1Filter::Util::Call(3pm)Perl Programmers Reference GuideFilter::Util::Call(3pm)
2
6 Filter::Util::Call - Perl Source Filter Utility Module
7
9 use Filter::Util::Call ;
10
12 This module provides you with the framework to write Source Filters in
13 Perl.
14 An alternate interface to Filter::Util::Call is now available. See
16 Filter::Simple for more details.
17 A Perl Source Filter is implemented as a Perl module. The structure of
19 the module can take one of two broadly similar formats. To distinguish
20 between them, the first will be referred to as method filter and the
21 second as closure filter.
22 Here is a skeleton for the method filter:
24 package MyFilter ;
26 use Filter::Util::Call ;
28 sub import
30 {
31 my($type, @arguments) = @_ ;
32 filter_add([]) ;
33 }
34 sub filter
36 {
37 my($self) = @_ ;
38 my($status) ;
39 $status = filter_read() ;
41 $status ;
42 }
43 1 ;
45 and this is the equivalent skeleton for the closure filter:
47 package MyFilter ;
49 use Filter::Util::Call ;
51 sub import
53 {
54 my($type, @arguments) = @_ ;
55 filter_add(
57 sub
58 {
59 my($status) ;
60 $status = filter_read() ;
61 $status ;
62 } )
63 }
64 1 ;
66 To make use of either of the two filter modules above, place the line
68 below in a Perl source file.
69 use MyFilter;
71 In fact, the skeleton modules shown above are fully functional Source
73 Filters, albeit fairly useless ones. All they does is filter the source
74 stream without modifying it at all.
75 As you can see both modules have a broadly similar structure. They both
77 make use of the "Filter::Util::Call" module and both have an "import"
78 method. The difference between them is that the method filter requires
79 a filter method, whereas the closure filter gets the equivalent of a
80 filter method with the anonymous sub passed to filter_add.
81 To make proper use of the closure filter shown above you need to have a
83 good understanding of the concept of a closure. See perlref for more
84 details on the mechanics of closures.
85 use Filter::Util::Call
87 The following functions are exported by "Filter::Util::Call":
88 filter_add()
90 filter_read()
91 filter_read_exact()
92 filter_del()
93 import()
95 The "import" method is used to create an instance of the filter. It is
96 called indirectly by Perl when it encounters the "use MyFilter" line in
97 a source file (See "import" in perlfunc for more details on "import").
98 It will always have at least one parameter automatically passed by Perl
100 - this corresponds to the name of the package. In the example above it
101 will be "MyFilter".
102 Apart from the first parameter, import can accept an optional list of
104 parameters. These can be used to pass parameters to the filter. For
105 example:
106 use MyFilter qw(a b c) ;
108 will result in the @_ array having the following values:
110 @_ [0] => "MyFilter"
112 @_ [1] => "a"
113 @_ [2] => "b"
114 @_ [3] => "c"
115 Before terminating, the "import" function must explicitly install the
117 filter by calling "filter_add".
118 filter_add()
120 The function, "filter_add", actually installs the filter. It takes one
122 parameter which should be a reference. The kind of reference used will
123 dictate which of the two filter types will be used.
124 If a CODE reference is used then a closure filter will be assumed.
126 If a CODE reference is not used, a method filter will be assumed. In a
128 method filter, the reference can be used to store context information.
129 The reference will be blessed into the package by "filter_add".
130 See the filters at the end of this documents for examples of using
132 context information using both method filters and closure filters.
133 filter() and anonymous sub
135 Both the "filter" method used with a method filter and the anonymous
136 sub used with a closure filter is where the main processing for the
137 filter is done.
138 The big difference between the two types of filter is that the method
140 filter uses the object passed to the method to store any context data,
141 whereas the closure filter uses the lexical variables that are
142 maintained by the closure.
143 Note that the single parameter passed to the method filter, $self, is
145 the same reference that was passed to "filter_add" blessed into the
146 filter's package. See the example filters later on for details of using
147 $self.
148 Here is a list of the common features of the anonymous sub and the
150 "filter()" method.
151 $_ Although $_ doesn't actually appear explicitly in the sample
153 filters above, it is implicitly used in a number of places.
154 Firstly, when either "filter" or the anonymous sub are called, a
156 local copy of $_ will automatically be created. It will always
157 contain the empty string at this point.
158 Next, both "filter_read" and "filter_read_exact" will append any
160 source data that is read to the end of $_.
161 Finally, when "filter" or the anonymous sub are finished
163 processing, they are expected to return the filtered source using
164 $_.
165 This implicit use of $_ greatly simplifies the filter.
167 $status
169 The status value that is returned by the user's "filter" method or
170 anonymous sub and the "filter_read" and "read_exact" functions
171 take the same set of values, namely:
172 < 0 Error
174 = 0 EOF
175 > 0 OK
176 filter_read and filter_read_exact
178 These functions are used by the filter to obtain either a line or
179 block from the next filter in the chain or the actual source file
180 if there aren't any other filters.
181 The function "filter_read" takes two forms:
183 $status = filter_read() ;
185 $status = filter_read($size) ;
186 The first form is used to request a line, the second requests a
188 block.
189 In line mode, "filter_read" will append the next source line to
191 the end of the $_ scalar.
192 In block mode, "filter_read" will append a block of data which is
194 <= $size to the end of the $_ scalar. It is important to emphasise
195 the that "filter_read" will not necessarily read a block which is
196 precisely $size bytes.
197 If you need to be able to read a block which has an exact size,
199 you can use the function "filter_read_exact". It works identically
200 to "filter_read" in block mode, except it will try to read a block
201 which is exactly $size bytes in length. The only circumstances
202 when it will not return a block which is $size bytes long is on
203 EOF or error.
204 It is very important to check the value of $status after every
206 call to "filter_read" or "filter_read_exact".
207 filter_del
209 The function, "filter_del", is used to disable the current filter.
210 It does not affect the running of the filter. All it does is tell
211 Perl not to call filter any more.
212 See "Example 4: Using filter_del" for details.
214
216 Here are a few examples which illustrate the key concepts - as such
217 most of them are of little practical use.
218 The "examples" sub-directory has copies of all these filters
220 implemented both as method filters and as closure filters.
221 Example 1: A simple filter.
223 Below is a method filter which is hard-wired to replace all occurrences
224 of the string "Joe" to "Jim". Not particularly Useful, but it is the
225 first example and I wanted to keep it simple.
226 package Joe2Jim ;
228 use Filter::Util::Call ;
230 sub import
232 {
233 my($type) = @_ ;
234 filter_add(bless []) ;
236 }
237 sub filter
239 {
240 my($self) = @_ ;
241 my($status) ;
242 s/Joe/Jim/g
244 if ($status = filter_read()) > 0 ;
245 $status ;
246 }
247 1 ;
249 Here is an example of using the filter:
251 use Joe2Jim ;
253 print "Where is Joe?\n" ;
254 And this is what the script above will print:
256 Where is Jim?
258 Example 2: Using the context
260 The previous example was not particularly useful. To make it more
261 general purpose we will make use of the context data and allow any
262 arbitrary from and to strings to be used. This time we will use a
263 closure filter. To reflect its enhanced role, the filter is called
264 "Subst".
265 package Subst ;
267 use Filter::Util::Call ;
269 use Carp ;
270 sub import
272 {
273 croak("usage: use Subst qw(from to)")
274 unless @_ == 3 ;
275 my ($self, $from, $to) = @_ ;
276 filter_add(
277 sub
278 {
279 my ($status) ;
280 s/$from/$to/
281 if ($status = filter_read()) > 0 ;
282 $status ;
283 })
284 }
285 1 ;
286 and is used like this:
288 use Subst qw(Joe Jim) ;
290 print "Where is Joe?\n" ;
291 Example 3: Using the context within the filter
293 Here is a filter which a variation of the "Joe2Jim" filter. As well as
294 substituting all occurrences of "Joe" to "Jim" it keeps a count of the
295 number of substitutions made in the context object.
296 Once EOF is detected ($status is zero) the filter will insert an extra
298 line into the source stream. When this extra line is executed it will
299 print a count of the number of substitutions actually made. Note that
300 $status is set to 1 in this case.
301 package Count ;
303 use Filter::Util::Call ;
305 sub filter
307 {
308 my ($self) = @_ ;
309 my ($status) ;
310 if (($status = filter_read()) > 0 ) {
312 s/Joe/Jim/g ;
313 ++ $$self ;
314 }
315 elsif ($$self >= 0) { # EOF
316 $_ = "print q[Made ${$self} substitutions\n]" ;
317 $status = 1 ;
318 $$self = -1 ;
319 }
320 $status ;
322 }
323 sub import
325 {
326 my ($self) = @_ ;
327 my ($count) = 0 ;
328 filter_add(\$count) ;
329 }
330 1 ;
332 Here is a script which uses it:
334 use Count ;
336 print "Hello Joe\n" ;
337 print "Where is Joe\n" ;
338 Outputs:
340 Hello Jim
342 Where is Jim
343 Made 2 substitutions
344 Example 4: Using filter_del
346 Another variation on a theme. This time we will modify the "Subst"
347 filter to allow a starting and stopping pattern to be specified as well
348 as the from and to patterns. If you know the vi editor, it is the
349 equivalent of this command:
350 :/start/,/stop/s/from/to/
352 When used as a filter we want to invoke it like this:
354 use NewSubst qw(start stop from to) ;
356 Here is the module.
358 package NewSubst ;
360 use Filter::Util::Call ;
362 use Carp ;
363 sub import
365 {
366 my ($self, $start, $stop, $from, $to) = @_ ;
367 my ($found) = 0 ;
368 croak("usage: use Subst qw(start stop from to)")
369 unless @_ == 5 ;
370 filter_add(
372 sub
373 {
374 my ($status) ;
375 if (($status = filter_read()) > 0) {
377 $found = 1
379 if $found == 0 and /$start/ ;
380 if ($found) {
382 s/$from/$to/ ;
383 filter_del() if /$stop/ ;
384 }
385 }
387 $status ;
388 } )
389 }
391 1 ;
393
395 If you intend using the Filter::Call functionality, I would strongly
396 recommend that you check out Damian Conway's excellent Filter::Simple
397 module. Damian's module provides a much cleaner interface than
398 Filter::Util::Call. Although it doesn't allow the fine control that
399 Filter::Util::Call does, it should be adequate for the majority of
400 applications. It's available at
401 http://www.cpan.org/modules/by-author/Damian_Conway/Filter-Simple.tar.gz
403 http://www.csse.monash.edu.au/~damian/CPAN/Filter-Simple.tar.gz
404
406 Paul Marquess
407
409 26th January 1996
410perl v5.10.1 2009-04-15 Filter::Util::Call(3pm)