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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
121 parameter which should be a reference. The kind of reference used will
122 dictate which of the two filter types will be used.
123 If a CODE reference is used then a closure filter will be assumed.
125 If a CODE reference is not used, a method filter will be assumed. In a
127 method filter, the reference can be used to store context information.
128 The reference will be blessed into the package by "filter_add".
129 See the filters at the end of this documents for examples of using
131 context information using both method filters and closure filters.
132 filter() and anonymous sub
134 Both the "filter" method used with a method filter and the anonymous
135 sub used with a closure filter is where the main processing for the
136 filter is done.
137 The big difference between the two types of filter is that the method
139 filter uses the object passed to the method to store any context data,
140 whereas the closure filter uses the lexical variables that are
141 maintained by the closure.
142 Note that the single parameter passed to the method filter, $self, is
144 the same reference that was passed to "filter_add" blessed into the
145 filter's package. See the example filters later on for details of using
146 $self.
147 Here is a list of the common features of the anonymous sub and the
149 "filter()" method.
150 $_ Although $_ doesn't actually appear explicitly in the sample
152 filters above, it is implicitly used in a number of places.
153 Firstly, when either "filter" or the anonymous sub are called, a
155 local copy of $_ will automatically be created. It will always
156 contain the empty string at this point.
157 Next, both "filter_read" and "filter_read_exact" will append any
159 source data that is read to the end of $_.
160 Finally, when "filter" or the anonymous sub are finished
162 processing, they are expected to return the filtered source using
163 $_.
164 This implicit use of $_ greatly simplifies the filter.
166 $status
168 The status value that is returned by the user's "filter" method or
169 anonymous sub and the "filter_read" and "read_exact" functions
170 take the same set of values, namely:
171 < 0 Error
173 = 0 EOF
174 > 0 OK
175 filter_read and filter_read_exact
177 These functions are used by the filter to obtain either a line or
178 block from the next filter in the chain or the actual source file
179 if there aren't any other filters.
180 The function "filter_read" takes two forms:
182 $status = filter_read() ;
184 $status = filter_read($size) ;
185 The first form is used to request a line, the second requests a
187 block.
188 In line mode, "filter_read" will append the next source line to
190 the end of the $_ scalar.
191 In block mode, "filter_read" will append a block of data which is
193 <= $size to the end of the $_ scalar. It is important to emphasise
194 the that "filter_read" will not necessarily read a block which is
195 precisely $size bytes.
196 If you need to be able to read a block which has an exact size,
198 you can use the function "filter_read_exact". It works identically
199 to "filter_read" in block mode, except it will try to read a block
200 which is exactly $size bytes in length. The only circumstances
201 when it will not return a block which is $size bytes long is on
202 EOF or error.
203 It is very important to check the value of $status after every
205 call to "filter_read" or "filter_read_exact".
206 filter_del
208 The function, "filter_del", is used to disable the current filter.
209 It does not affect the running of the filter. All it does is tell
210 Perl not to call filter any more.
211 See "Example 4: Using filter_del" for details.
213 real_import
215 Internal function which adds the filter, based on the filter_add
216 argument type.
217
219 Here are a few examples which illustrate the key concepts - as such
220 most of them are of little practical use.
221 The "examples" sub-directory has copies of all these filters
223 implemented both as method filters and as closure filters.
224 Example 1: A simple filter.
226 Below is a method filter which is hard-wired to replace all occurrences
227 of the string "Joe" to "Jim". Not particularly Useful, but it is the
228 first example and I wanted to keep it simple.
229 package Joe2Jim ;
231 use Filter::Util::Call ;
233 sub import
235 {
236 my($type) = @_ ;
237 filter_add(bless []) ;
239 }
240 sub filter
242 {
243 my($self) = @_ ;
244 my($status) ;
245 s/Joe/Jim/g
247 if ($status = filter_read()) > 0 ;
248 $status ;
249 }
250 1 ;
252 Here is an example of using the filter:
254 use Joe2Jim ;
256 print "Where is Joe?\n" ;
257 And this is what the script above will print:
259 Where is Jim?
261 Example 2: Using the context
263 The previous example was not particularly useful. To make it more
264 general purpose we will make use of the context data and allow any
265 arbitrary from and to strings to be used. This time we will use a
266 closure filter. To reflect its enhanced role, the filter is called
267 "Subst".
268 package Subst ;
270 use Filter::Util::Call ;
272 use Carp ;
273 sub import
275 {
276 croak("usage: use Subst qw(from to)")
277 unless @_ == 3 ;
278 my ($self, $from, $to) = @_ ;
279 filter_add(
280 sub
281 {
282 my ($status) ;
283 s/$from/$to/
284 if ($status = filter_read()) > 0 ;
285 $status ;
286 })
287 }
288 1 ;
289 and is used like this:
291 use Subst qw(Joe Jim) ;
293 print "Where is Joe?\n" ;
294 Example 3: Using the context within the filter
296 Here is a filter which a variation of the "Joe2Jim" filter. As well as
297 substituting all occurrences of "Joe" to "Jim" it keeps a count of the
298 number of substitutions made in the context object.
299 Once EOF is detected ($status is zero) the filter will insert an extra
301 line into the source stream. When this extra line is executed it will
302 print a count of the number of substitutions actually made. Note that
303 $status is set to 1 in this case.
304 package Count ;
306 use Filter::Util::Call ;
308 sub filter
310 {
311 my ($self) = @_ ;
312 my ($status) ;
313 if (($status = filter_read()) > 0 ) {
315 s/Joe/Jim/g ;
316 ++ $$self ;
317 }
318 elsif ($$self >= 0) { # EOF
319 $_ = "print q[Made ${$self} substitutions\n]" ;
320 $status = 1 ;
321 $$self = -1 ;
322 }
323 $status ;
325 }
326 sub import
328 {
329 my ($self) = @_ ;
330 my ($count) = 0 ;
331 filter_add(\$count) ;
332 }
333 1 ;
335 Here is a script which uses it:
337 use Count ;
339 print "Hello Joe\n" ;
340 print "Where is Joe\n" ;
341 Outputs:
343 Hello Jim
345 Where is Jim
346 Made 2 substitutions
347 Example 4: Using filter_del
349 Another variation on a theme. This time we will modify the "Subst"
350 filter to allow a starting and stopping pattern to be specified as well
351 as the from and to patterns. If you know the vi editor, it is the
352 equivalent of this command:
353 :/start/,/stop/s/from/to/
355 When used as a filter we want to invoke it like this:
357 use NewSubst qw(start stop from to) ;
359 Here is the module.
361 package NewSubst ;
363 use Filter::Util::Call ;
365 use Carp ;
366 sub import
368 {
369 my ($self, $start, $stop, $from, $to) = @_ ;
370 my ($found) = 0 ;
371 croak("usage: use Subst qw(start stop from to)")
372 unless @_ == 5 ;
373 filter_add(
375 sub
376 {
377 my ($status) ;
378 if (($status = filter_read()) > 0) {
380 $found = 1
382 if $found == 0 and /$start/ ;
383 if ($found) {
385 s/$from/$to/ ;
386 filter_del() if /$stop/ ;
387 }
388 }
390 $status ;
391 } )
392 }
394 1 ;
396
398 If you intend using the Filter::Call functionality, I would strongly
399 recommend that you check out Damian Conway's excellent Filter::Simple
400 module. Damian's module provides a much cleaner interface than
401 Filter::Util::Call. Although it doesn't allow the fine control that
402 Filter::Util::Call does, it should be adequate for the majority of
403 applications. It's available at
404 http://search.cpan.org/dist/Filter-Simple/
406
408 Paul Marquess
409
411 26th January 1996
412perl v5.16.3 2013-04-02 Call(3)