B::Deparse
B::Deparse(3) Perl Programmers Reference Guide B::Deparse(3)
NAME
B::Deparse - Perl compiler backend to produce perl code
SYNOPSIS
perl -MO=Deparse[,-d][,-fFILE][,-p][,-q][,-l]
[,-sLETTERS][,-xLEVEL] prog.pl
DESCRIPTION
B::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 corre-
spondence between perl's syntactical constructions and their compiled
form, but it will often be close. When you use the -p option, the out-
put also includes parentheses even when they are not required by prece-
dence, which can make it easy to see if perl is parsing your expres-
sions 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 section 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 program, 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.
-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 function 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 combinations 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 $me . '!');
Note that the expanded form represents the way perl handles such
constructions internally -- this option actually turns off the
reverse translation that B::Deparse usually does. On the other
hand, note that "$x = "$y"" is not the same as "$x = $y": the for-
mer makes the value of $y into a string before doing the assign-
ment.
-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 deter-
mined 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
expression, generally a constant. Note that unless it's a num-
ber, it probably needs to be quoted, and on a command line
quotes need to be protected from the shell. Some conventional
values include 0, 1, 42, '', 'foo', and 'Useless use of con-
stant omitted' (which may need to be -sv"'Useless use of con-
stant 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 con-
stant 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 opera-
tions.
If LEVEL is at least 3, "for" loops will be translated into equiva-
lent 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 perfectly car-
ried 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
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 pro-
grams.
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 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:
o use strict;
o use warnings;
o Assigning to the special variable $[
o use integer;
o use bytes;
o use utf8;
o use re;
Ordinarily, if you use B::Deparse on a subroutine which has been com-
piled in the presence of one or more of these pragmas, 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 particu-
lar 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
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 permis-
sible 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 warnings.
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.
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
o The only pragmas to be completely supported are: "use warnings",
"use strict 'refs'", "use bytes", and "use integer". ($[, which
behaves like a pragma, is also supported.)
Excepting those listed above, we're currently unable to guarantee
that B::Deparse will produce a pragma at the correct point in the
program. (Specifically, pragmas at the beginning of a block often
appear right before the start of the block instead.) Since the
effects of pragmas are often lexically scoped, this can mean that
the pragma holds sway over a different portion of the program than
in the input file.
o In fact, the above is a specific instance of a more general prob-
lem: we can't guarantee to produce BEGIN blocks or "use" declara-
tions in exactly the right place. So if you use a module which
affects compilation (such as by over-riding keywords, overloading
constants or whatever) then the output code might not work as
intended.
This is the most serious outstanding problem, and will require some
help from the Perl core to fix.
o If a keyword is over-ridden, and your program explicitly calls the
built-in version by using CORE::keyword, the output of B::Deparse
will not reflect this. If you run the resulting code, it will call
the over-ridden version rather than the built-in one. (Maybe there
should be an option to always print keyword calls as "CORE::name".)
o Some constants don't print correctly either with or without -d.
For instance, neither B::Deparse nor Data::Dumper know how to print
dual-valued scalars correctly, as in:
use constant E2BIG => ($!=7); $y = E2BIG; print $y, 0+$y;
o An input file that uses source filtering probably won't be deparsed
into runnable code, because it will still include the use declara-
tion for the source filtering module, even though the code that is
produced is already ordinary Perl which shouldn't be filtered
again.
o 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.
o There are probably many more bugs on non-ASCII platforms (EBCDIC).
AUTHOR
Stephen McCamant <smcc@CSUA.Berkeley.EDU>, based on an earlier 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.
perl v5.8.6 2001-09-21 B::Deparse(3)
