B::Deparse - man - phpman

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

SYNOPSIS
    perl -MO=Deparse[,-d][,-f*FILE*][,-p][,-q][,-l] [,-s*LETTERS*][,-x*LEVEL*] *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).

    -f*FILE*
        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.

    -s*LETTERS*
        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.

        i*NUMBER*
            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.

        v*STRING*.
            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.

    -x*LEVEL*
        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:

    *   use strict;

    *   use warnings;

    *   Assigning to the special variable $[

    *   use integer;

    *   use bytes;

    *   use utf8;

    *   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 $[. Obsolete: cannot be non-zero.

    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 warnings 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
    *   The only pragmas to be completely supported are: "use warnings", "use strict", "use bytes",
        "use integer" and "use feature".

        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.

    *   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.

    *   Optimized-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 coderef2text
        output text as package variables. This is a tricky problem, as perl has no native facility
        for referring to a lexical variable defined within a different scope, although PadWalker is
        a good start.

        See also Data::Dump::Streamer, which combines B::Deparse and PadWalker to serialize closures
        properly.

    *   There are probably many more bugs on non-ASCII platforms (EBCDIC).

AUTHOR
    Stephen McCamant <smcc AT CSUA.EDU>, based on an earlier version by Malcolm Beattie
    <mbeattie AT sable.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.