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:

-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¶

    use B::Deparse;
    $deparse = B::Deparse->new("-p", "-sC");
    $body = $deparse->coderef2text(\&func);
    eval "sub func $body"; # the inverse operation

    $deparse = B::Deparse->new(OPTIONS)

    $deparse->ambient_pragmas(strict => 'all', '$[' => $[);

    $body = $deparse->coderef2text(\&func)
    $body = $deparse->coderef2text(sub ($$) { ... })

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 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.

• 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.

• 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:

      use constant E2BIG => ($!=7); $y = E2BIG; print $y, 0+$y;
      use constant H => { "#" => 1 }; H->{"#"};
      

• 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.

      foreach my $i (@_) { 0 }
    =>
      foreach my $i (@_) { '???' }
      

• 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.
• 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.