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B::Deparse(3pm) Perl Programmers Reference Guide B::Deparse(3pm)
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 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 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 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 expression, 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 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 perfectly 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
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
separate strings.
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 compiled 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 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
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 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.
%^H This parameter is used to specify the ambient pragmas which are stored in the special hash %^H.
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".
($[, which behaves like a pragma, is also supported.)
Excepting those listed above, we're currently unable to guarantee that B::Deparse will produce a pragma at the
correct point in the program. (Specifically, pragmas at the beginning of a block often appear right before the start
of the block instead.) Since the effects of pragmas are often lexically scoped, this can mean that the pragma holds
sway over a different portion of the program than in the input file.
o In fact, the above is a specific instance of a more general problem: we can't guarantee to produce BEGIN blocks or
"use" declarations in exactly the right place. So if you use a module which affects compilation (such as by over-
riding keywords, overloading constants or whatever) then the output code might not work as intended.
This is the most serious outstanding problem, and will require some help from the Perl core to fix.
o If a keyword is over-ridden, and your program explicitly calls the built-in version by using CORE::keyword, the
output of B::Deparse will not reflect this. If you run the resulting code, it will call the over-ridden version
rather than the built-in one. (Maybe there should be an option to always print keyword calls as "CORE::name".)
o Some constants don't print correctly either with or without -d. For instance, neither B::Deparse nor Data::Dumper
know how to print dual-valued scalars correctly, as in:
use constant E2BIG => ($!=7); $y = E2BIG; print $y, 0+$y;
use constant H => { "#" => 1 }; H->{"#"};
o 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.
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.
foreach my $i (@_) { 0 }
=>
foreach my $i (@_) { '???' }
o Lexical (my) variables declared in scopes external to a subroutine appear in code2ref output text as package
variables. This is a tricky problem, as perl has no native facility for refering to a lexical variable defined within
a different scope, although PadWalker is a good start.
o There are probably many more bugs on non-ASCII platforms (EBCDIC).
AUTHOR
Stephen McCamant <smccATCSUA.EDU>, based on an earlier version by Malcolm Beattie <mbeattieATsable.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 v5.12.4 2011-06-07 B::Deparse(3pm)
