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B::Deparse(3perl)                Perl Programmers Reference Guide               B::Deparse(3perl)

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

       perl -MO=Deparse[,-d][,-fFILE][,-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.

       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.

       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).

           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 (\@) {

           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!";


               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:

           C   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.

           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.

               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 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 {

           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.

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

       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:

           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 $[.  Cannot be non-zero on Perl 5.15.3 or

           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');

           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

               $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:

                   warnings => 'all',
                   warnings => [FATAL => qw/void io/],

           See warnings 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.

       %^H This parameter is used to specify the ambient pragmas which are stored in the special
           hash %^H.

           $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", "use
           bytes", "use integer" and "use feature".  ($[, which behaves like a pragma, is also

           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.

       ·   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 referring 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).

       ·   Lexical "my" subroutines are not deparsed properly at the moment.  They are emitted as
           pure declarations, without their body; and the declaration may appear in the wrong
           place (before any lexicals the body closes over, or before the "use feature"
           declaration that permits use of this feature).

           We expect to resolve this before the lexical-subroutine feature is no longer
           considered experimental.

       ·   Lexical "state" subroutines are not deparsed at all at the moment.

           We expect to resolve this before the lexical-subroutine feature is no longer
           considered experimental.

       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.

perl v5.20.2                                2014-12-27                          B::Deparse(3perl)

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