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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 |
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8 | facilitate this, Perl helps you code up your output page |
9 | 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 on, when to |
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11 | print page headers, etc. Keywords are borrowed from FORTRAN: |
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12 | format() to declare and write() to execute; see their entries in |
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13 | L<perlfunc>. Fortunately, the layout is much more legible, more like |
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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 | |
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93 | Using caret fields can produce variable length records. If the text |
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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. |
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103 | It's triggered at the top of each page. See L<perlfunc/write>. |
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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). |
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159 | Whether to autoflush output on this handle is stored in C<$|> |
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160 | ($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 | |
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201 | Since the values line may contain arbitrary expressions (for at fields, |
202 | not caret fields), you can farm out more sophisticated processing |
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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 $FORMAT_LINES_LEFT before each write() and print the footer |
271 | yourself if necessary. |
272 | |
273 | Here's another strategy; open a pipe to yourself, using C<open(MESELF, "|-")> |
274 | (see L<perlfunc/open()>) and always write() to MESELF instead of |
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275 | STDOUT. Have your child process massage its STDIN to rearrange |
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276 | headers and footers however you like. Not very convenient, but doable. |
277 | |
278 | =head2 Accessing Formatting Internals |
279 | |
280 | For low-level access to the formatting mechanism. you may use formline() |
281 | and access C<$^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 | |
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294 | use Carp; |
295 | sub swrite { |
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296 | croak "usage: swrite PICTURE ARGS" unless @_; |
297 | my $format = shift; |
298 | $^A = ""; |
299 | formline($format,@_); |
300 | return $^A; |
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301 | } |
302 | |
303 | $string = swrite(<<'END', 1, 2, 3); |
304 | Check me out |
305 | @<<< @||| @>>> |
306 | END |
307 | print $string; |
308 | |
309 | =head1 WARNING |
310 | |
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311 | Lexical variables (declared with "my") are not visible within a |
312 | format unless the format is declared within the scope of the lexical |
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313 | variable. (They weren't visible at all before version 5.001.) Furthermore, |
314 | lexical aliases will not be compiled correctly: see |
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315 | L<perlfunc/my> for other issues. |