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1 | package Filter::Util::Call ; |
2 | ||
3 | require 5.002 ; | |
4 | require DynaLoader; | |
5 | require Exporter; | |
6 | use Carp ; | |
7 | use strict; | |
8 | use vars qw($VERSION @ISA @EXPORT) ; | |
9 | ||
10 | @ISA = qw(Exporter DynaLoader); | |
11 | @EXPORT = qw( filter_add filter_del filter_read filter_read_exact) ; | |
12 | $VERSION = "1.04" ; | |
13 | ||
14 | sub filter_read_exact($) | |
15 | { | |
16 | my ($size) = @_ ; | |
17 | my ($left) = $size ; | |
18 | my ($status) ; | |
19 | ||
20 | croak ("filter_read_exact: size parameter must be > 0") | |
21 | unless $size > 0 ; | |
22 | ||
23 | # try to read a block which is exactly $size bytes long | |
24 | while ($left and ($status = filter_read($left)) > 0) { | |
25 | $left = $size - length $_ ; | |
26 | } | |
27 | ||
28 | # EOF with pending data is a special case | |
29 | return 1 if $status == 0 and length $_ ; | |
30 | ||
31 | return $status ; | |
32 | } | |
33 | ||
34 | sub filter_add($) | |
35 | { | |
36 | my($obj) = @_ ; | |
37 | ||
38 | # Did we get a code reference? | |
39 | my $coderef = (ref $obj eq 'CODE') ; | |
40 | ||
41 | # If the parameter isn't already a reference, make it one. | |
42 | $obj = \$obj unless ref $obj ; | |
43 | ||
44 | $obj = bless ($obj, (caller)[0]) unless $coderef ; | |
45 | ||
46 | # finish off the installation of the filter in C. | |
47 | Filter::Util::Call::real_import($obj, (caller)[0], $coderef) ; | |
48 | } | |
49 | ||
50 | bootstrap Filter::Util::Call ; | |
51 | ||
52 | 1; | |
53 | __END__ | |
54 | ||
55 | =head1 NAME | |
56 | ||
57 | Filter::Util::Call - Perl Source Filter Utility Module | |
58 | ||
59 | =head1 DESCRIPTION | |
60 | ||
61 | This module provides you with the framework to write I<Source Filters> | |
62 | in Perl. | |
63 | ||
64 | A I<Perl Source Filter> is implemented as a Perl module. The structure | |
65 | of the module can take one of two broadly similar formats. To | |
66 | distinguish between them, the first will be referred to as I<method | |
67 | filter> and the second as I<closure filter>. | |
68 | ||
69 | Here is a skeleton for the I<method filter>: | |
70 | ||
71 | package MyFilter ; | |
72 | ||
73 | use Filter::Util::Call ; | |
74 | ||
75 | sub import | |
76 | { | |
77 | my($type, @arguments) = @_ ; | |
78 | filter_add([]) ; | |
79 | } | |
80 | ||
81 | sub filter | |
82 | { | |
83 | my($self) = @_ ; | |
84 | my($status) ; | |
85 | ||
86 | $status = filter_read() ; | |
87 | $status ; | |
88 | } | |
89 | ||
90 | 1 ; | |
91 | ||
92 | and this is the equivalent skeleton for the I<closure filter>: | |
93 | ||
94 | package MyFilter ; | |
95 | ||
96 | use Filter::Util::Call ; | |
97 | ||
98 | sub import | |
99 | { | |
100 | my($type, @arguments) = @_ ; | |
101 | ||
102 | filter_add( | |
103 | sub | |
104 | { | |
105 | my($status) ; | |
106 | $status = filter_read() ; | |
107 | $status ; | |
108 | } ) | |
109 | } | |
110 | ||
111 | 1 ; | |
112 | ||
113 | To make use of either of the two filter modules above, place the line | |
114 | below in a Perl source file. | |
115 | ||
116 | use MyFilter; | |
117 | ||
118 | In fact, the skeleton modules shown above are fully functional I<Source | |
119 | Filters>, albeit fairly useless ones. All they does is filter the | |
120 | source stream without modifying it at all. | |
121 | ||
122 | As you can see both modules have a broadly similar structure. They both | |
123 | make use of the C<Filter::Util::Call> module and both have an C<import> | |
124 | method. The difference between them is that the I<method filter> | |
125 | requires a I<filter> method, whereas the I<closure filter> gets the | |
126 | equivalent of a I<filter> method with the anonymous sub passed to | |
127 | I<filter_add>. | |
128 | ||
129 | To make proper use of the I<closure filter> shown above you need to | |
130 | have a good understanding of the concept of a I<closure>. See | |
131 | L<perlref> for more details on the mechanics of I<closures>. | |
132 | ||
133 | =head2 B<use Filter::Util::Call> | |
134 | ||
135 | The following functions are exported by C<Filter::Util::Call>: | |
136 | ||
137 | filter_add() | |
138 | filter_read() | |
139 | filter_read_exact() | |
140 | filter_del() | |
141 | ||
142 | =head2 B<import()> | |
143 | ||
144 | The C<import> method is used to create an instance of the filter. It is | |
145 | called indirectly by Perl when it encounters the C<use MyFilter> line | |
146 | in a source file (See L<perlfunc/import> for more details on | |
147 | C<import>). | |
148 | ||
149 | It will always have at least one parameter automatically passed by Perl | |
150 | - this corresponds to the name of the package. In the example above it | |
151 | will be C<"MyFilter">. | |
152 | ||
153 | Apart from the first parameter, import can accept an optional list of | |
154 | parameters. These can be used to pass parameters to the filter. For | |
155 | example: | |
156 | ||
157 | use MyFilter qw(a b c) ; | |
158 | ||
159 | will result in the C<@_> array having the following values: | |
160 | ||
161 | @_ [0] => "MyFilter" | |
162 | @_ [1] => "a" | |
163 | @_ [2] => "b" | |
164 | @_ [3] => "c" | |
165 | ||
166 | Before terminating, the C<import> function must explicitly install the | |
167 | filter by calling C<filter_add>. | |
168 | ||
169 | B<filter_add()> | |
170 | ||
171 | The function, C<filter_add>, actually installs the filter. It takes one | |
172 | parameter which should be a reference. The kind of reference used will | |
173 | dictate which of the two filter types will be used. | |
174 | ||
175 | If a CODE reference is used then a I<closure filter> will be assumed. | |
176 | ||
177 | If a CODE reference is not used, a I<method filter> will be assumed. | |
178 | In a I<method filter>, the reference can be used to store context | |
179 | information. The reference will be I<blessed> into the package by | |
180 | C<filter_add>. | |
181 | ||
182 | See the filters at the end of this documents for examples of using | |
183 | context information using both I<method filters> and I<closure | |
184 | filters>. | |
185 | ||
186 | =head2 B<filter() and anonymous sub> | |
187 | ||
188 | Both the C<filter> method used with a I<method filter> and the | |
189 | anonymous sub used with a I<closure filter> is where the main | |
190 | processing for the filter is done. | |
191 | ||
192 | The big difference between the two types of filter is that the I<method | |
193 | filter> uses the object passed to the method to store any context data, | |
194 | whereas the I<closure filter> uses the lexical variables that are | |
195 | maintained by the closure. | |
196 | ||
197 | Note that the single parameter passed to the I<method filter>, | |
198 | C<$self>, is the same reference that was passed to C<filter_add> | |
199 | blessed into the filter's package. See the example filters later on for | |
200 | details of using C<$self>. | |
201 | ||
202 | Here is a list of the common features of the anonymous sub and the | |
203 | C<filter()> method. | |
204 | ||
205 | =over 5 | |
206 | ||
207 | =item B<$_> | |
208 | ||
209 | Although C<$_> doesn't actually appear explicitly in the sample filters | |
210 | above, it is implicitly used in a number of places. | |
211 | ||
212 | Firstly, when either C<filter> or the anonymous sub are called, a local | |
213 | copy of C<$_> will automatically be created. It will always contain the | |
214 | empty string at this point. | |
215 | ||
216 | Next, both C<filter_read> and C<filter_read_exact> will append any | |
217 | source data that is read to the end of C<$_>. | |
218 | ||
219 | Finally, when C<filter> or the anonymous sub are finished processing, | |
220 | they are expected to return the filtered source using C<$_>. | |
221 | ||
222 | This implicit use of C<$_> greatly simplifies the filter. | |
223 | ||
224 | =item B<$status> | |
225 | ||
226 | The status value that is returned by the user's C<filter> method or | |
227 | anonymous sub and the C<filter_read> and C<read_exact> functions take | |
228 | the same set of values, namely: | |
229 | ||
230 | < 0 Error | |
231 | = 0 EOF | |
232 | > 0 OK | |
233 | ||
234 | =item B<filter_read> and B<filter_read_exact> | |
235 | ||
236 | These functions are used by the filter to obtain either a line or block | |
237 | from the next filter in the chain or the actual source file if there | |
238 | aren't any other filters. | |
239 | ||
240 | The function C<filter_read> takes two forms: | |
241 | ||
242 | $status = filter_read() ; | |
243 | $status = filter_read($size) ; | |
244 | ||
245 | The first form is used to request a I<line>, the second requests a | |
246 | I<block>. | |
247 | ||
248 | In line mode, C<filter_read> will append the next source line to the | |
249 | end of the C<$_> scalar. | |
250 | ||
251 | In block mode, C<filter_read> will append a block of data which is <= | |
252 | C<$size> to the end of the C<$_> scalar. It is important to emphasise | |
253 | the that C<filter_read> will not necessarily read a block which is | |
254 | I<precisely> C<$size> bytes. | |
255 | ||
256 | If you need to be able to read a block which has an exact size, you can | |
257 | use the function C<filter_read_exact>. It works identically to | |
258 | C<filter_read> in block mode, except it will try to read a block which | |
259 | is exactly C<$size> bytes in length. The only circumstances when it | |
260 | will not return a block which is C<$size> bytes long is on EOF or | |
261 | error. | |
262 | ||
263 | It is I<very> important to check the value of C<$status> after I<every> | |
264 | call to C<filter_read> or C<filter_read_exact>. | |
265 | ||
266 | =item B<filter_del> | |
267 | ||
268 | The function, C<filter_del>, is used to disable the current filter. It | |
269 | does not affect the running of the filter. All it does is tell Perl not | |
270 | to call filter any more. | |
271 | ||
272 | See L<Example 4: Using filter_del> for details. | |
273 | ||
274 | =back | |
275 | ||
276 | =head1 EXAMPLES | |
277 | ||
278 | Here are a few examples which illustrate the key concepts - as such | |
279 | most of them are of little practical use. | |
280 | ||
281 | The C<examples> sub-directory has copies of all these filters | |
282 | implemented both as I<method filters> and as I<closure filters>. | |
283 | ||
284 | =head2 Example 1: A simple filter. | |
285 | ||
286 | Below is a I<method filter> which is hard-wired to replace all | |
287 | occurrences of the string C<"Joe"> to C<"Jim">. Not particularly | |
288 | Useful, but it is the first example and I wanted to keep it simple. | |
289 | ||
290 | package Joe2Jim ; | |
291 | ||
292 | use Filter::Util::Call ; | |
293 | ||
294 | sub import | |
295 | { | |
296 | my($type) = @_ ; | |
297 | ||
298 | filter_add(bless []) ; | |
299 | } | |
300 | ||
301 | sub filter | |
302 | { | |
303 | my($self) = @_ ; | |
304 | my($status) ; | |
305 | ||
306 | s/Joe/Jim/g | |
307 | if ($status = filter_read()) > 0 ; | |
308 | $status ; | |
309 | } | |
310 | ||
311 | 1 ; | |
312 | ||
313 | Here is an example of using the filter: | |
314 | ||
315 | use Joe2Jim ; | |
316 | print "Where is Joe?\n" ; | |
317 | ||
318 | And this is what the script above will print: | |
319 | ||
320 | Where is Jim? | |
321 | ||
322 | =head2 Example 2: Using the context | |
323 | ||
324 | The previous example was not particularly useful. To make it more | |
325 | general purpose we will make use of the context data and allow any | |
326 | arbitrary I<from> and I<to> strings to be used. This time we will use a | |
327 | I<closure filter>. To reflect its enhanced role, the filter is called | |
328 | C<Subst>. | |
329 | ||
330 | package Subst ; | |
331 | ||
332 | use Filter::Util::Call ; | |
333 | use Carp ; | |
334 | ||
335 | sub import | |
336 | { | |
337 | croak("usage: use Subst qw(from to)") | |
338 | unless @_ == 3 ; | |
339 | my ($self, $from, $to) = @_ ; | |
340 | filter_add( | |
341 | sub | |
342 | { | |
343 | my ($status) ; | |
344 | s/$from/$to/ | |
345 | if ($status = filter_read()) > 0 ; | |
346 | $status ; | |
347 | }) | |
348 | } | |
349 | 1 ; | |
350 | ||
351 | and is used like this: | |
352 | ||
353 | use Subst qw(Joe Jim) ; | |
354 | print "Where is Joe?\n" ; | |
355 | ||
356 | ||
357 | =head2 Example 3: Using the context within the filter | |
358 | ||
359 | Here is a filter which a variation of the C<Joe2Jim> filter. As well as | |
360 | substituting all occurrences of C<"Joe"> to C<"Jim"> it keeps a count | |
361 | of the number of substitutions made in the context object. | |
362 | ||
363 | Once EOF is detected (C<$status> is zero) the filter will insert an | |
364 | extra line into the source stream. When this extra line is executed it | |
365 | will print a count of the number of substitutions actually made. | |
366 | Note that C<$status> is set to C<1> in this case. | |
367 | ||
368 | package Count ; | |
369 | ||
370 | use Filter::Util::Call ; | |
371 | ||
372 | sub filter | |
373 | { | |
374 | my ($self) = @_ ; | |
375 | my ($status) ; | |
376 | ||
377 | if (($status = filter_read()) > 0 ) { | |
378 | s/Joe/Jim/g ; | |
379 | ++ $$self ; | |
380 | } | |
381 | elsif ($$self >= 0) { # EOF | |
382 | $_ = "print q[Made ${$self} substitutions\n]" ; | |
383 | $status = 1 ; | |
384 | $$self = -1 ; | |
385 | } | |
386 | ||
387 | $status ; | |
388 | } | |
389 | ||
390 | sub import | |
391 | { | |
392 | my ($self) = @_ ; | |
393 | my ($count) = 0 ; | |
394 | filter_add(\$count) ; | |
395 | } | |
396 | ||
397 | 1 ; | |
398 | ||
399 | Here is a script which uses it: | |
400 | ||
401 | use Count ; | |
402 | print "Hello Joe\n" ; | |
403 | print "Where is Joe\n" ; | |
404 | ||
405 | Outputs: | |
406 | ||
407 | Hello Jim | |
408 | Where is Jim | |
409 | Made 2 substitutions | |
410 | ||
411 | =head2 Example 4: Using filter_del | |
412 | ||
413 | Another variation on a theme. This time we will modify the C<Subst> | |
414 | filter to allow a starting and stopping pattern to be specified as well | |
415 | as the I<from> and I<to> patterns. If you know the I<vi> editor, it is | |
416 | the equivalent of this command: | |
417 | ||
418 | :/start/,/stop/s/from/to/ | |
419 | ||
420 | When used as a filter we want to invoke it like this: | |
421 | ||
422 | use NewSubst qw(start stop from to) ; | |
423 | ||
424 | Here is the module. | |
425 | ||
426 | package NewSubst ; | |
427 | ||
428 | use Filter::Util::Call ; | |
429 | use Carp ; | |
430 | ||
431 | sub import | |
432 | { | |
433 | my ($self, $start, $stop, $from, $to) = @_ ; | |
434 | my ($found) = 0 ; | |
435 | croak("usage: use Subst qw(start stop from to)") | |
436 | unless @_ == 5 ; | |
437 | ||
438 | filter_add( | |
439 | sub | |
440 | { | |
441 | my ($status) ; | |
442 | ||
443 | if (($status = filter_read()) > 0) { | |
444 | ||
445 | $found = 1 | |
446 | if $found == 0 and /$start/ ; | |
447 | ||
448 | if ($found) { | |
449 | s/$from/$to/ ; | |
450 | filter_del() if /$stop/ ; | |
451 | } | |
452 | ||
453 | } | |
454 | $status ; | |
455 | } ) | |
456 | ||
457 | } | |
458 | ||
459 | 1 ; | |
460 | ||
461 | =head1 AUTHOR | |
462 | ||
463 | Paul Marquess | |
464 | ||
465 | =head1 DATE | |
466 | ||
467 | 26th January 1996 | |
468 | ||
469 | =cut | |
470 |