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B::Deparse - Perl compiler backend to produce perl code


perl -MO=Deparse[,-uPACKAGE][,-p][,-q][,-l] [,-sLETTERS][,-xLEVEL]


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.


Add '#line' declarations to the output based on the line and file locations of the original code.


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).


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!";


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.


Normally, B::Deparse deparses the main code of a program, all the subs called by the main program (and all the subs called by them, recursively), and any other subs in the main:: package. To include subs in other packages that aren't called directly, such as AUTOLOAD, DESTROY, other subs called automatically by perl, and methods (which aren't resolved to subs until runtime), use the -u option. The argument to -u is the name of a package, and should follow directly after the 'u'. Multiple -u options may be given, separated by commas. Note that unlike some other backends, B::Deparse doesn't (yet) try to guess automatically when -u is needed -- you must invoke it yourself.


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.


Indent lines by multiples of NUMBER columns. The default is 4 columns.


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.


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.


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 {

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.


$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.


See the 'to do' list at the beginning of the module file.


Stephen McCamant <smcc@CSUA.Berkeley.EDU>, based on an earlier version by Malcolm Beattie <>, with contributions from Gisle Aas, James Duncan, Albert Dvornik, Hugo van der Sanden, Gurusamy Sarathy, and Nick Ing-Simmons.