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 $[. Cannot be non-zero
on Perl 5.15.3 or later.
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
o In Perl 5.20 and earlier, the only pragmas to be completely
supported are: "use warnings", "use strict", "use bytes", "use
integer" and "use feature". ($[, which behaves like a pragma, is
also supported.)
Excepting those listed above, we're currently unable to guarantee
that B::Deparse will produce a pragma at the correct point in the
program. (Specifically, pragmas at the beginning of a block often
appear right before the start of the block instead.) Since the
effects of pragmas are often lexically scoped, this can mean that
the pragma holds sway over a different portion of the program than
in the input file.
o In fact, the above is a specific instance of a more general
problem: we can't guarantee to produce BEGIN blocks or "use"
declarations in exactly the right place. So if you use a module
which affects compilation (such as by over-riding keywords,
overloading constants or whatever) then the output code might not
work as intended.
This is the most serious problem in Perl 5.20 and earlier. Fixing
this required internal changes in Perl 5.22.
o Some constants don't print correctly either with or without -d.
For instance, neither B::Deparse nor Data::Dumper know how to print
dual-valued scalars correctly, as in:
use constant E2BIG => ($!=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 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 (@_) { '???' }
o 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.
o There are probably many more bugs on non-ASCII platforms (EBCDIC).
o Prior to Perl 5.22, lexical "my" subroutines were not deparsed
properly. They were emitted as pure declarations, sometimes in the
wrong place. Lexical "state" subroutines were not deparsed at all.
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.
perl v5.26.3 2023-05-18 B::Deparse(3pm)
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