b::deparse(3)
NAME
B::Deparse - Perl compiler backend to produce perl code
SYNOPSIS
perl -MO=Deparse[,-uPACKAGE][,-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 pro
gram. 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 cor
respondence 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.
Please note that this module is mainly new and untested
code and is still under development, so it may change in
the future.
OPTIONS
As with all compiler backend options, these must follow
directly after the '-MO=Deparse', separated by a comma but
not any white space.
- -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) when
ever 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), "0;
$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, $gentlemen e!"; - 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. - -fFILE
- Normally, B::Deparse deparses the main code of a pro
gram, 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. - -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"'Use less use of constant omitted'." or something sim ilar 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 equiva
lent 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 7, if statements will be trans
lated 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 opera
tion 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-sepa
- rated list of values. The values "all" and "none" are
special. It's also permissible to pass an array refer
ence here.
$deparser->ambient_pragmas(re => 'eval'); - warnings
- Takes a string, possibly containing a whitespace-sepa
rated 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[voidio]]); - 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 oth
ers 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. - 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". ($[, which behaves like a pragma, is also
supported.) - Excepting those listed above, we're currently unable
to guarantee that B::Deparse will produce a pragma at
the correct point in the program. Since the effects
of pragmas are often lexically scoped, this can mean
that the pragma holds sway over a different portion of
the program than in the input file. - · In fact, the above is a specific instance of a more
- general problem: we can't guarantee to produce BEGIN
blocks or "use" declarations in exactly the right
place. So if you use a module which affects compila
tion (such as by over-riding keywords, overloading
constants or whatever) then the output code might not
work as intended. - This is the most serious outstanding problem, and will
be very hard to fix. - · If a keyword is over-ridden, and your program explic
- itly calls the built-in version by using CORE::key
word, the output of B::Deparse will not reflect this.
If you run the resulting code, it will call the overridden version rather than the built-in one. (Maybe
there should be an option to always print keyword
calls as "CORE::name".) - · "sort foo (1, 2, 3)" comes out as "sort (foo 1, 2,
- 3)", which causes perl to issue a warning.
- The obvious fix doesn't work, because these are dif
ferent:
print (FOO 1, 2, 3), 4, 5, 6;
print FOO (1, 2, 3), 4, 5, 6; - · Constants (other than simple strings or numbers) don't
- work properly. Pathological examples that fail (and
probably always will) include:
use constant E2BIG => ($!=7);
use constant x=>; print x - The following could (and should) be made to work:
use constant regex => qr/blah/;
print regex; - · 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 fil
tering module, even though the code that is produced
is already ordinary Perl which shouldn't be filtered
again. - · There are probably many more bugs on non-ASCII plat
- forms (EBCDIC).
AUTHOR
- Stephen McCamant <smcc@CSUA.Berkeley.EDU>, based on an
earlier version by Malcolm Beattie <mbeat
tie@sable.ox.ac.uk>, with contributions from Gisle Aas,
James Duncan, Albert Dvornik, Robin Houston, Hugo van der
Sanden, Gurusamy Sarathy, Nick Ing-Simmons, and Rafael
Garcia-Suarez.