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 perlfunc. 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.
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:
A comment, indicated by putting a '#' in the first column.
A ``picture'' line giving the format for one output line.
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
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 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 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 ``write in perlfunc.
format STDOUT_TOP = Bug Reports @ 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.
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 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.
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 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 perllocale for further discussion of locale handling.