ext::B::B::DeparsPerl)Programmers Referenceext::B::B::Deparse(3p)NAMEB::Deparse - Perl compiler backend to produce perl code
SYNOPSIS
perl -MO=Deparse[,-d][,-fFILE][,-p][,-q][,-l]
[,-sLETTERS][,-xLEVEL] prog.pl
DESCRIPTIONB::Deparse is a backend module for the Perl compiler that
generates perl source code, based on the internal compiled
structure that perl itself creates after parsing a program.
The output of B::Deparse won't be exactly the same as the
original source, since perl doesn't keep track of comments
or whitespace, and there isn't a one-to-one correspondence
between perl's syntactical constructions and their compiled
form, but it will often be close. When you use the -p
option, the output also includes parentheses even when they
are not required by precedence, which can make it easy to
see if perl is parsing your expressions the way you
intended.
While B::Deparse goes to some lengths to try to figure out
what your original program was doing, some parts of the
language can still trip it up; it still fails even on some
parts of Perl's own test suite. If you encounter a failure
other than the most common ones described in the BUGS sec-
tion below, you can help contribute to B::Deparse's ongoing
development by submitting a bug report with a small example.
OPTIONS
As with all compiler backend options, these must follow
directly after the '-MO=Deparse', separated by a comma but
not any white space.
-d Output data values (when they appear as constants) using
Data::Dumper. Without this option, B::Deparse will use
some simple routines of its own for the same purpose.
Currently, Data::Dumper is better for some kinds of data
(such as complex structures with sharing and
self-reference) while the built-in routines are better
for others (such as odd floating-point values).
-fFILE
Normally, B::Deparse deparses the main code of a pro-
gram, and all the subs defined in the same file. To
include subs defined in other files, pass the -f option
with the filename. You can pass the -f option several
times, to include more than one secondary file. (Most
of the time you don't want to use it at all.) You can
also use this option to include subs which are defined
in the scope of a #line directive with two parameters.
perl v5.8.8 2005-02-05 1
ext::B::B::DeparsPerl)Programmers Referenceext::B::B::Deparse(3p)-l Add '#line' declarations to the output based on the line
and file locations of the original code.
-p Print extra parentheses. Without this option, B::Deparse
includes parentheses in its output only when they are
needed, based on the structure of your program. With -p,
it uses parentheses (almost) whenever they would be
legal. This can be useful if you are used to LISP, or if
you want to see how perl parses your input. If you say
if ($var & 0x7f == 65) {print "Gimme an A!"}
print ($which ? $a : $b), "\n";
$name = $ENV{USER} or "Bob";
"B::Deparse,-p" will print
if (($var & 0)) {
print('Gimme an A!')
};
(print(($which ? $a : $b)), '???');
(($name = $ENV{'USER'}) or '???')
which probably isn't what you intended (the '???' is a
sign that perl optimized away a constant value).
-P Disable prototype checking. With this option, all func-
tion calls are deparsed as if no prototype was defined
for them. In other words,
perl -MO=Deparse,-P -e 'sub foo (\@) { 1 } foo @x'
will print
sub foo (\@) {
1;
}
&foo(\@x);
making clear how the parameters are actually passed to
"foo".
-q Expand double-quoted strings into the corresponding com-
binations of concatenation, uc, ucfirst, lc, lcfirst,
quotemeta, and join. For instance, print
print "Hello, $world, @ladies, \u$gentlemen\E, \u\L$me!";
as
print 'Hello, ' . $world . ', ' . join($", @ladies) . ', '
. ucfirst($gentlemen) . ', ' . ucfirst(lc $me . '!');
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Note that the expanded form represents the way perl han-
dles such constructions internally -- this option actu-
ally turns off the reverse translation that B::Deparse
usually does. On the other hand, note that "$x = "$y""
is not the same as "$x = $y": the former makes the value
of $y into a string before doing the assignment.
-sLETTERS
Tweak the style of B::Deparse's output. The letters
should follow directly after the 's', with no space or
punctuation. The following options are available:
C Cuddle "elsif", "else", and "continue" blocks. For
example, print
if (...) {
...
} else {
...
}
instead of
if (...) {
...
}
else {
...
}
The default is not to cuddle.
iNUMBER
Indent lines by multiples of NUMBER columns. The
default is 4 columns.
T Use tabs for each 8 columns of indent. The default
is to use only spaces. For instance, if the style
options are -si4T, a line that's indented 3 times
will be preceded by one tab and four spaces; if the
options were -si8T, the same line would be preceded
by three tabs.
vSTRING.
Print STRING for the value of a constant that can't
be determined because it was optimized away
(mnemonic: this happens when a constant is used in
void context). The end of the string is marked by a
period. The string should be a valid perl expres-
sion, generally a constant. Note that unless it's a
number, it probably needs to be quoted, and on a
command line quotes need to be protected from the
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shell. Some conventional values include 0, 1, 42,
'', 'foo', and 'Useless use of constant omitted'
(which may need to be -sv"'Useless use of constant
omitted'." or something similar depending on your
shell). The default is '???'. If you're using
B::Deparse on a module or other file that's
require'd, you shouldn't use a value that evaluates
to false, since the customary true constant at the
end of a module will be in void context when the
file is compiled as a main program.
-xLEVEL
Expand conventional syntax constructions into equivalent
ones that expose their internal operation. LEVEL should
be a digit, with higher values meaning more expansion.
As with -q, this actually involves turning off special
cases in B::Deparse's normal operations.
If LEVEL is at least 3, "for" loops will be translated
into equivalent while loops with continue blocks; for
instance
for ($i = 0; $i < 10; ++$i) {
print $i;
}
turns into
$i = 0;
while ($i < 10) {
print $i;
} continue {
++$i
}
Note that in a few cases this translation can't be per-
fectly carried back into the source code -- if the
loop's initializer declares a my variable, for instance,
it won't have the correct scope outside of the loop.
If LEVEL is at least 5, "use" declarations will be
translated into "BEGIN" blocks containing calls to
"require" and "import"; for instance,
use strict 'refs';
turns into
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sub BEGIN {
require strict;
do {
'strict'->import('refs')
};
}
If LEVEL is at least 7, "if" statements will be
translated into equivalent expressions using "&&", "?:"
and "do {}"; for instance
print 'hi' if $nice;
if ($nice) {
print 'hi';
}
if ($nice) {
print 'hi';
} else {
print 'bye';
}
turns into
$nice and print 'hi';
$nice and do { print 'hi' };
$nice ? do { print 'hi' } : do { print 'bye' };
Long sequences of elsifs will turn into nested ternary
operators, which B::Deparse doesn't know how to indent
nicely.
USING B::Deparse AS A MODULE
Synopsis
use B::Deparse;
$deparse = B::Deparse->new("-p", "-sC");
$body = $deparse->coderef2text(\&func);
eval "sub func $body"; # the inverse operation
Description
B::Deparse can also be used on a sub-by-sub basis from other
perl programs.
new
$deparse = B::Deparse->new(OPTIONS)
Create an object to store the state of a deparsing operation
and any options. The options are the same as those that can
be given on the command line (see "OPTIONS"); options that
are separated by commas after -MO=Deparse should be given as
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separate strings. Some options, like -u, don't make sense
for a single subroutine, so don't pass them.
ambient_pragmas
$deparse->ambient_pragmas(strict => 'all', '$[' => $[);
The compilation of a subroutine can be affected by a few
compiler directives, pragmas. These are:
+ use strict;
+ use warnings;
+ Assigning to the special variable $[
+ use integer;
+ use bytes;
+ use utf8;
+ use re;
Ordinarily, if you use B::Deparse on a subroutine which has
been compiled in the presence of one or more of these prag-
mas, the output will include statements to turn on the
appropriate directives. So if you then compile the code
returned by coderef2text, it will behave the same way as the
subroutine which you deparsed.
However, you may know that you intend to use the results in
a particular context, where some pragmas are already in
scope. In this case, you use the ambient_pragmas method to
describe the assumptions you wish to make.
Not all of the options currently have any useful effect. See
"BUGS" for more details.
The parameters it accepts are:
strict
Takes a string, possibly containing several values
separated by whitespace. The special values "all" and
"none" mean what you'd expect.
$deparse->ambient_pragmas(strict => 'subs refs');
$[ Takes a number, the value of the array base $[.
bytes
utf8
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integer
If the value is true, then the appropriate pragma is
assumed to be in the ambient scope, otherwise not.
re Takes a string, possibly containing a whitespace-
separated list of values. The values "all" and "none"
are special. It's also permissible to pass an array
reference here.
$deparser->ambient_pragmas(re => 'eval');
warnings
Takes a string, possibly containing a whitespace-
separated list of values. The values "all" and "none"
are special, again. It's also permissible to pass an
array reference here.
$deparser->ambient_pragmas(warnings => [qw[void io]]);
If one of the values is the string "FATAL", then all the
warnings in that list will be considered fatal, just as
with the warnings pragma itself. Should you need to
specify that some warnings are fatal, and others are
merely enabled, you can pass the warnings parameter
twice:
$deparser->ambient_pragmas(
warnings => 'all',
warnings => [FATAL => qw/void io/],
);
See perllexwarn for more information about lexical warn-
ings.
hint_bits
warning_bits
These two parameters are used to specify the ambient
pragmas in the format used by the special variables $^H
and ${^WARNING_BITS}.
They exist principally so that you can write code like:
{ my ($hint_bits, $warning_bits);
BEGIN {($hint_bits, $warning_bits) = ($^H, ${^WARNING_BITS})}
$deparser->ambient_pragmas (
hint_bits => $hint_bits,
warning_bits => $warning_bits,
'$[' => 0 + $[
); }
which specifies that the ambient pragmas are exactly
those which are in scope at the point of calling.
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coderef2text
$body = $deparse->coderef2text(\&func)
$body = $deparse->coderef2text(sub ($$) { ... })
Return source code for the body of a subroutine (a block,
optionally preceded by a prototype in parens), given a
reference to the sub. Because a subroutine can have no
names, or more than one name, this method doesn't return a
complete subroutine definition -- if you want to eval the
result, you should prepend "sub subname ", or "sub " for an
anonymous function constructor. Unless the sub was defined
in the main:: package, the code will include a package
declaration.
BUGS
+ The only pragmas to be completely supported are: "use
warnings", "use strict 'refs'", "use bytes", and "use
integer". ($[, which behaves like a pragma, is also sup-
ported.)
Excepting those listed above, we're currently unable to
guarantee that B::Deparse will produce a pragma at the
correct point in the program. (Specifically, pragmas at
the beginning of a block often appear right before the
start of the block instead.) Since the effects of prag-
mas are often lexically scoped, this can mean that the
pragma holds sway over a different portion of the pro-
gram than in the input file.
+ In fact, the above is a specific instance of a more gen-
eral problem: we can't guarantee to produce BEGIN blocks
or "use" declarations in exactly the right place. So if
you use a module which affects compilation (such as by
over-riding keywords, overloading constants or whatever)
then the output code might not work as intended.
This is the most serious outstanding problem, and will
require some help from the Perl core to fix.
+ If a keyword is over-ridden, and your program explicitly
calls the built-in version by using CORE::keyword, the
output of B::Deparse will not reflect this. If you run
the resulting code, it will call the over-ridden version
rather than the built-in one. (Maybe there should be an
option to always print keyword calls as "CORE::name".)
+ Some constants don't print correctly either with or
without -d. For instance, neither B::Deparse nor
Data::Dumper know how to print dual-valued scalars
correctly, as in:
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use constant E2BIG => ($!=7); $y = E2BIG; print $y, 0+$y;
+ An input file that uses source filtering probably won't
be deparsed into runnable code, because it will still
include the use declaration for the source filtering
module, even though the code that is produced is already
ordinary Perl which shouldn't be filtered again.
+ Optimised away statements are rendered as '???'. This
includes statements that have a compile-time
side-effect, such as the obscure
my $x if 0;
which is not, consequently, deparsed correctly.
+ There are probably many more bugs on non-ASCII platforms
(EBCDIC).
AUTHOR
Stephen McCamant <smcc@CSUA.Berkeley.EDU>, based on an ear-
lier version by Malcolm Beattie <mbeattie@sable.ox.ac.uk>,
with contributions from Gisle Aas, James Duncan, Albert
Dvornik, Robin Houston, Dave Mitchell, Hugo van der Sanden,
Gurusamy Sarathy, Nick Ing-Simmons, and Rafael
Garcia-Suarez.
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