3 ms·
I am a Lisp newbie. Here is, effectively, the Lisp equivalent, along with the typical Lisp style guidelines: (defconstant +logging-enabled+ t) (defmacro
by Xurinos 15y ago
I am a Lisp newbie. Here is, effectively, the Lisp equivalent, along with the typical Lisp style guidelines:
(defconstant +logging-enabled+ t)
(defmacro log-message (message)
(when +logging-enabled+
`(err-say ,message)))
(log-message (crazily-expensive-computation))
Like other structures, they can be abused or used with great success. Based on my limited experience with a number of quicklisp-downloaded packages, I have found macros helpful and unnoticeable as macros.
Irony: Fewer bracing symbols.
In the next blog post, I am hoping to see how perl 6's macro feature handles the parsing of passed-in code. Specifically, I would like to see an example for do-in-reverse:
(defmacro do-in-reverse (&body commands)
`(progn ,@(reverse commands)))
(do-in-reverse
(princ "First")
(princ "Second")
(princ "Third")))
Prints "ThirdSecondFirst"
- perlgeek 15y agoThere 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 => $code.statements.reverse); } do-in-reverse { say 'First'; say 'Second'; say 'Third'; }; But it might turn out not be that simple. The diffulcty comes from the fact that unlike Lisp, in Perl 6 not everything is a list, so you can't treat the AST as a list (... of lists, possibly) either. The .clone would take care to preserve things like outer lexicals and the signature of the block (you don't see it; it's implicit) and other non-listy things.
- draegtun 15y agoSo potentially not to different to how you might do it in Io (www.iolanguage.com): doInReverse := method ( m := call argAt(0) stmts := list() // list of statements (ie. messages) loop ( rest := m next // rest of messages after current stmts append(m setNext) // get current message m := rest if (m == nil, break) // exhausted statements when "nil" ) stmts reverseForeach (n, doMessage(n)) ) doInReverse( writeln("First") writeln("Second") writeln("Third") ) Of course this is easy in Io because everything is just a message. NB. The above code doesn't actually amend the AST but it could. Here are some previous examples of amending AST in Io posted here: http://news.ycombinator.com/item?id=1810480 http://news.ycombinator.com/item?id=1810480 http://news.ycombinator.com/item?id=1804599 http://news.ycombinator.com/item?id=1804599
- Xurinos 15y agoI like your proposal, especially if it could be expanded a bit: macro do_in_reverse($value, @code) ...