Commit | Line | Data |
---|---|---|
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 | |
8 | facilitate this, Perl helps you lay out your output page in your code in a | |
9 | fashion that's close to how it will look when it's printed. It can keep | |
10 | track of things like how many lines on a page, what page you're, when to | |
748a9306 | 11 | print page headers, etc. Keywords are borrowed from FORTRAN: |
a0d0e21e | 12 | format() to declare and write() to execute; see their entries in |
748a9306 | 13 | L<perlfunc>. Fortunately, the layout is much more legible, more like |
a0d0e21e LW |
14 | BASIC's PRINT USING statement. Think of it as a poor man's nroff(1). |
15 | ||
16 | Formats, like packages and subroutines, are declared rather than executed, | |
17 | so they may occur at any point in your program. (Usually it's best to | |
18 | keep them all together though.) They have their own namespace apart from | |
19 | all the other "types" in Perl. This means that if you have a function | |
20 | named "Foo", it is not the same thing as having a format named "Foo". | |
21 | However, the default name for the format associated with a given | |
22 | filehandle is the same as the name of the filehandle. Thus, the default | |
23 | format for STDOUT is name "STDOUT", and the default format for filehandle | |
24 | TEMP is name "TEMP". They just look the same. They aren't. | |
25 | ||
26 | Output record formats are declared as follows: | |
27 | ||
28 | format NAME = | |
29 | FORMLIST | |
30 | . | |
31 | ||
32 | If name is omitted, format "STDOUT" is defined. FORMLIST consists of a | |
33 | sequence of lines, each of which may be of one of three types: | |
34 | ||
35 | =over 4 | |
36 | ||
37 | =item 1. | |
38 | ||
39 | A comment, indicated by putting a '#' in the first column. | |
40 | ||
41 | =item 2. | |
42 | ||
43 | A "picture" line giving the format for one output line. | |
44 | ||
45 | =item 3. | |
46 | ||
47 | An argument line supplying values to plug into the previous picture line. | |
48 | ||
49 | =back | |
50 | ||
51 | Picture lines are printed exactly as they look, except for certain fields | |
52 | that substitute values into the line. Each field in a picture line starts | |
53 | with either "@" (at) or "^" (caret). These lines do not undergo any kind | |
54 | of variable interpolation. The at field (not to be confused with the array | |
55 | marker @) is the normal kind of field; the other kind, caret fields, are used | |
56 | to do rudimentary multi-line text block filling. The length of the field | |
57 | is supplied by padding out the field with multiple "<", ">", or "|" | |
58 | characters to specify, respectively, left justification, right | |
59 | justification, or centering. If the variable would exceed the width | |
60 | specified, it is truncated. | |
61 | ||
62 | As an alternate form of right justification, you may also use "#" | |
63 | characters (with an optional ".") to specify a numeric field. This way | |
64 | you can line up the decimal points. If any value supplied for these | |
65 | fields contains a newline, only the text up to the newline is printed. | |
66 | Finally, the special field "@*" can be used for printing multi-line, | |
67 | non-truncated values; it should appear by itself on a line. | |
68 | ||
69 | The values are specified on the following line in the same order as | |
70 | the picture fields. The expressions providing the values should be | |
71 | separated by commas. The expressions are all evaluated in a list context | |
72 | before the line is processed, so a single list expression could produce | |
73 | multiple list elements. The expressions may be spread out to more than | |
74 | one line if enclosed in braces. If so, the opening brace must be the first | |
75 | token on the first line. | |
76 | ||
77 | Picture fields that begin with ^ rather than @ are treated specially. | |
78 | With a # field, the field is blanked out if the value is undefined. For | |
79 | other field types, the caret enables a kind of fill mode. Instead of an | |
80 | arbitrary expression, the value supplied must be a scalar variable name | |
81 | that contains a text string. Perl puts as much text as it can into the | |
82 | field, and then chops off the front of the string so that the next time | |
83 | the variable is referenced, more of the text can be printed. (Yes, this | |
84 | means that the variable itself is altered during execution of the write() | |
85 | call, and is not returned.) Normally you would use a sequence of fields | |
86 | in a vertical stack to print out a block of text. You might wish to end | |
87 | the final field with the text "...", which will appear in the output if | |
88 | the text was too long to appear in its entirety. You can change which | |
89 | characters are legal to break on by changing the variable C<$:> (that's | |
90 | $FORMAT_LINE_BREAK_CHARACTERS if you're using the English module) to a | |
91 | list of the desired characters. | |
92 | ||
748a9306 | 93 | Using caret fields can produce variable length records. If the text |
a0d0e21e LW |
94 | to be formatted is short, you can suppress blank lines by putting a |
95 | "~" (tilde) character anywhere in the line. The tilde will be translated | |
96 | to a space upon output. If you put a second tilde contiguous to the | |
97 | first, the line will be repeated until all the fields on the line are | |
98 | exhausted. (If you use a field of the at variety, the expression you | |
99 | supply had better not give the same value every time forever!) | |
100 | ||
101 | Top-of-form processing is by default handled by a format with the | |
102 | same name as the current filehandle with "_TOP" concatenated to it. | |
103 | It's triggered at the top of each page. See <perlfunc/write()>. | |
104 | ||
105 | Examples: | |
106 | ||
107 | # a report on the /etc/passwd file | |
108 | format STDOUT_TOP = | |
109 | Passwd File | |
110 | Name Login Office Uid Gid Home | |
111 | ------------------------------------------------------------------ | |
112 | . | |
113 | format STDOUT = | |
114 | @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<< | |
115 | $name, $login, $office,$uid,$gid, $home | |
116 | . | |
117 | ||
118 | ||
119 | # a report from a bug report form | |
120 | format STDOUT_TOP = | |
121 | Bug Reports | |
122 | @<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>> | |
123 | $system, $%, $date | |
124 | ------------------------------------------------------------------ | |
125 | . | |
126 | format STDOUT = | |
127 | Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
128 | $subject | |
129 | Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
130 | $index, $description | |
131 | Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
132 | $priority, $date, $description | |
133 | From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
134 | $from, $description | |
135 | Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
136 | $programmer, $description | |
137 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
138 | $description | |
139 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
140 | $description | |
141 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
142 | $description | |
143 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
144 | $description | |
145 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<... | |
146 | $description | |
147 | . | |
148 | ||
149 | It is possible to intermix print()s with write()s on the same output | |
150 | channel, but you'll have to handle $- ($FORMAT_LINES_LEFT) | |
151 | yourself. | |
152 | ||
153 | =head2 Format Variables | |
154 | ||
155 | The current format name is stored in the variable C<$~> ($FORMAT_NAME), | |
156 | and the current top of form format name is in C<$^> ($FORMAT_TOP_NAME). | |
157 | The current output page number is stored in C<$%> ($FORMAT_PAGE_NUMBER), | |
158 | and the number of lines on the page is in C<$=> ($FORMAT_LINES_PER_PAGE). | |
748a9306 | 159 | Whether to autoflush output on this handle is stored in C<$|> |
a0d0e21e LW |
160 | ($OUTPUT_AUTOFLUSH). The string output before each top of page (except |
161 | the first) is stored in C<$^L> ($FORMAT_FORMFEED). These variables are | |
162 | set on a per-filehandle basis, so you'll need to select() into a different | |
163 | one to affect them: | |
164 | ||
165 | select((select(OUTF), | |
166 | $~ = "My_Other_Format", | |
167 | $^ = "My_Top_Format" | |
168 | )[0]); | |
169 | ||
170 | Pretty ugly, eh? It's a common idiom though, so don't be too surprised | |
171 | when you see it. You can at least use a temporary variable to hold | |
172 | the previous filehandle: (this is a much better approach in general, | |
173 | because not only does legibility improve, you now have intermediary | |
174 | stage in the expression to single-step the debugger through): | |
175 | ||
176 | $ofh = select(OUTF); | |
177 | $~ = "My_Other_Format"; | |
178 | $^ = "My_Top_Format"; | |
179 | select($ofh); | |
180 | ||
181 | If you use the English module, you can even read the variable names: | |
182 | ||
183 | use English; | |
184 | $ofh = select(OUTF); | |
185 | $FORMAT_NAME = "My_Other_Format"; | |
186 | $FORMAT_TOP_NAME = "My_Top_Format"; | |
187 | select($ofh); | |
188 | ||
189 | But you still have those funny select()s. So just use the FileHandle | |
190 | module. Now, you can access these special variables using lower-case | |
191 | method names instead: | |
192 | ||
193 | use FileHandle; | |
194 | format_name OUTF "My_Other_Format"; | |
195 | format_top_name OUTF "My_Top_Format"; | |
196 | ||
197 | Much better! | |
198 | ||
199 | =head1 NOTES | |
200 | ||
748a9306 LW |
201 | Since the values line may contain arbitrary expressions (for at fields, |
202 | not caret fields), you can farm out more sophisticated processing | |
a0d0e21e LW |
203 | to other functions, like sprintf() or one of your own. For example: |
204 | ||
205 | format Ident = | |
206 | @<<<<<<<<<<<<<<< | |
207 | &commify($n) | |
208 | . | |
209 | ||
210 | To get a real at or caret into the field, do this: | |
211 | ||
212 | format Ident = | |
213 | I have an @ here. | |
214 | "@" | |
215 | . | |
216 | ||
217 | To center a whole line of text, do something like this: | |
218 | ||
219 | format Ident = | |
220 | @||||||||||||||||||||||||||||||||||||||||||||||| | |
221 | "Some text line" | |
222 | . | |
223 | ||
224 | There is no builtin way to say "float this to the right hand side | |
225 | of the page, however wide it is." You have to specify where it goes. | |
226 | The truly desperate can generate their own format on the fly, based | |
227 | on the current number of columns, and then eval() it: | |
228 | ||
229 | $format = "format STDOUT = \n"; | |
230 | . '^' . '<' x $cols . "\n"; | |
231 | . '$entry' . "\n"; | |
232 | . "\t^" . "<" x ($cols-8) . "~~\n"; | |
233 | . '$entry' . "\n"; | |
234 | . ".\n"; | |
235 | print $format if $Debugging; | |
236 | eval $format; | |
237 | die $@ if $@; | |
238 | ||
239 | Which would generate a format looking something like this: | |
240 | ||
241 | format STDOUT = | |
242 | ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
243 | $entry | |
244 | ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~ | |
245 | $entry | |
246 | . | |
247 | ||
248 | Here's a little program that's somewhat like fmt(1): | |
249 | ||
250 | format = | |
251 | ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~ | |
252 | $_ | |
253 | ||
254 | . | |
255 | ||
256 | $/ = ''; | |
257 | while (<>) { | |
258 | s/\s*\n\s*/ /g; | |
259 | write; | |
260 | } | |
261 | ||
262 | =head2 Footers | |
263 | ||
264 | While $FORMAT_TOP_NAME contains the name of the current header format, | |
265 | there is no corresponding mechanism to automatically do the same thing | |
266 | for a footer. Not knowing how big a format is going to be until you | |
267 | evaluate it is one of the major problems. It's on the TODO list. | |
268 | ||
269 | Here's one strategy: If you have a fixed-size footer, you can get footers | |
270 | by checking $FORMAT_LINES_LEFT before each write() and print the footer | |
271 | yourself if necessary. | |
272 | ||
273 | Here's another strategy; open a pipe to yourself, using C<open(MESELF, "|-")> | |
274 | (see L<perlfunc/open()>) and always write() to MESELF instead of | |
275 | STDOUT. Have your child process postprocesses its STDIN to rearrange | |
276 | headers and footers however you like. Not very convenient, but doable. | |
277 | ||
278 | =head2 Accessing Formatting Internals | |
279 | ||
280 | For low-level access to the formatting mechanism. you may use formline() | |
281 | and access C<$^A> (the $ACCUMULATOR variable) directly. | |
282 | ||
283 | For example: | |
284 | ||
285 | $str = formline <<'END', 1,2,3; | |
286 | @<<< @||| @>>> | |
287 | END | |
288 | ||
289 | print "Wow, I just stored `$^A' in the accumulator!\n"; | |
290 | ||
291 | Or to make an swrite() subroutine which is to write() what sprintf() | |
292 | is to printf(), do this: | |
293 | ||
a0d0e21e LW |
294 | use Carp; |
295 | sub swrite { | |
748a9306 LW |
296 | croak "usage: swrite PICTURE ARGS" unless @_; |
297 | my $format = shift; | |
298 | $^A = ""; | |
299 | formline($format,@_); | |
300 | return $^A; | |
a0d0e21e LW |
301 | } |
302 | ||
303 | $string = swrite(<<'END', 1, 2, 3); | |
304 | Check me out | |
305 | @<<< @||| @>>> | |
306 | END | |
307 | print $string; | |
308 | ||
309 | =head1 WARNING | |
310 | ||
748a9306 LW |
311 | Lexical variables (declared with "my") are not visible within a |
312 | format unless the format is declared within the scope of the lexical | |
313 | variable. (They weren't visiblie at all before version 5.001.) |