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| 128 | .rm #[ #] #H #V #F C |
| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "Filter::Util::Call 3" |
| 132 | .TH Filter::Util::Call 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | Filter::Util::Call \- Perl Source Filter Utility Module |
| 135 | .SH "SYNOPSIS" |
| 136 | .IX Header "SYNOPSIS" |
| 137 | .Vb 1 |
| 138 | \& use Filter::Util::Call ; |
| 139 | .Ve |
| 140 | .SH "DESCRIPTION" |
| 141 | .IX Header "DESCRIPTION" |
| 142 | This module provides you with the framework to write \fISource Filters\fR |
| 143 | in Perl. |
| 144 | .PP |
| 145 | An alternate interface to Filter::Util::Call is now available. See |
| 146 | Filter::Simple for more details. |
| 147 | .PP |
| 148 | A \fIPerl Source Filter\fR is implemented as a Perl module. The structure |
| 149 | of the module can take one of two broadly similar formats. To |
| 150 | distinguish between them, the first will be referred to as \fImethod |
| 151 | filter\fR and the second as \fIclosure filter\fR. |
| 152 | .PP |
| 153 | Here is a skeleton for the \fImethod filter\fR: |
| 154 | .PP |
| 155 | .Vb 1 |
| 156 | \& package MyFilter ; |
| 157 | .Ve |
| 158 | .PP |
| 159 | .Vb 1 |
| 160 | \& use Filter::Util::Call ; |
| 161 | .Ve |
| 162 | .PP |
| 163 | .Vb 5 |
| 164 | \& sub import |
| 165 | \& { |
| 166 | \& my($type, @arguments) = @_ ; |
| 167 | \& filter_add([]) ; |
| 168 | \& } |
| 169 | .Ve |
| 170 | .PP |
| 171 | .Vb 4 |
| 172 | \& sub filter |
| 173 | \& { |
| 174 | \& my($self) = @_ ; |
| 175 | \& my($status) ; |
| 176 | .Ve |
| 177 | .PP |
| 178 | .Vb 3 |
| 179 | \& $status = filter_read() ; |
| 180 | \& $status ; |
| 181 | \& } |
| 182 | .Ve |
| 183 | .PP |
| 184 | .Vb 1 |
| 185 | \& 1 ; |
| 186 | .Ve |
| 187 | .PP |
| 188 | and this is the equivalent skeleton for the \fIclosure filter\fR: |
| 189 | .PP |
| 190 | .Vb 1 |
| 191 | \& package MyFilter ; |
| 192 | .Ve |
| 193 | .PP |
| 194 | .Vb 1 |
| 195 | \& use Filter::Util::Call ; |
| 196 | .Ve |
| 197 | .PP |
| 198 | .Vb 3 |
| 199 | \& sub import |
| 200 | \& { |
| 201 | \& my($type, @arguments) = @_ ; |
| 202 | .Ve |
| 203 | .PP |
| 204 | .Vb 8 |
| 205 | \& filter_add( |
| 206 | \& sub |
| 207 | \& { |
| 208 | \& my($status) ; |
| 209 | \& $status = filter_read() ; |
| 210 | \& $status ; |
| 211 | \& } ) |
| 212 | \& } |
| 213 | .Ve |
| 214 | .PP |
| 215 | .Vb 1 |
| 216 | \& 1 ; |
| 217 | .Ve |
| 218 | .PP |
| 219 | To make use of either of the two filter modules above, place the line |
| 220 | below in a Perl source file. |
| 221 | .PP |
| 222 | .Vb 1 |
| 223 | \& use MyFilter; |
| 224 | .Ve |
| 225 | .PP |
| 226 | In fact, the skeleton modules shown above are fully functional \fISource |
| 227 | Filters\fR, albeit fairly useless ones. All they does is filter the |
| 228 | source stream without modifying it at all. |
| 229 | .PP |
| 230 | As you can see both modules have a broadly similar structure. They both |
| 231 | make use of the \f(CW\*(C`Filter::Util::Call\*(C'\fR module and both have an \f(CW\*(C`import\*(C'\fR |
| 232 | method. The difference between them is that the \fImethod filter\fR |
| 233 | requires a \fIfilter\fR method, whereas the \fIclosure filter\fR gets the |
| 234 | equivalent of a \fIfilter\fR method with the anonymous sub passed to |
| 235 | \&\fIfilter_add\fR. |
| 236 | .PP |
| 237 | To make proper use of the \fIclosure filter\fR shown above you need to |
| 238 | have a good understanding of the concept of a \fIclosure\fR. See |
| 239 | perlref for more details on the mechanics of \fIclosures\fR. |
| 240 | .Sh "\fBuse Filter::Util::Call\fP" |
| 241 | .IX Subsection "use Filter::Util::Call" |
| 242 | The following functions are exported by \f(CW\*(C`Filter::Util::Call\*(C'\fR: |
| 243 | .PP |
| 244 | .Vb 4 |
| 245 | \& filter_add() |
| 246 | \& filter_read() |
| 247 | \& filter_read_exact() |
| 248 | \& filter_del() |
| 249 | .Ve |
| 250 | .Sh "\fB\fP\f(BIimport()\fP\fB\fP" |
| 251 | .IX Subsection "import()" |
| 252 | The \f(CW\*(C`import\*(C'\fR method is used to create an instance of the filter. It is |
| 253 | called indirectly by Perl when it encounters the \f(CW\*(C`use MyFilter\*(C'\fR line |
| 254 | in a source file (See \*(L"import\*(R" in perlfunc for more details on |
| 255 | \&\f(CW\*(C`import\*(C'\fR). |
| 256 | .PP |
| 257 | It will always have at least one parameter automatically passed by Perl |
| 258 | \&\- this corresponds to the name of the package. In the example above it |
| 259 | will be \f(CW"MyFilter"\fR. |
| 260 | .PP |
| 261 | Apart from the first parameter, import can accept an optional list of |
| 262 | parameters. These can be used to pass parameters to the filter. For |
| 263 | example: |
| 264 | .PP |
| 265 | .Vb 1 |
| 266 | \& use MyFilter qw(a b c) ; |
| 267 | .Ve |
| 268 | .PP |
| 269 | will result in the \f(CW@_\fR array having the following values: |
| 270 | .PP |
| 271 | .Vb 4 |
| 272 | \& @_ [0] => "MyFilter" |
| 273 | \& @_ [1] => "a" |
| 274 | \& @_ [2] => "b" |
| 275 | \& @_ [3] => "c" |
| 276 | .Ve |
| 277 | .PP |
| 278 | Before terminating, the \f(CW\*(C`import\*(C'\fR function must explicitly install the |
| 279 | filter by calling \f(CW\*(C`filter_add\*(C'\fR. |
| 280 | .PP |
| 281 | \&\fB\f(BIfilter_add()\fB\fR |
| 282 | .PP |
| 283 | The function, \f(CW\*(C`filter_add\*(C'\fR, actually installs the filter. It takes one |
| 284 | parameter which should be a reference. The kind of reference used will |
| 285 | dictate which of the two filter types will be used. |
| 286 | .PP |
| 287 | If a \s-1CODE\s0 reference is used then a \fIclosure filter\fR will be assumed. |
| 288 | .PP |
| 289 | If a \s-1CODE\s0 reference is not used, a \fImethod filter\fR will be assumed. |
| 290 | In a \fImethod filter\fR, the reference can be used to store context |
| 291 | information. The reference will be \fIblessed\fR into the package by |
| 292 | \&\f(CW\*(C`filter_add\*(C'\fR. |
| 293 | .PP |
| 294 | See the filters at the end of this documents for examples of using |
| 295 | context information using both \fImethod filters\fR and \fIclosure |
| 296 | filters\fR. |
| 297 | .Sh "\fB\fP\f(BIfilter()\fP\fB and anonymous sub\fP" |
| 298 | .IX Subsection "filter() and anonymous sub" |
| 299 | Both the \f(CW\*(C`filter\*(C'\fR method used with a \fImethod filter\fR and the |
| 300 | anonymous sub used with a \fIclosure filter\fR is where the main |
| 301 | processing for the filter is done. |
| 302 | .PP |
| 303 | The big difference between the two types of filter is that the \fImethod |
| 304 | filter\fR uses the object passed to the method to store any context data, |
| 305 | whereas the \fIclosure filter\fR uses the lexical variables that are |
| 306 | maintained by the closure. |
| 307 | .PP |
| 308 | Note that the single parameter passed to the \fImethod filter\fR, |
| 309 | \&\f(CW$self\fR, is the same reference that was passed to \f(CW\*(C`filter_add\*(C'\fR |
| 310 | blessed into the filter's package. See the example filters later on for |
| 311 | details of using \f(CW$self\fR. |
| 312 | .PP |
| 313 | Here is a list of the common features of the anonymous sub and the |
| 314 | \&\f(CW\*(C`filter()\*(C'\fR method. |
| 315 | .IP "\fB$_\fR" 5 |
| 316 | .IX Item "$_" |
| 317 | Although \f(CW$_\fR doesn't actually appear explicitly in the sample filters |
| 318 | above, it is implicitly used in a number of places. |
| 319 | .Sp |
| 320 | Firstly, when either \f(CW\*(C`filter\*(C'\fR or the anonymous sub are called, a local |
| 321 | copy of \f(CW$_\fR will automatically be created. It will always contain the |
| 322 | empty string at this point. |
| 323 | .Sp |
| 324 | Next, both \f(CW\*(C`filter_read\*(C'\fR and \f(CW\*(C`filter_read_exact\*(C'\fR will append any |
| 325 | source data that is read to the end of \f(CW$_\fR. |
| 326 | .Sp |
| 327 | Finally, when \f(CW\*(C`filter\*(C'\fR or the anonymous sub are finished processing, |
| 328 | they are expected to return the filtered source using \f(CW$_\fR. |
| 329 | .Sp |
| 330 | This implicit use of \f(CW$_\fR greatly simplifies the filter. |
| 331 | .IP "\fB$status\fR" 5 |
| 332 | .IX Item "$status" |
| 333 | The status value that is returned by the user's \f(CW\*(C`filter\*(C'\fR method or |
| 334 | anonymous sub and the \f(CW\*(C`filter_read\*(C'\fR and \f(CW\*(C`read_exact\*(C'\fR functions take |
| 335 | the same set of values, namely: |
| 336 | .Sp |
| 337 | .Vb 3 |
| 338 | \& < 0 Error |
| 339 | \& = 0 EOF |
| 340 | \& > 0 OK |
| 341 | .Ve |
| 342 | .IP "\fBfilter_read\fR and \fBfilter_read_exact\fR" 5 |
| 343 | .IX Item "filter_read and filter_read_exact" |
| 344 | These functions are used by the filter to obtain either a line or block |
| 345 | from the next filter in the chain or the actual source file if there |
| 346 | aren't any other filters. |
| 347 | .Sp |
| 348 | The function \f(CW\*(C`filter_read\*(C'\fR takes two forms: |
| 349 | .Sp |
| 350 | .Vb 2 |
| 351 | \& $status = filter_read() ; |
| 352 | \& $status = filter_read($size) ; |
| 353 | .Ve |
| 354 | .Sp |
| 355 | The first form is used to request a \fIline\fR, the second requests a |
| 356 | \&\fIblock\fR. |
| 357 | .Sp |
| 358 | In line mode, \f(CW\*(C`filter_read\*(C'\fR will append the next source line to the |
| 359 | end of the \f(CW$_\fR scalar. |
| 360 | .Sp |
| 361 | In block mode, \f(CW\*(C`filter_read\*(C'\fR will append a block of data which is <= |
| 362 | \&\f(CW$size\fR to the end of the \f(CW$_\fR scalar. It is important to emphasise |
| 363 | the that \f(CW\*(C`filter_read\*(C'\fR will not necessarily read a block which is |
| 364 | \&\fIprecisely\fR \f(CW$size\fR bytes. |
| 365 | .Sp |
| 366 | If you need to be able to read a block which has an exact size, you can |
| 367 | use the function \f(CW\*(C`filter_read_exact\*(C'\fR. It works identically to |
| 368 | \&\f(CW\*(C`filter_read\*(C'\fR in block mode, except it will try to read a block which |
| 369 | is exactly \f(CW$size\fR bytes in length. The only circumstances when it |
| 370 | will not return a block which is \f(CW$size\fR bytes long is on \s-1EOF\s0 or |
| 371 | error. |
| 372 | .Sp |
| 373 | It is \fIvery\fR important to check the value of \f(CW$status\fR after \fIevery\fR |
| 374 | call to \f(CW\*(C`filter_read\*(C'\fR or \f(CW\*(C`filter_read_exact\*(C'\fR. |
| 375 | .IP "\fBfilter_del\fR" 5 |
| 376 | .IX Item "filter_del" |
| 377 | The function, \f(CW\*(C`filter_del\*(C'\fR, is used to disable the current filter. It |
| 378 | does not affect the running of the filter. All it does is tell Perl not |
| 379 | to call filter any more. |
| 380 | .Sp |
| 381 | See \*(L"Example 4: Using filter_del\*(R" for details. |
| 382 | .SH "EXAMPLES" |
| 383 | .IX Header "EXAMPLES" |
| 384 | Here are a few examples which illustrate the key concepts \- as such |
| 385 | most of them are of little practical use. |
| 386 | .PP |
| 387 | The \f(CW\*(C`examples\*(C'\fR sub-directory has copies of all these filters |
| 388 | implemented both as \fImethod filters\fR and as \fIclosure filters\fR. |
| 389 | .Sh "Example 1: A simple filter." |
| 390 | .IX Subsection "Example 1: A simple filter." |
| 391 | Below is a \fImethod filter\fR which is hard-wired to replace all |
| 392 | occurrences of the string \f(CW"Joe"\fR to \f(CW"Jim"\fR. Not particularly |
| 393 | Useful, but it is the first example and I wanted to keep it simple. |
| 394 | .PP |
| 395 | .Vb 1 |
| 396 | \& package Joe2Jim ; |
| 397 | .Ve |
| 398 | .PP |
| 399 | .Vb 1 |
| 400 | \& use Filter::Util::Call ; |
| 401 | .Ve |
| 402 | .PP |
| 403 | .Vb 3 |
| 404 | \& sub import |
| 405 | \& { |
| 406 | \& my($type) = @_ ; |
| 407 | .Ve |
| 408 | .PP |
| 409 | .Vb 2 |
| 410 | \& filter_add(bless []) ; |
| 411 | \& } |
| 412 | .Ve |
| 413 | .PP |
| 414 | .Vb 4 |
| 415 | \& sub filter |
| 416 | \& { |
| 417 | \& my($self) = @_ ; |
| 418 | \& my($status) ; |
| 419 | .Ve |
| 420 | .PP |
| 421 | .Vb 4 |
| 422 | \& s/Joe/Jim/g |
| 423 | \& if ($status = filter_read()) > 0 ; |
| 424 | \& $status ; |
| 425 | \& } |
| 426 | .Ve |
| 427 | .PP |
| 428 | .Vb 1 |
| 429 | \& 1 ; |
| 430 | .Ve |
| 431 | .PP |
| 432 | Here is an example of using the filter: |
| 433 | .PP |
| 434 | .Vb 2 |
| 435 | \& use Joe2Jim ; |
| 436 | \& print "Where is Joe?\en" ; |
| 437 | .Ve |
| 438 | .PP |
| 439 | And this is what the script above will print: |
| 440 | .PP |
| 441 | .Vb 1 |
| 442 | \& Where is Jim? |
| 443 | .Ve |
| 444 | .Sh "Example 2: Using the context" |
| 445 | .IX Subsection "Example 2: Using the context" |
| 446 | The previous example was not particularly useful. To make it more |
| 447 | general purpose we will make use of the context data and allow any |
| 448 | arbitrary \fIfrom\fR and \fIto\fR strings to be used. This time we will use a |
| 449 | \&\fIclosure filter\fR. To reflect its enhanced role, the filter is called |
| 450 | \&\f(CW\*(C`Subst\*(C'\fR. |
| 451 | .PP |
| 452 | .Vb 1 |
| 453 | \& package Subst ; |
| 454 | .Ve |
| 455 | .PP |
| 456 | .Vb 2 |
| 457 | \& use Filter::Util::Call ; |
| 458 | \& use Carp ; |
| 459 | .Ve |
| 460 | .PP |
| 461 | .Vb 15 |
| 462 | \& sub import |
| 463 | \& { |
| 464 | \& croak("usage: use Subst qw(from to)") |
| 465 | \& unless @_ == 3 ; |
| 466 | \& my ($self, $from, $to) = @_ ; |
| 467 | \& filter_add( |
| 468 | \& sub |
| 469 | \& { |
| 470 | \& my ($status) ; |
| 471 | \& s/$from/$to/ |
| 472 | \& if ($status = filter_read()) > 0 ; |
| 473 | \& $status ; |
| 474 | \& }) |
| 475 | \& } |
| 476 | \& 1 ; |
| 477 | .Ve |
| 478 | .PP |
| 479 | and is used like this: |
| 480 | .PP |
| 481 | .Vb 2 |
| 482 | \& use Subst qw(Joe Jim) ; |
| 483 | \& print "Where is Joe?\en" ; |
| 484 | .Ve |
| 485 | .Sh "Example 3: Using the context within the filter" |
| 486 | .IX Subsection "Example 3: Using the context within the filter" |
| 487 | Here is a filter which a variation of the \f(CW\*(C`Joe2Jim\*(C'\fR filter. As well as |
| 488 | substituting all occurrences of \f(CW"Joe"\fR to \f(CW"Jim"\fR it keeps a count |
| 489 | of the number of substitutions made in the context object. |
| 490 | .PP |
| 491 | Once \s-1EOF\s0 is detected (\f(CW$status\fR is zero) the filter will insert an |
| 492 | extra line into the source stream. When this extra line is executed it |
| 493 | will print a count of the number of substitutions actually made. |
| 494 | Note that \f(CW$status\fR is set to \f(CW1\fR in this case. |
| 495 | .PP |
| 496 | .Vb 1 |
| 497 | \& package Count ; |
| 498 | .Ve |
| 499 | .PP |
| 500 | .Vb 1 |
| 501 | \& use Filter::Util::Call ; |
| 502 | .Ve |
| 503 | .PP |
| 504 | .Vb 4 |
| 505 | \& sub filter |
| 506 | \& { |
| 507 | \& my ($self) = @_ ; |
| 508 | \& my ($status) ; |
| 509 | .Ve |
| 510 | .PP |
| 511 | .Vb 9 |
| 512 | \& if (($status = filter_read()) > 0 ) { |
| 513 | \& s/Joe/Jim/g ; |
| 514 | \& ++ $$self ; |
| 515 | \& } |
| 516 | \& elsif ($$self >= 0) { # EOF |
| 517 | \& $_ = "print q[Made ${$self} substitutions\en]" ; |
| 518 | \& $status = 1 ; |
| 519 | \& $$self = -1 ; |
| 520 | \& } |
| 521 | .Ve |
| 522 | .PP |
| 523 | .Vb 2 |
| 524 | \& $status ; |
| 525 | \& } |
| 526 | .Ve |
| 527 | .PP |
| 528 | .Vb 6 |
| 529 | \& sub import |
| 530 | \& { |
| 531 | \& my ($self) = @_ ; |
| 532 | \& my ($count) = 0 ; |
| 533 | \& filter_add(\e$count) ; |
| 534 | \& } |
| 535 | .Ve |
| 536 | .PP |
| 537 | .Vb 1 |
| 538 | \& 1 ; |
| 539 | .Ve |
| 540 | .PP |
| 541 | Here is a script which uses it: |
| 542 | .PP |
| 543 | .Vb 3 |
| 544 | \& use Count ; |
| 545 | \& print "Hello Joe\en" ; |
| 546 | \& print "Where is Joe\en" ; |
| 547 | .Ve |
| 548 | .PP |
| 549 | Outputs: |
| 550 | .PP |
| 551 | .Vb 3 |
| 552 | \& Hello Jim |
| 553 | \& Where is Jim |
| 554 | \& Made 2 substitutions |
| 555 | .Ve |
| 556 | .Sh "Example 4: Using filter_del" |
| 557 | .IX Subsection "Example 4: Using filter_del" |
| 558 | Another variation on a theme. This time we will modify the \f(CW\*(C`Subst\*(C'\fR |
| 559 | filter to allow a starting and stopping pattern to be specified as well |
| 560 | as the \fIfrom\fR and \fIto\fR patterns. If you know the \fIvi\fR editor, it is |
| 561 | the equivalent of this command: |
| 562 | .PP |
| 563 | .Vb 1 |
| 564 | \& :/start/,/stop/s/from/to/ |
| 565 | .Ve |
| 566 | .PP |
| 567 | When used as a filter we want to invoke it like this: |
| 568 | .PP |
| 569 | .Vb 1 |
| 570 | \& use NewSubst qw(start stop from to) ; |
| 571 | .Ve |
| 572 | .PP |
| 573 | Here is the module. |
| 574 | .PP |
| 575 | .Vb 1 |
| 576 | \& package NewSubst ; |
| 577 | .Ve |
| 578 | .PP |
| 579 | .Vb 2 |
| 580 | \& use Filter::Util::Call ; |
| 581 | \& use Carp ; |
| 582 | .Ve |
| 583 | .PP |
| 584 | .Vb 6 |
| 585 | \& sub import |
| 586 | \& { |
| 587 | \& my ($self, $start, $stop, $from, $to) = @_ ; |
| 588 | \& my ($found) = 0 ; |
| 589 | \& croak("usage: use Subst qw(start stop from to)") |
| 590 | \& unless @_ == 5 ; |
| 591 | .Ve |
| 592 | .PP |
| 593 | .Vb 4 |
| 594 | \& filter_add( |
| 595 | \& sub |
| 596 | \& { |
| 597 | \& my ($status) ; |
| 598 | .Ve |
| 599 | .PP |
| 600 | .Vb 1 |
| 601 | \& if (($status = filter_read()) > 0) { |
| 602 | .Ve |
| 603 | .PP |
| 604 | .Vb 2 |
| 605 | \& $found = 1 |
| 606 | \& if $found == 0 and /$start/ ; |
| 607 | .Ve |
| 608 | .PP |
| 609 | .Vb 4 |
| 610 | \& if ($found) { |
| 611 | \& s/$from/$to/ ; |
| 612 | \& filter_del() if /$stop/ ; |
| 613 | \& } |
| 614 | .Ve |
| 615 | .PP |
| 616 | .Vb 3 |
| 617 | \& } |
| 618 | \& $status ; |
| 619 | \& } ) |
| 620 | .Ve |
| 621 | .PP |
| 622 | .Vb 1 |
| 623 | \& } |
| 624 | .Ve |
| 625 | .PP |
| 626 | .Vb 1 |
| 627 | \& 1 ; |
| 628 | .Ve |
| 629 | .SH "Filter::Simple" |
| 630 | .IX Header "Filter::Simple" |
| 631 | If you intend using the Filter::Call functionality, I would strongly |
| 632 | recommend that you check out Damian Conway's excellent Filter::Simple |
| 633 | module. Damian's module provides a much cleaner interface than |
| 634 | Filter::Util::Call. Although it doesn't allow the fine control that |
| 635 | Filter::Util::Call does, it should be adequate for the majority of |
| 636 | applications. It's available at |
| 637 | .PP |
| 638 | .Vb 2 |
| 639 | \& http://www.cpan.org/modules/by-author/Damian_Conway/Filter-Simple.tar.gz |
| 640 | \& http://www.csse.monash.edu.au/~damian/CPAN/Filter-Simple.tar.gz |
| 641 | .Ve |
| 642 | .SH "AUTHOR" |
| 643 | .IX Header "AUTHOR" |
| 644 | Paul Marquess |
| 645 | .SH "DATE" |
| 646 | .IX Header "DATE" |
| 647 | 26th January 1996 |