B::Generate 1.06 review

B::Generate is a Perl module which you can create your own op trees. SYNOPSIS use B::Generate; # Do nothing, slowly.

License: Perl Artistic License File size: 12K Developer: Simon Cozens

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B::Generate is a Perl module which you can create your own op trees.


SYNOPSIS

use B::Generate;
# Do nothing, slowly.
CHECK {
my $null = new B::OP("null",0);
my $enter = new B::OP("enter",0);
my $cop = new B::COP(0, "hiya", 0);
my $leave = new B::LISTOP("leave", 0, $enter, $null);
$leave->children(3);
$enter->sibling($cop);
$enter->next($cop);
$cop->sibling($null);
$null->next($leave);
$cop->next($leave);

# Tell Perl where to find our tree.
B::main_root($leave);
B::main_start($enter);
}

WARNING

This module will create segmentation faults if you don't know how to use it properly. Further warning: sometimes I don't know how to use it properly.

There are lots of other methods and utility functions, but they are not documented here. This is deliberate, rather than just through laziness. You are expected to have read the Perl and XS sources to this module before attempting to do anything with it.
Patches welcome.

Malcolm Beattie's B module allows you to examine the Perl op tree at runtime, in Perl space; it's the basis of the Perl compiler. But what it doesn't let you do is manipulate that op tree: it won't let you create new ops, or modify old ones. Now you can.
Well, if you're intimately familiar with Perl's internals, you can.

B::Generate turns B's accessor methods into get-set methods. Hence, instead of merely saying

$op2 = $op->next;

you can now say

$op->next($op2);

to set the next op in the chain. It also adds constructor methods to create new ops. This is where it gets really hairy.

new B::OP ( type, flags )
new B::UNOP ( type, flags, first )
new B::BINOP ( type, flags, first, last )
new B::LOGOP ( type, flags, first, other )
new B::LISTOP ( type, flags, first, last )
new B::COP ( flags, name, first )

In all of the above constructors, type is either a numeric value representing the op type (62 is the addition operator, for instance) or the name of the op. ("add")
(Incidentally, if you know about custom ops and have registed them properly with the interpreter, you can create custom ops by name: new B::OP("mycustomop",0), or whatever.)

first, last and other are ops to be attached to the current op; these should be B::OP objects. If you haven't created the ops yet, don't worry; give a false value, and fill them in later:

$x = new B::UNOP("negate", 0, undef);
# ... create some more ops ...
$x->first($y);

In addition, one may create a new nextstate operator with

newstate B::op ( flags, label, op)

in the same manner as B::COP::new - this will also, however, add the lineseq op.
Finally, you can set the main root and the starting op by passing ops to the B::main_root and B::main_start functions.

This module can obviously be used for all sorts of fun purposes. The best one will be in conjuction with source filters; have your source filter parse an input file in a foreign language, create an op tree for it and get Perl to execute it. Then email me and tell me how you did it. And why.

Requirements:
Perl