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

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.

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

       The compilation of a subroutine can be affected by a few compiler
       directives, pragmas. These are:

       o   use strict;

       o   use warnings;

       o   Assigning to the special variable $[

       o   use integer;

       o   use bytes;

       o   use utf8;

       o   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
       o   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.

       o   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.

       o   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".)

       o   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->{"#"};

       o   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.

       o   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 (@_) { '???' }

       o   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.

       o   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 v5.12.1                      2010-05-07                   B::Deparse(3pm)
 

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