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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 | |
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9 | package Filter::Util::Call ; |
10 | |
11 | require 5.002 ; |
12 | require DynaLoader; |
13 | require Exporter; |
14 | use Carp ; |
15 | use strict; |
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16 | use warnings; |
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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) ; |
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21 | $VERSION = "1.0601" ; |
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22 | |
23 | sub filter_read_exact($) |
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], $coderef) ; |
57 | } |
58 | |
59 | bootstrap Filter::Util::Call ; |
60 | |
61 | 1; |
62 | __END__ |
63 | |
64 | =head1 NAME |
65 | |
66 | Filter::Util::Call - Perl Source Filter Utility Module |
67 | |
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68 | =head1 SYNOPSIS |
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69 | |
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70 | use Filter::Util::Call ; |
71 | |
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72 | =head1 DESCRIPTION |
73 | |
74 | This module provides you with the framework to write I<Source Filters> |
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75 | in Perl. |
76 | |
77 | An alternate interface to Filter::Util::Call is now available. See |
78 | L<Filter::Simple> for more details. |
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79 | |
80 | A I<Perl Source Filter> is implemented as a Perl module. The structure |
81 | of the module can take one of two broadly similar formats. To |
82 | distinguish between them, the first will be referred to as I<method |
83 | filter> and the second as I<closure filter>. |
84 | |
85 | Here is a skeleton for the I<method filter>: |
86 | |
87 | package MyFilter ; |
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88 | |
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89 | use Filter::Util::Call ; |
90 | |
91 | sub import |
92 | { |
93 | my($type, @arguments) = @_ ; |
94 | filter_add([]) ; |
95 | } |
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96 | |
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97 | sub filter |
98 | { |
99 | my($self) = @_ ; |
100 | my($status) ; |
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101 | |
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102 | $status = filter_read() ; |
103 | $status ; |
104 | } |
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105 | |
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106 | 1 ; |
107 | |
108 | and this is the equivalent skeleton for the I<closure filter>: |
109 | |
110 | package MyFilter ; |
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111 | |
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112 | use Filter::Util::Call ; |
113 | |
114 | sub import |
115 | { |
116 | my($type, @arguments) = @_ ; |
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117 | |
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118 | filter_add( |
119 | sub |
120 | { |
121 | my($status) ; |
122 | $status = filter_read() ; |
123 | $status ; |
124 | } ) |
125 | } |
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126 | |
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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<$status> after I<every> |
280 | call to C<filter_read> or C<filter_read_exact>. |
281 | |
282 | =item B<filter_del> |
283 | |
284 | The function, C<filter_del>, is used to disable the current filter. It |
285 | does not affect the running of the filter. All it does is tell Perl not |
286 | to call filter any more. |
287 | |
288 | See L<Example 4: Using filter_del> for details. |
289 | |
290 | =back |
291 | |
292 | =head1 EXAMPLES |
293 | |
294 | Here are a few examples which illustrate the key concepts - as such |
295 | most of them are of little practical use. |
296 | |
297 | The C<examples> sub-directory has copies of all these filters |
298 | implemented both as I<method filters> and as I<closure filters>. |
299 | |
300 | =head2 Example 1: A simple filter. |
301 | |
302 | Below is a I<method filter> which is hard-wired to replace all |
303 | occurrences of the string C<"Joe"> to C<"Jim">. Not particularly |
304 | Useful, but it is the first example and I wanted to keep it simple. |
305 | |
306 | package Joe2Jim ; |
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307 | |
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308 | use Filter::Util::Call ; |
309 | |
310 | sub import |
311 | { |
312 | my($type) = @_ ; |
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313 | |
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314 | filter_add(bless []) ; |
315 | } |
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316 | |
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317 | sub filter |
318 | { |
319 | my($self) = @_ ; |
320 | my($status) ; |
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321 | |
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322 | s/Joe/Jim/g |
323 | if ($status = filter_read()) > 0 ; |
324 | $status ; |
325 | } |
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326 | |
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327 | 1 ; |
328 | |
329 | Here is an example of using the filter: |
330 | |
331 | use Joe2Jim ; |
332 | print "Where is Joe?\n" ; |
333 | |
334 | And this is what the script above will print: |
335 | |
336 | Where is Jim? |
337 | |
338 | =head2 Example 2: Using the context |
339 | |
340 | The previous example was not particularly useful. To make it more |
341 | general purpose we will make use of the context data and allow any |
342 | arbitrary I<from> and I<to> strings to be used. This time we will use a |
343 | I<closure filter>. To reflect its enhanced role, the filter is called |
344 | C<Subst>. |
345 | |
346 | package Subst ; |
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347 | |
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348 | use Filter::Util::Call ; |
349 | use Carp ; |
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350 | |
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351 | sub import |
352 | { |
353 | croak("usage: use Subst qw(from to)") |
354 | unless @_ == 3 ; |
355 | my ($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<$status> is set to C<1> in this case. |
383 | |
384 | package Count ; |
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385 | |
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386 | use Filter::Util::Call ; |
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387 | |
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388 | sub filter |
389 | { |
390 | my ($self) = @_ ; |
391 | my ($status) ; |
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392 | |
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393 | if (($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 | $status ; |
404 | } |
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405 | |
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406 | sub import |
407 | { |
408 | my ($self) = @_ ; |
409 | my ($count) = 0 ; |
410 | filter_add(\$count) ; |
411 | } |
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412 | |
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413 | 1 ; |
414 | |
415 | Here is a script which uses it: |
416 | |
417 | use Count ; |
418 | print "Hello Joe\n" ; |
419 | print "Where is Joe\n" ; |
420 | |
421 | Outputs: |
422 | |
423 | Hello Jim |
424 | Where is Jim |
425 | Made 2 substitutions |
426 | |
427 | =head2 Example 4: Using filter_del |
428 | |
429 | Another variation on a theme. This time we will modify the C<Subst> |
430 | filter to allow a starting and stopping pattern to be specified as well |
431 | as the I<from> and I<to> patterns. If you know the I<vi> editor, it is |
432 | the equivalent of this command: |
433 | |
434 | :/start/,/stop/s/from/to/ |
435 | |
436 | When used as a filter we want to invoke it like this: |
437 | |
438 | use NewSubst qw(start stop from to) ; |
439 | |
440 | Here is the module. |
441 | |
442 | package NewSubst ; |
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443 | |
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444 | use Filter::Util::Call ; |
445 | use Carp ; |
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446 | |
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447 | sub import |
448 | { |
449 | my ($self, $start, $stop, $from, $to) = @_ ; |
450 | my ($found) = 0 ; |
451 | croak("usage: use Subst qw(start stop from to)") |
452 | unless @_ == 5 ; |
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453 | |
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454 | filter_add( |
455 | sub |
456 | { |
457 | my ($status) ; |
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458 | |
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459 | if (($status = filter_read()) > 0) { |
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460 | |
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461 | $found = 1 |
462 | if $found == 0 and /$start/ ; |
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463 | |
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464 | if ($found) { |
465 | s/$from/$to/ ; |
466 | filter_del() if /$stop/ ; |
467 | } |
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468 | |
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469 | } |
470 | $status ; |
471 | } ) |
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472 | |
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473 | } |
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474 | |
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475 | 1 ; |
476 | |
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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 |
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481 | module. Damian's module provides a much cleaner interface than |
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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 | |
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489 | =head1 AUTHOR |
490 | |
491 | Paul Marquess |
492 | |
493 | =head1 DATE |
494 | |
495 | 26th January 1996 |
496 | |
497 | =cut |
498 | |