot is not on the left margin; this will prevent email 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 which unconditionally use information
from a program's locale; if a program's 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.
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