B::Deparse - Perl compiler backend to produce perl code
perl -MO=Deparse[,-uPACKAGE][,-p][,-q][,-l] [,-sLETTERS][,-xLEVEL] prog.pl
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.
Please note that this module is mainly new and untested code and is still under development, so it may change in the future.
As with all compiler backend options, these must follow directly after the '-MO=Deparse', separated by a comma but not any white space.
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).
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.
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 $y into a string before doing the assignment.
Tweak the style of B::Deparse's output. The letters should follow directly after the 's', with no space or punctuation. The following options are available:
Cuddle elsif, else, and continue blocks. For example, print
if (...) {
...
} else {
...
}
instead of
if (...) {
...
}
else {
...
}
The default is not to cuddle.
Expand conventional syntax constructions into equivalent ones that expose their internal operation. LEVEL should be a digit, with higher values meaning more expansion. As with -q, this actually involves turning off special cases in B::Deparse's normal operations.
If LEVEL is at least 3, for loops will be translated into equivalent while loops with continue blocks; for instance
for ($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 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.
use B::Deparse;
$deparse = B::Deparse->new("-p", "-sC");
$body = $deparse->coderef2text(\&func);
eval "sub func $body"; # the inverse operation
B::Deparse can also be used on a sub-by-sub basis from other perl programs.
$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.
$deparse->ambient_pragmas(strict => 'all', '$[' => $[);
The compilation of a subroutine can be affected by a few compiler directives, pragmas. These are:
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:
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 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');
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.
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.
$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.
The only pragmas to be completely supported are: use warnings,
use strict 'refs', use bytes, and use integer. ($[, which
behaves like a pragma, is also supported.)
Excepting those listed above, we're currently unable to guarantee that B::Deparse will produce a pragma at the correct point in the program. Since the effects of pragmas are often lexically scoped, this can mean that the pragma holds sway over a different portion of the program than in the input file.
In fact, the above is a specific instance of a more general problem:
we can't guarantee to produce BEGIN blocks or use declarations in
exactly the right place. So if you use a module which affects compilation
(such as by over-riding keywords, overloading constants or whatever)
then the output code might not work as intended.
This is the most serious outstanding problem, and will be very hard to fix.
CORE::name.)
sort foo (1, 2, 3) comes out as sort (foo 1, 2, 3), which
causes perl to issue a warning.
The obvious fix doesn't work, because these are different:
print (FOO 1, 2, 3), 4, 5, 6;
print FOO (1, 2, 3), 4, 5, 6;
Constants (other than simple strings or numbers) don't work properly. Pathological examples that fail (and probably always will) include:
use constant E2BIG => ($!=7);
use constant x=>\$x; print x
The following could (and should) be made to work:
use constant regex => qr/blah/;
print regex;
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, Hugo van der Sanden, Gurusamy Sarathy, Nick Ing-Simmons, and Rafael Garcia-Suarez.