perlform - Perl formats

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.

Output record formats are declared as follows:

If the name is omitted, format "STDOUT" is defined. A single "." in column 1 is used to terminate a format. FORMLIST consists of a sequence of lines, each of which may be one of three types:

Picture lines contain output field definitions, intermingled with literal text. These lines do not undergo any kind of variable interpolation. Field definitions are made up from a set of characters, for starting and extending a field to its desired width. This is the complete set of characters for field definitions:

   @    start of regular field
   ^    start of special field
   <    pad character for left adjustification
   |    pad character for centering
   >    pad character for right adjustificat
   #    pad character for a right justified numeric field
   0    instead of first #: pad number with leading zeroes
   .    decimal point within a numeric field
   ...  terminate a text field, show "..." as truncation evidence
   @*   variable width field for a multi-line value
   ^*   variable width field for next line of a multi-line value
   ~    suppress line with all fields empty
   ~~   repeat line until all fields are exhausted

Each field in a picture line starts with either "@" (at) or "^" (caret), indicating what we'll call, respectively, a "regular" or "special" field. The choice of pad characters determines whether a field is textual or numeric. The tilde operators are not part of a field. Let's look at the various possibilities in detail.

The length of the field is supplied by padding out the field with multiple "<", ">", or "|" characters to specify a non-numeric field with, respectively, left justification, right justification, or centering. For a regular field, the value (up to the first newline) is taken and printed according to the selected justification, truncating excess characters. If you terminate a text field with "...", three dots will be shown if the value is truncated. A special text field may be used to do rudimentary multi-line text block filling; see "Using Fill Mode" for details.

Using "#" as a padding character specifies a numeric field, with right justification. An optional "." defines the position of the decimal point. With a "0" (zero) instead of the first "#", the formatted number will be padded with leading zeroes if necessary. A special numeric field is blanked out if the value is undefined. If the resulting value would exceed the width specified the field is filled with "#" as overflow evidence.

The field "@*" can be used for printing multi-line, nontruncated values; it should (but need not) appear by itself on a line. A final line feed is chomped off, but all other characters are emitted verbatim.

Like "@*", this is a variable width field. The value supplied must be a scalar variable. Perl puts the first line (up to the first "\n") of the text 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. The variable will not be restored.

The values are specified on the following format line in the same order as the picture fields. The expressions providing the values must be separated by commas. They 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.

On text fields the caret enables a kind of fill mode. Instead of an arbitrary expression, the value supplied must be a scalar variable that contains a text string. Perl puts the next portion of the text 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 restored.) The next portion of text is determined by a crude line breaking algorithm. You may use the carriage return character (\r) to force a line break. 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.

Normally you would use a sequence of fields in a vertical stack associated with the same scalar variable 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.

Using caret fields can produce lines where all fields are blank. You can suppress such lines by putting a "~" (tilde) character anywhere in the line. The tilde will be translated to a space upon output.

If you put two contiguous tilde characters "~~" anywhere into a line, the line will be repeated until all the fields on the line are exhausted, i.e. undefined. For special (caret) text fields this will occur sooner or later, but if you use a text field of the at variety, the expression you supply had better not give the same value every time forever! (shift(@f) is a simple example that would work.) Don't use a regular (at) numeric field in such lines, because it will never go blank.

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.

Examples:

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.

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:

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):

If you use the English module, you can even read the variable names:

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:

Much better!

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:

To get a real at or caret into the field, do this:

To center a whole line of text, do something like this:

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:

Which would generate a format looking something like this:

Here's a little program that's somewhat like fmt(1):

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.

For low-level access to the formatting mechanism. you may use formline() and access $^A (the $ACCUMULATOR variable) directly.

For example:

Or to make an swrite() subroutine, which is to write() what sprintf() is to printf(), do this:

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.

Within strings that are to be displayed in a fixed length text field, each control character is substituted by a space. (But remember the special meaning of \r when using fill mode.) This is done to avoid misalignment when control characters "disappear" on some output media.