1B::Deparse(3pm) Perl Programmers Reference Guide B::Deparse(3pm)
2
6 B::Deparse - Perl compiler backend to produce perl code
7
9 perl -MO=Deparse[,-d][,-fFILE][,-p][,-q][,-l]
10 [,-sLETTERS][,-xLEVEL] prog.pl
11
13 B::Deparse is a backend module for the Perl compiler that generates
14 perl source code, based on the internal compiled structure that perl
15 itself creates after parsing a program. The output of B::Deparse won't
16 be exactly the same as the original source, since perl doesn't keep
17 track of comments or whitespace, and there isn't a one-to-one corre‐
18 spondence between perl's syntactical constructions and their compiled
19 form, but it will often be close. When you use the -p option, the out‐
20 put also includes parentheses even when they are not required by prece‐
21 dence, which can make it easy to see if perl is parsing your expres‐
22 sions the way you intended.
23 While B::Deparse goes to some lengths to try to figure out what your
25 original program was doing, some parts of the language can still trip
26 it up; it still fails even on some parts of Perl's own test suite. If
27 you encounter a failure other than the most common ones described in
28 the BUGS section below, you can help contribute to B::Deparse's ongoing
29 development by submitting a bug report with a small example.
30
32 As with all compiler backend options, these must follow directly after
33 the '-MO=Deparse', separated by a comma but not any white space.
34 -d Output data values (when they appear as constants) using
36 Data::Dumper. Without this option, B::Deparse will use some simple
37 routines of its own for the same purpose. Currently, Data::Dumper
38 is better for some kinds of data (such as complex structures with
39 sharing and self-reference) while the built-in routines are better
40 for others (such as odd floating-point values).
41 -fFILE
43 Normally, B::Deparse deparses the main code of a program, and all
44 the subs defined in the same file. To include subs defined in other
45 files, pass the -f option with the filename. You can pass the -f
46 option several times, to include more than one secondary file.
47 (Most of the time you don't want to use it at all.) You can also
48 use this option to include subs which are defined in the scope of a
49 #line directive with two parameters.
50 -l Add '#line' declarations to the output based on the line and file
52 locations of the original code.
53 -p Print extra parentheses. Without this option, B::Deparse includes
55 parentheses in its output only when they are needed, based on the
56 structure of your program. With -p, it uses parentheses (almost)
57 whenever they would be legal. This can be useful if you are used to
58 LISP, or if you want to see how perl parses your input. If you say
59 if ($var & 0x7f == 65) {print "Gimme an A!"}
61 print ($which ? $a : $b), "\n";
62 $name = $ENV{USER} or "Bob";
63 "B::Deparse,-p" will print
65 if (($var & 0)) {
67 print('Gimme an A!')
68 };
69 (print(($which ? $a : $b)), '???');
70 (($name = $ENV{'USER'}) or '???')
71 which probably isn't what you intended (the '???' is a sign that
73 perl optimized away a constant value).
74 -P Disable prototype checking. With this option, all function calls
76 are deparsed as if no prototype was defined for them. In other
77 words,
78 perl -MO=Deparse,-P -e 'sub foo (\@) { 1 } foo @x'
80 will print
82 sub foo (\@) {
84 1;
85 }
86 &foo(\@x);
87 making clear how the parameters are actually passed to "foo".
89 -q Expand double-quoted strings into the corresponding combinations of
91 concatenation, uc, ucfirst, lc, lcfirst, quotemeta, and join. For
92 instance, print
93 print "Hello, $world, @ladies, \u$gentlemen\E, \u\L$me!";
95 as
97 print 'Hello, ' . $world . ', ' . join($", @ladies) . ', '
99 . ucfirst($gentlemen) . ', ' . ucfirst(lc $me . '!');
100 Note that the expanded form represents the way perl handles such
102 constructions internally -- this option actually turns off the
103 reverse translation that B::Deparse usually does. On the other
104 hand, note that "$x = "$y"" is not the same as "$x = $y": the for‐
105 mer makes the value of $y into a string before doing the assign‐
106 ment.
107 -sLETTERS
109 Tweak the style of B::Deparse's output. The letters should follow
110 directly after the 's', with no space or punctuation. The following
111 options are available:
112 C Cuddle "elsif", "else", and "continue" blocks. For example,
114 print
115 if (...) {
117 ...
118 } else {
119 ...
120 }
121 instead of
123 if (...) {
125 ...
126 }
127 else {
128 ...
129 }
130 The default is not to cuddle.
132 iNUMBER
134 Indent lines by multiples of NUMBER columns. The default is 4
135 columns.
136 T Use tabs for each 8 columns of indent. The default is to use
138 only spaces. For instance, if the style options are -si4T, a
139 line that's indented 3 times will be preceded by one tab and
140 four spaces; if the options were -si8T, the same line would be
141 preceded by three tabs.
142 vSTRING.
144 Print STRING for the value of a constant that can't be deter‐
145 mined because it was optimized away (mnemonic: this happens
146 when a constant is used in void context). The end of the string
147 is marked by a period. The string should be a valid perl
148 expression, generally a constant. Note that unless it's a num‐
149 ber, it probably needs to be quoted, and on a command line
150 quotes need to be protected from the shell. Some conventional
151 values include 0, 1, 42, '', 'foo', and 'Useless use of con‐
152 stant omitted' (which may need to be -sv"'Useless use of con‐
153 stant omitted'." or something similar depending on your
154 shell). The default is '???'. If you're using B::Deparse on a
155 module or other file that's require'd, you shouldn't use a
156 value that evaluates to false, since the customary true con‐
157 stant at the end of a module will be in void context when the
158 file is compiled as a main program.
159 -xLEVEL
161 Expand conventional syntax constructions into equivalent ones that
162 expose their internal operation. LEVEL should be a digit, with
163 higher values meaning more expansion. As with -q, this actually
164 involves turning off special cases in B::Deparse's normal opera‐
165 tions.
166 If LEVEL is at least 3, "for" loops will be translated into equiva‐
168 lent while loops with continue blocks; for instance
169 for ($i = 0; $i < 10; ++$i) {
171 print $i;
172 }
173 turns into
175 $i = 0;
177 while ($i < 10) {
178 print $i;
179 } continue {
180 ++$i
181 }
182 Note that in a few cases this translation can't be perfectly car‐
184 ried back into the source code -- if the loop's initializer
185 declares a my variable, for instance, it won't have the correct
186 scope outside of the loop.
187 If LEVEL is at least 5, "use" declarations will be translated into
189 "BEGIN" blocks containing calls to "require" and "import"; for
190 instance,
191 use strict 'refs';
193 turns into
195 sub BEGIN {
197 require strict;
198 do {
199 'strict'->import('refs')
200 };
201 }
202 If LEVEL is at least 7, "if" statements will be translated into
204 equivalent expressions using "&&", "?:" and "do {}"; for instance
205 print 'hi' if $nice;
207 if ($nice) {
208 print 'hi';
209 }
210 if ($nice) {
211 print 'hi';
212 } else {
213 print 'bye';
214 }
215 turns into
217 $nice and print 'hi';
219 $nice and do { print 'hi' };
220 $nice ? do { print 'hi' } : do { print 'bye' };
221 Long sequences of elsifs will turn into nested ternary operators,
223 which B::Deparse doesn't know how to indent nicely.
224
226 Synopsis
227 use B::Deparse;
229 $deparse = B::Deparse->new("-p", "-sC");
230 $body = $deparse->coderef2text(\&func);
231 eval "sub func $body"; # the inverse operation
232 Description
234 B::Deparse can also be used on a sub-by-sub basis from other perl pro‐
236 grams.
237 new
239 $deparse = B::Deparse->new(OPTIONS)
241 Create an object to store the state of a deparsing operation and any
243 options. The options are the same as those that can be given on the
244 command line (see "OPTIONS"); options that are separated by commas
245 after -MO=Deparse should be given as separate strings. Some options,
246 like -u, don't make sense for a single subroutine, so don't pass them.
247 ambient_pragmas
249 $deparse->ambient_pragmas(strict => 'all', '$[' => $[);
251 The compilation of a subroutine can be affected by a few compiler
253 directives, pragmas. These are:
254 · use strict;
256 · use warnings;
258 · Assigning to the special variable $[
260 · use integer;
262 · use bytes;
264 · use utf8;
266 · use re;
268 Ordinarily, if you use B::Deparse on a subroutine which has been com‐
270 piled in the presence of one or more of these pragmas, the output will
271 include statements to turn on the appropriate directives. So if you
272 then compile the code returned by coderef2text, it will behave the same
273 way as the subroutine which you deparsed.
274 However, you may know that you intend to use the results in a particu‐
276 lar context, where some pragmas are already in scope. In this case, you
277 use the ambient_pragmas method to describe the assumptions you wish to
278 make.
279 Not all of the options currently have any useful effect. See "BUGS" for
281 more details.
282 The parameters it accepts are:
284 strict
286 Takes a string, possibly containing several values separated by
287 whitespace. The special values "all" and "none" mean what you'd
288 expect.
289 $deparse->ambient_pragmas(strict => 'subs refs');
291 $[ Takes a number, the value of the array base $[.
293 bytes
295 utf8
296 integer
297 If the value is true, then the appropriate pragma is assumed to be
298 in the ambient scope, otherwise not.
299 re Takes a string, possibly containing a whitespace-separated list of
301 values. The values "all" and "none" are special. It's also permis‐
302 sible to pass an array reference here.
303 $deparser->ambient_pragmas(re => 'eval');
305 warnings
307 Takes a string, possibly containing a whitespace-separated list of
308 values. The values "all" and "none" are special, again. It's also
309 permissible to pass an array reference here.
310 $deparser->ambient_pragmas(warnings => [qw[void io]]);
312 If one of the values is the string "FATAL", then all the warnings
314 in that list will be considered fatal, just as with the warnings
315 pragma itself. Should you need to specify that some warnings are
316 fatal, and others are merely enabled, you can pass the warnings
317 parameter twice:
318 $deparser->ambient_pragmas(
320 warnings => 'all',
321 warnings => [FATAL => qw/void io/],
322 );
323 See perllexwarn for more information about lexical warnings.
325 hint_bits
327 warning_bits
328 These two parameters are used to specify the ambient pragmas in the
329 format used by the special variables $^H and ${^WARNING_BITS}.
330 They exist principally so that you can write code like:
332 { my ($hint_bits, $warning_bits);
334 BEGIN {($hint_bits, $warning_bits) = ($^H, ${^WARNING_BITS})}
335 $deparser->ambient_pragmas (
336 hint_bits => $hint_bits,
337 warning_bits => $warning_bits,
338 '$[' => 0 + $[
339 ); }
340 which specifies that the ambient pragmas are exactly those which
342 are in scope at the point of calling.
343 coderef2text
345 $body = $deparse->coderef2text(\&func)
347 $body = $deparse->coderef2text(sub ($$) { ... })
348 Return source code for the body of a subroutine (a block, optionally
350 preceded by a prototype in parens), given a reference to the sub.
351 Because a subroutine can have no names, or more than one name, this
352 method doesn't return a complete subroutine definition -- if you want
353 to eval the result, you should prepend "sub subname ", or "sub " for an
354 anonymous function constructor. Unless the sub was defined in the
355 main:: package, the code will include a package declaration.
356
358 · The only pragmas to be completely supported are: "use warnings",
359 "use strict 'refs'", "use bytes", and "use integer". ($[, which
360 behaves like a pragma, is also supported.)
361 Excepting those listed above, we're currently unable to guarantee
363 that B::Deparse will produce a pragma at the correct point in the
364 program. (Specifically, pragmas at the beginning of a block often
365 appear right before the start of the block instead.) Since the
366 effects of pragmas are often lexically scoped, this can mean that
367 the pragma holds sway over a different portion of the program than
368 in the input file.
369 · In fact, the above is a specific instance of a more general prob‐
371 lem: we can't guarantee to produce BEGIN blocks or "use" declara‐
372 tions in exactly the right place. So if you use a module which
373 affects compilation (such as by over-riding keywords, overloading
374 constants or whatever) then the output code might not work as
375 intended.
376 This is the most serious outstanding problem, and will require some
378 help from the Perl core to fix.
379 · If a keyword is over-ridden, and your program explicitly calls the
381 built-in version by using CORE::keyword, the output of B::Deparse
382 will not reflect this. If you run the resulting code, it will call
383 the over-ridden version rather than the built-in one. (Maybe there
384 should be an option to always print keyword calls as "CORE::name".)
385 · Some constants don't print correctly either with or without -d.
387 For instance, neither B::Deparse nor Data::Dumper know how to print
388 dual-valued scalars correctly, as in:
389 use constant E2BIG => ($!=7); $y = E2BIG; print $y, 0+$y;
391 · An input file that uses source filtering probably won't be deparsed
393 into runnable code, because it will still include the use declara‐
394 tion for the source filtering module, even though the code that is
395 produced is already ordinary Perl which shouldn't be filtered
396 again.
397 · Optimised away statements are rendered as '???'. This includes
399 statements that have a compile-time side-effect, such as the
400 obscure
401 my $x if 0;
403 which is not, consequently, deparsed correctly.
405 · There are probably many more bugs on non-ASCII platforms (EBCDIC).
407
409 Stephen McCamant <smcc@CSUA.Berkeley.EDU>, based on an earlier version
410 by Malcolm Beattie <mbeattie@sable.ox.ac.uk>, with contributions from
411 Gisle Aas, James Duncan, Albert Dvornik, Robin Houston, Dave Mitchell,
412 Hugo van der Sanden, Gurusamy Sarathy, Nick Ing-Simmons, and Rafael
413 Garcia-Suarez.
414perl v5.8.8 2001-09-21 B::Deparse(3pm)