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1 | |
2 | # Call.pm | |
3 | # | |
4 | # Copyright (c) 1995-2001 Paul Marquess. All rights reserved. | |
5 | # | |
6 | # This program is free software; you can redistribute it and/or | |
7 | # modify it under the same terms as Perl itself. | |
8 | ||
9 | package Filter::Util::Call ; | |
10 | ||
11 | require 5.002 ; | |
12 | require DynaLoader; | |
13 | require Exporter; | |
14 | use Carp ; | |
15 | use strict; | |
16 | use warnings; | |
17 | use vars qw($VERSION @ISA @EXPORT) ; | |
18 | ||
19 | @ISA = qw(Exporter DynaLoader); | |
20 | @EXPORT = qw( filter_add filter_del filter_read filter_read_exact) ; | |
21 | $VERSION = "1.0601" ; | |
22 | ||
23 | sub filter_read_exact($) | |
24 | { | |
25 | my ($size) = @_ ; | |
26 | my ($left) = $size ; | |
27 | my ($status) ; | |
28 | ||
29 | croak ("filter_read_exact: size parameter must be > 0") | |
30 | unless $size > 0 ; | |
31 | ||
32 | # try to read a block which is exactly $size bytes long | |
33 | while ($left and ($status = filter_read($left)) > 0) { | |
34 | $left = $size - length $_ ; | |
35 | } | |
36 | ||
37 | # EOF with pending data is a special case | |
38 | return 1 if $status == 0 and length $_ ; | |
39 | ||
40 | return $status ; | |
41 | } | |
42 | ||
43 | sub filter_add($) | |
44 | { | |
45 | my($obj) = @_ ; | |
46 | ||
47 | # Did we get a code reference? | |
48 | my $coderef = (ref $obj eq 'CODE') ; | |
49 | ||
50 | # If the parameter isn't already a reference, make it one. | |
51 | $obj = \$obj unless ref $obj ; | |
52 | ||
53 | $obj = bless ($obj, (caller)[0]) unless $coderef ; | |
54 | ||
55 | # finish off the installation of the filter in C. | |
56 | Filter::Util::Call::real_import($obj, (caller)[0], $coderef) ; | |
57 | } | |
58 | ||
59 | bootstrap Filter::Util::Call ; | |
60 | ||
61 | 1; | |
62 | __END__ | |
63 | ||
64 | =head1 NAME | |
65 | ||
66 | Filter::Util::Call - Perl Source Filter Utility Module | |
67 | ||
68 | =head1 SYNOPSIS | |
69 | ||
70 | use Filter::Util::Call ; | |
71 | ||
72 | =head1 DESCRIPTION | |
73 | ||
74 | This module provides you with the framework to write I<Source Filters> | |
75 | in Perl. | |
76 | ||
77 | An alternate interface to Filter::Util::Call is now available. See | |
78 | L<Filter::Simple> for more details. | |
79 | ||
80 | A I<Perl Source Filter> is implemented as a Perl module. The structure | |
81 | of the module can take one of two broadly similar formats. To | |
82 | distinguish between them, the first will be referred to as I<method | |
83 | filter> and the second as I<closure filter>. | |
84 | ||
85 | Here is a skeleton for the I<method filter>: | |
86 | ||
87 | package MyFilter ; | |
88 | ||
89 | use Filter::Util::Call ; | |
90 | ||
91 | sub import | |
92 | { | |
93 | my($type, @arguments) = @_ ; | |
94 | filter_add([]) ; | |
95 | } | |
96 | ||
97 | sub filter | |
98 | { | |
99 | my($self) = @_ ; | |
100 | my($status) ; | |
101 | ||
102 | $status = filter_read() ; | |
103 | $status ; | |
104 | } | |
105 | ||
106 | 1 ; | |
107 | ||
108 | and this is the equivalent skeleton for the I<closure filter>: | |
109 | ||
110 | package MyFilter ; | |
111 | ||
112 | use Filter::Util::Call ; | |
113 | ||
114 | sub import | |
115 | { | |
116 | my($type, @arguments) = @_ ; | |
117 | ||
118 | filter_add( | |
119 | sub | |
120 | { | |
121 | my($status) ; | |
122 | $status = filter_read() ; | |
123 | $status ; | |
124 | } ) | |
125 | } | |
126 | ||
127 | 1 ; | |
128 | ||
129 | To make use of either of the two filter modules above, place the line | |
130 | below in a Perl source file. | |
131 | ||
132 | use MyFilter; | |
133 | ||
134 | In fact, the skeleton modules shown above are fully functional I<Source | |
135 | Filters>, albeit fairly useless ones. All they does is filter the | |
136 | source stream without modifying it at all. | |
137 | ||
138 | As you can see both modules have a broadly similar structure. They both | |
139 | make use of the C<Filter::Util::Call> module and both have an C<import> | |
140 | method. The difference between them is that the I<method filter> | |
141 | requires a I<filter> method, whereas the I<closure filter> gets the | |
142 | equivalent of a I<filter> method with the anonymous sub passed to | |
143 | I<filter_add>. | |
144 | ||
145 | To make proper use of the I<closure filter> shown above you need to | |
146 | have a good understanding of the concept of a I<closure>. See | |
147 | L<perlref> for more details on the mechanics of I<closures>. | |
148 | ||
149 | =head2 B<use Filter::Util::Call> | |
150 | ||
151 | The following functions are exported by C<Filter::Util::Call>: | |
152 | ||
153 | filter_add() | |
154 | filter_read() | |
155 | filter_read_exact() | |
156 | filter_del() | |
157 | ||
158 | =head2 B<import()> | |
159 | ||
160 | The C<import> method is used to create an instance of the filter. It is | |
161 | called indirectly by Perl when it encounters the C<use MyFilter> line | |
162 | in a source file (See L<perlfunc/import> for more details on | |
163 | C<import>). | |
164 | ||
165 | It will always have at least one parameter automatically passed by Perl | |
166 | - this corresponds to the name of the package. In the example above it | |
167 | will be C<"MyFilter">. | |
168 | ||
169 | Apart from the first parameter, import can accept an optional list of | |
170 | parameters. These can be used to pass parameters to the filter. For | |
171 | example: | |
172 | ||
173 | use MyFilter qw(a b c) ; | |
174 | ||
175 | will result in the C<@_> array having the following values: | |
176 | ||
177 | @_ [0] => "MyFilter" | |
178 | @_ [1] => "a" | |
179 | @_ [2] => "b" | |
180 | @_ [3] => "c" | |
181 | ||
182 | Before terminating, the C<import> function must explicitly install the | |
183 | filter by calling C<filter_add>. | |
184 | ||
185 | B<filter_add()> | |
186 | ||
187 | The function, C<filter_add>, actually installs the filter. It takes one | |
188 | parameter which should be a reference. The kind of reference used will | |
189 | dictate which of the two filter types will be used. | |
190 | ||
191 | If a CODE reference is used then a I<closure filter> will be assumed. | |
192 | ||
193 | If a CODE reference is not used, a I<method filter> will be assumed. | |
194 | In a I<method filter>, the reference can be used to store context | |
195 | information. The reference will be I<blessed> into the package by | |
196 | C<filter_add>. | |
197 | ||
198 | See the filters at the end of this documents for examples of using | |
199 | context information using both I<method filters> and I<closure | |
200 | filters>. | |
201 | ||
202 | =head2 B<filter() and anonymous sub> | |
203 | ||
204 | Both the C<filter> method used with a I<method filter> and the | |
205 | anonymous sub used with a I<closure filter> is where the main | |
206 | processing for the filter is done. | |
207 | ||
208 | The big difference between the two types of filter is that the I<method | |
209 | filter> uses the object passed to the method to store any context data, | |
210 | whereas the I<closure filter> uses the lexical variables that are | |
211 | maintained by the closure. | |
212 | ||
213 | Note that the single parameter passed to the I<method filter>, | |
214 | C<$self>, is the same reference that was passed to C<filter_add> | |
215 | blessed into the filter's package. See the example filters later on for | |
216 | details of using C<$self>. | |
217 | ||
218 | Here is a list of the common features of the anonymous sub and the | |
219 | C<filter()> method. | |
220 | ||
221 | =over 5 | |
222 | ||
223 | =item B<$_> | |
224 | ||
225 | Although C<$_> doesn't actually appear explicitly in the sample filters | |
226 | above, it is implicitly used in a number of places. | |
227 | ||
228 | Firstly, when either C<filter> or the anonymous sub are called, a local | |
229 | copy of C<$_> will automatically be created. It will always contain the | |
230 | empty string at this point. | |
231 | ||
232 | Next, both C<filter_read> and C<filter_read_exact> will append any | |
233 | source data that is read to the end of C<$_>. | |
234 | ||
235 | Finally, when C<filter> or the anonymous sub are finished processing, | |
236 | they are expected to return the filtered source using C<$_>. | |
237 | ||
238 | This implicit use of C<$_> greatly simplifies the filter. | |
239 | ||
240 | =item B<$status> | |
241 | ||
242 | The status value that is returned by the user's C<filter> method or | |
243 | anonymous sub and the C<filter_read> and C<read_exact> functions take | |
244 | the same set of values, namely: | |
245 | ||
246 | < 0 Error | |
247 | = 0 EOF | |
248 | > 0 OK | |
249 | ||
250 | =item B<filter_read> and B<filter_read_exact> | |
251 | ||
252 | These functions are used by the filter to obtain either a line or block | |
253 | from the next filter in the chain or the actual source file if there | |
254 | aren't any other filters. | |
255 | ||
256 | The function C<filter_read> takes two forms: | |
257 | ||
258 | $status = filter_read() ; | |
259 | $status = filter_read($size) ; | |
260 | ||
261 | The first form is used to request a I<line>, the second requests a | |
262 | I<block>. | |
263 | ||
264 | In line mode, C<filter_read> will append the next source line to the | |
265 | end of the C<$_> scalar. | |
266 | ||
267 | In block mode, C<filter_read> will append a block of data which is <= | |
268 | C<$size> to the end of the C<$_> scalar. It is important to emphasise | |
269 | the that C<filter_read> will not necessarily read a block which is | |
270 | I<precisely> C<$size> bytes. | |
271 | ||
272 | If you need to be able to read a block which has an exact size, you can | |
273 | use the function C<filter_read_exact>. It works identically to | |
274 | C<filter_read> in block mode, except it will try to read a block which | |
275 | is exactly C<$size> bytes in length. The only circumstances when it | |
276 | will not return a block which is C<$size> bytes long is on EOF or | |
277 | error. | |
278 | ||
279 | It is I<very> important to check the value of C<$status> after I<every> | |
280 | call to C<filter_read> or C<filter_read_exact>. | |
281 | ||
282 | =item B<filter_del> | |
283 | ||
284 | The function, C<filter_del>, is used to disable the current filter. It | |
285 | does not affect the running of the filter. All it does is tell Perl not | |
286 | to call filter any more. | |
287 | ||
288 | See L<Example 4: Using filter_del> for details. | |
289 | ||
290 | =back | |
291 | ||
292 | =head1 EXAMPLES | |
293 | ||
294 | Here are a few examples which illustrate the key concepts - as such | |
295 | most of them are of little practical use. | |
296 | ||
297 | The C<examples> sub-directory has copies of all these filters | |
298 | implemented both as I<method filters> and as I<closure filters>. | |
299 | ||
300 | =head2 Example 1: A simple filter. | |
301 | ||
302 | Below is a I<method filter> which is hard-wired to replace all | |
303 | occurrences of the string C<"Joe"> to C<"Jim">. Not particularly | |
304 | Useful, but it is the first example and I wanted to keep it simple. | |
305 | ||
306 | package Joe2Jim ; | |
307 | ||
308 | use Filter::Util::Call ; | |
309 | ||
310 | sub import | |
311 | { | |
312 | my($type) = @_ ; | |
313 | ||
314 | filter_add(bless []) ; | |
315 | } | |
316 | ||
317 | sub filter | |
318 | { | |
319 | my($self) = @_ ; | |
320 | my($status) ; | |
321 | ||
322 | s/Joe/Jim/g | |
323 | if ($status = filter_read()) > 0 ; | |
324 | $status ; | |
325 | } | |
326 | ||
327 | 1 ; | |
328 | ||
329 | Here is an example of using the filter: | |
330 | ||
331 | use Joe2Jim ; | |
332 | print "Where is Joe?\n" ; | |
333 | ||
334 | And this is what the script above will print: | |
335 | ||
336 | Where is Jim? | |
337 | ||
338 | =head2 Example 2: Using the context | |
339 | ||
340 | The previous example was not particularly useful. To make it more | |
341 | general purpose we will make use of the context data and allow any | |
342 | arbitrary I<from> and I<to> strings to be used. This time we will use a | |
343 | I<closure filter>. To reflect its enhanced role, the filter is called | |
344 | C<Subst>. | |
345 | ||
346 | package Subst ; | |
347 | ||
348 | use Filter::Util::Call ; | |
349 | use Carp ; | |
350 | ||
351 | sub import | |
352 | { | |
353 | croak("usage: use Subst qw(from to)") | |
354 | unless @_ == 3 ; | |
355 | my ($self, $from, $to) = @_ ; | |
356 | filter_add( | |
357 | sub | |
358 | { | |
359 | my ($status) ; | |
360 | s/$from/$to/ | |
361 | if ($status = filter_read()) > 0 ; | |
362 | $status ; | |
363 | }) | |
364 | } | |
365 | 1 ; | |
366 | ||
367 | and is used like this: | |
368 | ||
369 | use Subst qw(Joe Jim) ; | |
370 | print "Where is Joe?\n" ; | |
371 | ||
372 | ||
373 | =head2 Example 3: Using the context within the filter | |
374 | ||
375 | Here is a filter which a variation of the C<Joe2Jim> filter. As well as | |
376 | substituting all occurrences of C<"Joe"> to C<"Jim"> it keeps a count | |
377 | of the number of substitutions made in the context object. | |
378 | ||
379 | Once EOF is detected (C<$status> is zero) the filter will insert an | |
380 | extra line into the source stream. When this extra line is executed it | |
381 | will print a count of the number of substitutions actually made. | |
382 | Note that C<$status> is set to C<1> in this case. | |
383 | ||
384 | package Count ; | |
385 | ||
386 | use Filter::Util::Call ; | |
387 | ||
388 | sub filter | |
389 | { | |
390 | my ($self) = @_ ; | |
391 | my ($status) ; | |
392 | ||
393 | if (($status = filter_read()) > 0 ) { | |
394 | s/Joe/Jim/g ; | |
395 | ++ $$self ; | |
396 | } | |
397 | elsif ($$self >= 0) { # EOF | |
398 | $_ = "print q[Made ${$self} substitutions\n]" ; | |
399 | $status = 1 ; | |
400 | $$self = -1 ; | |
401 | } | |
402 | ||
403 | $status ; | |
404 | } | |
405 | ||
406 | sub import | |
407 | { | |
408 | my ($self) = @_ ; | |
409 | my ($count) = 0 ; | |
410 | filter_add(\$count) ; | |
411 | } | |
412 | ||
413 | 1 ; | |
414 | ||
415 | Here is a script which uses it: | |
416 | ||
417 | use Count ; | |
418 | print "Hello Joe\n" ; | |
419 | print "Where is Joe\n" ; | |
420 | ||
421 | Outputs: | |
422 | ||
423 | Hello Jim | |
424 | Where is Jim | |
425 | Made 2 substitutions | |
426 | ||
427 | =head2 Example 4: Using filter_del | |
428 | ||
429 | Another variation on a theme. This time we will modify the C<Subst> | |
430 | filter to allow a starting and stopping pattern to be specified as well | |
431 | as the I<from> and I<to> patterns. If you know the I<vi> editor, it is | |
432 | the equivalent of this command: | |
433 | ||
434 | :/start/,/stop/s/from/to/ | |
435 | ||
436 | When used as a filter we want to invoke it like this: | |
437 | ||
438 | use NewSubst qw(start stop from to) ; | |
439 | ||
440 | Here is the module. | |
441 | ||
442 | package NewSubst ; | |
443 | ||
444 | use Filter::Util::Call ; | |
445 | use Carp ; | |
446 | ||
447 | sub import | |
448 | { | |
449 | my ($self, $start, $stop, $from, $to) = @_ ; | |
450 | my ($found) = 0 ; | |
451 | croak("usage: use Subst qw(start stop from to)") | |
452 | unless @_ == 5 ; | |
453 | ||
454 | filter_add( | |
455 | sub | |
456 | { | |
457 | my ($status) ; | |
458 | ||
459 | if (($status = filter_read()) > 0) { | |
460 | ||
461 | $found = 1 | |
462 | if $found == 0 and /$start/ ; | |
463 | ||
464 | if ($found) { | |
465 | s/$from/$to/ ; | |
466 | filter_del() if /$stop/ ; | |
467 | } | |
468 | ||
469 | } | |
470 | $status ; | |
471 | } ) | |
472 | ||
473 | } | |
474 | ||
475 | 1 ; | |
476 | ||
477 | =head1 Filter::Simple | |
478 | ||
479 | If you intend using the Filter::Call functionality, I would strongly | |
480 | recommend that you check out Damian Conway's excellent Filter::Simple | |
481 | module. Damian's module provides a much cleaner interface than | |
482 | Filter::Util::Call. Although it doesn't allow the fine control that | |
483 | Filter::Util::Call does, it should be adequate for the majority of | |
484 | applications. It's available at | |
485 | ||
486 | http://www.cpan.org/modules/by-author/Damian_Conway/Filter-Simple.tar.gz | |
487 | http://www.csse.monash.edu.au/~damian/CPAN/Filter-Simple.tar.gz | |
488 | ||
489 | =head1 AUTHOR | |
490 | ||
491 | Paul Marquess | |
492 | ||
493 | =head1 DATE | |
494 | ||
495 | 26th January 1996 | |
496 | ||
497 | =cut | |
498 |