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

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:

	   C   Cuddle "elsif", "else", and "continue" blocks. For example,
	       print

		   if (...) {
			...
		   } else {
			...
		   }

	       instead of

		   if (...) {
			...
		   }
		   else {
			...
		   }

	       The default is not to cuddle.

	   iNUMBER
	       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.

	   vSTRING.
	       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.

       -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
       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. Some options,
       like -u, don't make sense for a single subroutine, so don't pass them.

       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 $[.

       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 perllexwarn 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 '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;

       ·   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.

       ·   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.

perl v5.10.0			  2007-12-18			 B::Deparse(3)
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