perlform
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: for
mat() to declare and write() to execute; see their entries
in perlfunc. Fortunately, the layout is much more legi
ble, 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 for
mat 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
text block filling. The length of the field is supplied
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 mul
tiple 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 perllocale and "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 refer
enced, 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_CHAR
ACTERS 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
@<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
$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
~ ^<<<<<<<<<<<<<<<<<<<<<<<...
$description
.
It is possible to intermix print()s with write()s on the
same output channel, but you'll have to handle "$-" ($FOR
MAT_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_FORM
FEED). These variables are set on a per-filehandle basis,
so you'll need to select() into a different one to affect
them:
$^ = "My_Top_Format";
select($ofh);
If you use the English module, you can even read the vari
able names:
use English '-no_match_vars';
$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"
.
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:
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 "open()" in perlfunc) and always
write() to MYSELF instead of STDOUT. Have your child pro
cess 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 $ACCUMULATOR variable)
directly.
For example:
formline($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-end
ing 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 envi
ronment specifies an LC_NUMERIC locale, it is always used
to specify the decimal point character in formatted out
put. 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 perllocale 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 print
ing of a new line if allowed by the picture line.
perl v5.8.1 2003-09-02 PERLFORM(1)
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