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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlform - Perl formats | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | Perl has a mechanism to help you generate simple reports and charts. To | |
54310121 | 8 | facilitate this, Perl helps you code up your output page close to how it |
9 | will look when it's printed. It can keep track of things like how many | |
10 | lines are on a page, what page you're on, when to print page headers, | |
11 | etc. Keywords are borrowed from FORTRAN: format() to declare and write() | |
12 | to execute; see their entries in L<perlfunc>. Fortunately, the layout is | |
13 | much more legible, more like BASIC's PRINT USING statement. Think of it | |
14 | as a poor man's nroff(1). | |
15 | ||
16 | Formats, like packages and subroutines, are declared rather than | |
17 | executed, so they may occur at any point in your program. (Usually it's | |
18 | best to keep them all together though.) They have their own namespace | |
19 | apart from all the other "types" in Perl. This means that if you have a | |
20 | function named "Foo", it is not the same thing as having a format named | |
21 | "Foo". However, the default name for the format associated with a given | |
a0d0e21e | 22 | filehandle is the same as the name of the filehandle. Thus, the default |
7b8d334a GS |
23 | format for STDOUT is named "STDOUT", and the default format for filehandle |
24 | TEMP is named "TEMP". They just look the same. They aren't. | |
a0d0e21e LW |
25 | |
26 | Output record formats are declared as follows: | |
27 | ||
28 | format NAME = | |
29 | FORMLIST | |
30 | . | |
31 | ||
a1b95068 WL |
32 | If the name is omitted, format "STDOUT" is defined. A single "." in |
33 | column 1 is used to terminate a format. FORMLIST consists of a sequence | |
34 | of lines, each of which may be one of three types: | |
a0d0e21e LW |
35 | |
36 | =over 4 | |
37 | ||
38 | =item 1. | |
39 | ||
40 | A comment, indicated by putting a '#' in the first column. | |
41 | ||
42 | =item 2. | |
43 | ||
44 | A "picture" line giving the format for one output line. | |
45 | ||
46 | =item 3. | |
47 | ||
48 | An argument line supplying values to plug into the previous picture line. | |
49 | ||
50 | =back | |
51 | ||
a1b95068 WL |
52 | Picture lines contain output field definitions, intermingled with |
53 | literal text. These lines do not undergo any kind of variable interpolation. | |
54 | Field definitions are made up from a set of characters, for starting and | |
55 | extending a field to its desired width. This is the complete set of | |
56 | characters for field definitions: | |
57 | ||
58 | @ start of regular field | |
59 | ^ start of special field | |
60 | < pad character for left adjustification | |
61 | | pad character for centering | |
62 | > pad character for right adjustificat | |
63 | # pad character for a right justified numeric field | |
64 | 0 instead of first #: pad number with leading zeroes | |
65 | . decimal point within a numeric field | |
66 | ... terminate a text field, show "..." as truncation evidence | |
67 | @* variable width field for a multi-line value | |
68 | ^* variable width field for next line of a multi-line value | |
69 | ~ suppress line with all fields empty | |
70 | ~~ repeat line until all fields are exhausted | |
71 | ||
72 | Each field in a picture line starts with either "@" (at) or "^" (caret), | |
73 | indicating what we'll call, respectively, a "regular" or "special" field. | |
74 | The choice of pad characters determines whether a field is textual or | |
75 | numeric. The tilde operators are not part of a field. Let's look at | |
76 | the various possibilities in detail. | |
77 | ||
78 | ||
79 | =head2 Text Fields | |
80 | ||
81 | The length of the field is supplied by padding out the field with multiple | |
82 | "E<lt>", "E<gt>", or "|" characters to specify a non-numeric field with, | |
83 | respectively, left justification, right justification, or centering. | |
84 | For a regular field, the value (up to the first newline) is taken and | |
85 | printed according to the selected justification, truncating excess characters. | |
86 | If you terminate a text field with "...", three dots will be shown if | |
87 | the value is truncated. A special text field may be used to do rudimentary | |
88 | multi-line text block filling; see L</Using Fill Mode> for details. | |
89 | ||
90 | Example: | |
91 | format STDOUT = | |
92 | @<<<<<< @|||||| @>>>>>> | |
93 | "left", "middle", "right" | |
94 | . | |
95 | Output: | |
96 | left middle right | |
97 | ||
98 | ||
99 | =head2 Numeric Fields | |
100 | ||
101 | Using "#" as a padding character specifies a numeric field, with | |
102 | right justification. An optional "." defines the position of the | |
103 | decimal point. With a "0" (zero) instead of the first "#", the | |
104 | formatted number will be padded with leading zeroes if necessary. | |
105 | A special numeric field is blanked out if the value is undefined. | |
106 | If the resulting value would exceed the width specified the field is | |
107 | filled with "#" as overflow evidence. | |
108 | ||
109 | Example: | |
110 | format STDOUT = | |
111 | @### @.### @##.### @### @### ^#### | |
112 | 42, 3.1415, undef, 0, 10000, undef | |
113 | . | |
114 | Output: | |
115 | 42 3.142 0.000 0 #### | |
116 | ||
117 | ||
118 | =head2 The Field @* for Variable Width Multi-Line Text | |
119 | ||
120 | The field "@*" can be used for printing multi-line, nontruncated | |
121 | values; it should (but need not) appear by itself on a line. A final | |
122 | line feed is chomped off, but all other characters are emitted verbatim. | |
123 | ||
124 | ||
125 | =head2 The Field ^* for Variable Width One-line-at-a-time Text | |
126 | ||
127 | Like "@*", this is a variable width field. The value supplied must be a | |
128 | scalar variable. Perl puts the first line (up to the first "\n") of the | |
129 | text into the field, and then chops off the front of the string so that | |
130 | the next time the variable is referenced, more of the text can be printed. | |
131 | The variable will I<not> be restored. | |
132 | ||
133 | Example: | |
134 | $text = "line 1\nline 2\nline 3"; | |
135 | format STDOUT = | |
136 | Text: ^* | |
137 | $text | |
138 | ~~ ^* | |
139 | $text | |
140 | . | |
141 | Output: | |
142 | Text: line 1 | |
143 | line 2 | |
144 | line 3 | |
145 | ||
146 | ||
147 | =head2 Specifying Values | |
148 | ||
149 | The values are specified on the following format line in the same order as | |
150 | the picture fields. The expressions providing the values must be | |
151 | separated by commas. They are all evaluated in a list context | |
a0d0e21e LW |
152 | before the line is processed, so a single list expression could produce |
153 | multiple list elements. The expressions may be spread out to more than | |
154 | one line if enclosed in braces. If so, the opening brace must be the first | |
a034a98d DD |
155 | token on the first line. If an expression evaluates to a number with a |
156 | decimal part, and if the corresponding picture specifies that the decimal | |
157 | part should appear in the output (that is, any picture except multiple "#" | |
158 | characters B<without> an embedded "."), the character used for the decimal | |
159 | point is B<always> determined by the current LC_NUMERIC locale. This | |
160 | means that, if, for example, the run-time environment happens to specify a | |
161 | German locale, "," will be used instead of the default ".". See | |
162 | L<perllocale> and L<"WARNINGS"> for more information. | |
a0d0e21e | 163 | |
a1b95068 WL |
164 | |
165 | =head2 Using Fill Mode | |
166 | ||
167 | On text fields the caret enables a kind of fill mode. Instead of an | |
168 | arbitrary expression, the value supplied must be a scalar variable | |
169 | that contains a text string. Perl puts the next portion of the text into | |
170 | the field, and then chops off the front of the string so that the next time | |
a0d0e21e LW |
171 | the variable is referenced, more of the text can be printed. (Yes, this |
172 | means that the variable itself is altered during execution of the write() | |
a1b95068 WL |
173 | call, and is not restored.) The next portion of text is determined by |
174 | a crude line breaking algorithm. You may use the carriage return character | |
175 | (C<\r>) to force a line break. You can change which characters are legal | |
176 | to break on by changing the variable C<$:> (that's | |
177 | $FORMAT_LINE_BREAK_CHARACTERS if you're using the English module) to a | |
a0d0e21e LW |
178 | list of the desired characters. |
179 | ||
a1b95068 WL |
180 | Normally you would use a sequence of fields in a vertical stack associated |
181 | with the same scalar variable to print out a block of text. You might wish | |
182 | to end the final field with the text "...", which will appear in the output | |
183 | if the text was too long to appear in its entirety. | |
184 | ||
185 | ||
186 | =head2 Suppressing Lines Where All Fields Are Void | |
187 | ||
188 | Using caret fields can produce lines where all fields are blank. You can | |
189 | suppress such lines by putting a "~" (tilde) character anywhere in the | |
190 | line. The tilde will be translated to a space upon output. | |
191 | ||
192 | ||
193 | =head2 Repeating Format Lines | |
194 | ||
195 | If you put two contiguous tilde characters "~~" anywhere into a line, | |
196 | the line will be repeated until all the fields on the line are exhausted, | |
197 | i.e. undefined. For special (caret) text fields this will occur sooner or | |
198 | later, but if you use a text field of the at variety, the expression you | |
199 | supply had better not give the same value every time forever! (C<shift(@f)> | |
200 | is a simple example that would work.) Don't use a regular (at) numeric | |
201 | field in such lines, because it will never go blank. | |
202 | ||
203 | ||
204 | =head2 Top of Form Processing | |
a0d0e21e | 205 | |
54310121 | 206 | Top-of-form processing is by default handled by a format with the |
a0d0e21e | 207 | same name as the current filehandle with "_TOP" concatenated to it. |
a2eb9003 | 208 | It's triggered at the top of each page. See L<perlfunc/write>. |
a0d0e21e LW |
209 | |
210 | Examples: | |
211 | ||
212 | # a report on the /etc/passwd file | |
213 | format STDOUT_TOP = | |
214 | Passwd File | |
215 | Name Login Office Uid Gid Home | |
216 | ------------------------------------------------------------------ | |
217 | . | |
218 | format STDOUT = | |
219 | @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<< | |
220 | $name, $login, $office,$uid,$gid, $home | |
221 | . | |
222 | ||
223 | ||
224 | # a report from a bug report form | |
225 | format STDOUT_TOP = | |
226 | Bug Reports | |
227 | @<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>> | |
228 | $system, $%, $date | |
229 | ------------------------------------------------------------------ | |
230 | . | |
231 | format STDOUT = | |
232 | Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
233 | $subject | |
234 | Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
235 | $index, $description | |
236 | Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
237 | $priority, $date, $description | |
238 | From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
239 | $from, $description | |
240 | Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
241 | $programmer, $description | |
242 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
243 | $description | |
244 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
245 | $description | |
246 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
247 | $description | |
248 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
249 | $description | |
250 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<... | |
251 | $description | |
252 | . | |
253 | ||
254 | It is possible to intermix print()s with write()s on the same output | |
1fef88e7 | 255 | channel, but you'll have to handle C<$-> (C<$FORMAT_LINES_LEFT>) |
a0d0e21e LW |
256 | yourself. |
257 | ||
258 | =head2 Format Variables | |
259 | ||
1fef88e7 JM |
260 | The current format name is stored in the variable C<$~> (C<$FORMAT_NAME>), |
261 | and the current top of form format name is in C<$^> (C<$FORMAT_TOP_NAME>). | |
262 | The current output page number is stored in C<$%> (C<$FORMAT_PAGE_NUMBER>), | |
263 | and the number of lines on the page is in C<$=> (C<$FORMAT_LINES_PER_PAGE>). | |
748a9306 | 264 | Whether to autoflush output on this handle is stored in C<$|> |
1fef88e7 JM |
265 | (C<$OUTPUT_AUTOFLUSH>). The string output before each top of page (except |
266 | the first) is stored in C<$^L> (C<$FORMAT_FORMFEED>). These variables are | |
a0d0e21e LW |
267 | set on a per-filehandle basis, so you'll need to select() into a different |
268 | one to affect them: | |
269 | ||
54310121 | 270 | select((select(OUTF), |
a0d0e21e LW |
271 | $~ = "My_Other_Format", |
272 | $^ = "My_Top_Format" | |
273 | )[0]); | |
274 | ||
275 | Pretty ugly, eh? It's a common idiom though, so don't be too surprised | |
276 | when you see it. You can at least use a temporary variable to hold | |
277 | the previous filehandle: (this is a much better approach in general, | |
278 | because not only does legibility improve, you now have intermediary | |
279 | stage in the expression to single-step the debugger through): | |
280 | ||
281 | $ofh = select(OUTF); | |
282 | $~ = "My_Other_Format"; | |
283 | $^ = "My_Top_Format"; | |
284 | select($ofh); | |
285 | ||
286 | If you use the English module, you can even read the variable names: | |
287 | ||
a1ce9542 | 288 | use English '-no_match_vars'; |
a0d0e21e LW |
289 | $ofh = select(OUTF); |
290 | $FORMAT_NAME = "My_Other_Format"; | |
291 | $FORMAT_TOP_NAME = "My_Top_Format"; | |
292 | select($ofh); | |
293 | ||
294 | But you still have those funny select()s. So just use the FileHandle | |
68dc0745 | 295 | module. Now, you can access these special variables using lowercase |
a0d0e21e LW |
296 | method names instead: |
297 | ||
298 | use FileHandle; | |
299 | format_name OUTF "My_Other_Format"; | |
300 | format_top_name OUTF "My_Top_Format"; | |
301 | ||
302 | Much better! | |
303 | ||
304 | =head1 NOTES | |
305 | ||
54310121 | 306 | Because the values line may contain arbitrary expressions (for at fields, |
748a9306 | 307 | not caret fields), you can farm out more sophisticated processing |
a0d0e21e LW |
308 | to other functions, like sprintf() or one of your own. For example: |
309 | ||
54310121 | 310 | format Ident = |
a0d0e21e LW |
311 | @<<<<<<<<<<<<<<< |
312 | &commify($n) | |
313 | . | |
314 | ||
315 | To get a real at or caret into the field, do this: | |
316 | ||
54310121 | 317 | format Ident = |
a0d0e21e LW |
318 | I have an @ here. |
319 | "@" | |
320 | . | |
321 | ||
322 | To center a whole line of text, do something like this: | |
323 | ||
54310121 | 324 | format Ident = |
a0d0e21e LW |
325 | @||||||||||||||||||||||||||||||||||||||||||||||| |
326 | "Some text line" | |
327 | . | |
328 | ||
329 | There is no builtin way to say "float this to the right hand side | |
330 | of the page, however wide it is." You have to specify where it goes. | |
331 | The truly desperate can generate their own format on the fly, based | |
332 | on the current number of columns, and then eval() it: | |
333 | ||
5a964f20 TC |
334 | $format = "format STDOUT = \n" |
335 | . '^' . '<' x $cols . "\n" | |
336 | . '$entry' . "\n" | |
337 | . "\t^" . "<" x ($cols-8) . "~~\n" | |
338 | . '$entry' . "\n" | |
a0d0e21e LW |
339 | . ".\n"; |
340 | print $format if $Debugging; | |
54310121 | 341 | eval $format; |
a0d0e21e LW |
342 | die $@ if $@; |
343 | ||
344 | Which would generate a format looking something like this: | |
345 | ||
54310121 | 346 | format STDOUT = |
a0d0e21e LW |
347 | ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
348 | $entry | |
349 | ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~ | |
350 | $entry | |
351 | . | |
352 | ||
353 | Here's a little program that's somewhat like fmt(1): | |
354 | ||
54310121 | 355 | format = |
a0d0e21e LW |
356 | ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~ |
357 | $_ | |
358 | ||
359 | . | |
360 | ||
361 | $/ = ''; | |
362 | while (<>) { | |
363 | s/\s*\n\s*/ /g; | |
364 | write; | |
54310121 | 365 | } |
a0d0e21e LW |
366 | |
367 | =head2 Footers | |
368 | ||
369 | While $FORMAT_TOP_NAME contains the name of the current header format, | |
370 | there is no corresponding mechanism to automatically do the same thing | |
371 | for a footer. Not knowing how big a format is going to be until you | |
372 | evaluate it is one of the major problems. It's on the TODO list. | |
373 | ||
374 | Here's one strategy: If you have a fixed-size footer, you can get footers | |
375 | by checking $FORMAT_LINES_LEFT before each write() and print the footer | |
376 | yourself if necessary. | |
377 | ||
54310121 | 378 | Here's another strategy: Open a pipe to yourself, using C<open(MYSELF, "|-")> |
379 | (see L<perlfunc/open()>) and always write() to MYSELF instead of STDOUT. | |
380 | Have your child process massage its STDIN to rearrange headers and footers | |
381 | however you like. Not very convenient, but doable. | |
a0d0e21e LW |
382 | |
383 | =head2 Accessing Formatting Internals | |
384 | ||
385 | For low-level access to the formatting mechanism. you may use formline() | |
386 | and access C<$^A> (the $ACCUMULATOR variable) directly. | |
387 | ||
388 | For example: | |
389 | ||
390 | $str = formline <<'END', 1,2,3; | |
391 | @<<< @||| @>>> | |
392 | END | |
393 | ||
394 | print "Wow, I just stored `$^A' in the accumulator!\n"; | |
395 | ||
5a964f20 | 396 | Or to make an swrite() subroutine, which is to write() what sprintf() |
a0d0e21e LW |
397 | is to printf(), do this: |
398 | ||
a0d0e21e LW |
399 | use Carp; |
400 | sub swrite { | |
748a9306 LW |
401 | croak "usage: swrite PICTURE ARGS" unless @_; |
402 | my $format = shift; | |
403 | $^A = ""; | |
404 | formline($format,@_); | |
405 | return $^A; | |
54310121 | 406 | } |
a0d0e21e LW |
407 | |
408 | $string = swrite(<<'END', 1, 2, 3); | |
409 | Check me out | |
410 | @<<< @||| @>>> | |
411 | END | |
412 | print $string; | |
413 | ||
a034a98d | 414 | =head1 WARNINGS |
a0d0e21e | 415 | |
5a964f20 | 416 | The lone dot that ends a format can also prematurely end a mail |
6aa7c346 CS |
417 | message passing through a misconfigured Internet mailer (and based on |
418 | experience, such misconfiguration is the rule, not the exception). So | |
5a964f20 | 419 | when sending format code through mail, you should indent it so that |
6aa7c346 | 420 | the format-ending dot is not on the left margin; this will prevent |
5a964f20 | 421 | SMTP cutoff. |
6aa7c346 | 422 | |
748a9306 LW |
423 | Lexical variables (declared with "my") are not visible within a |
424 | format unless the format is declared within the scope of the lexical | |
6aa7c346 | 425 | variable. (They weren't visible at all before version 5.001.) |
a034a98d | 426 | |
5a964f20 | 427 | Formats are the only part of Perl that unconditionally use information |
a034a98d DD |
428 | from a program's locale; if a program's environment specifies an |
429 | LC_NUMERIC locale, it is always used to specify the decimal point | |
430 | character in formatted output. Perl ignores all other aspects of locale | |
431 | handling unless the C<use locale> pragma is in effect. Formatted output | |
432 | cannot be controlled by C<use locale> because the pragma is tied to the | |
433 | block structure of the program, and, for historical reasons, formats | |
434 | exist outside that block structure. See L<perllocale> for further | |
435 | discussion of locale handling. | |
c380484f | 436 | |
a1b95068 WL |
437 | Within strings that are to be displayed in a fixed length text field, |
438 | each control character is substituted by a space. (But remember the | |
439 | special meaning of C<\r> when using fill mode.) This is done to avoid | |
440 | misalignment when control characters "disappear" on some output media. | |
c380484f | 441 |