There isn't yet any API for introspecting ASTs ("parsing of passed-in code" as you call it), nor for programmatically constructing new AST nodes. Such things will hopefully emerge once macro implementation and specification become more concrete.
So I can only guess what it would look like; take the following with a rock of salt:
macro do-in-reverse($code) {
$code.clone(statements => $code.statements.reverse);
}
do-in-reverse {
say 'First';
say 'Second';
say 'Third';
};
But it might turn out not be that simple.
The diffulcty comes from the fact that unlike Lisp, in Perl 6 not everything is a list, so you can't treat the AST as a list (... of lists, possibly) either. The .clone would take care to preserve things like outer lexicals and the signature of the block (you don't see it; it's implicit) and other non-listy things.
So potentially not to different to how you might do it in Io (www.iolanguage.com):
doInReverse := method (
m := call argAt(0)
stmts := list() // list of statements (ie. messages)
loop (
rest := m next // rest of messages after current
stmts append(m setNext) // get current message
m := rest
if (m == nil, break) // exhausted statements when "nil"
)
stmts reverseForeach (n, doMessage(n))
)
doInReverse(
writeln("First")
writeln("Second")
writeln("Third")
)
Of course this is easy in Io because everything is just a message.
Comments
There isn't yet any API for introspecting ASTs ("parsing of passed-in code" as you call it), nor for programmatically constructing new AST nodes. Such things will hopefully emerge once macro implementation and specification become more concrete.
So I can only guess what it would look like; take the following with a rock of salt:
But it might turn out not be that simple.The diffulcty comes from the fact that unlike Lisp, in Perl 6 not everything is a list, so you can't treat the AST as a list (... of lists, possibly) either. The .clone would take care to preserve things like outer lexicals and the signature of the block (you don't see it; it's implicit) and other non-listy things.
So potentially not to different to how you might do it in Io (www.iolanguage.com):
Of course this is easy in Io because everything is just a message.NB. The above code doesn't actually amend the AST but it could. Here are some previous examples of amending AST in Io posted here: http://news.ycombinator.com/item?id=1810480 http://news.ycombinator.com/item?id=1804599
I like your proposal, especially if it could be expanded a bit: