It was just there for a brief pause between the macroexpansions of declare and undeclare.
If we want to change the behavior of other macros for a certain region of code, then that pattern might be useful. Since we seem to be talking about static type declarations, which I presume would be taken into account at macro-expansion time, I think the "between the macroexpansions" behavior is the whole point.
Thank you for the insight. It's probably the most lucid I've been all thread. It didn't seem deliberate to me, but it could have feasibly been written that way to control other macros' expansions. This also pushes computation to expansion time, which might clarify ylando's objections about "wasting run time". Except those still confuse me: macro expansion happens once, inside a function's body or outside of it.
arc> (mac m (expr)
(prn "macro m has expanded")
expr)
#(tagged mac #<procedure: m>)
arc> (def f (x)
(m (+ x 1)))
macro m has expanded
#<procedure: f>
arc> (f 1)
2
But the original point seems lost because declare's story keeps changing. So, ylando: why do we need "ignores"?
Try building a macro that change global value,
expand code (with macros) and then change the value back.
I think that this macro must use another macro to
change the value back; like the undeclare macro above.
The second macro expands into unnecessary code; so
if you put it inside a function this unnecessary code
will waste run time.
If we have "ignore" macro, we can write macros that do not
produce unnecessary code.
This introduces a redundant nil in the after block, and using after is a bit slower than just a do1. But we can't use do1 because this "do all the work at macro-expansion" approach is so touchy that it breaks:
arc> (load "macdebug.arc") ; see http://arclanguage.org/item?id=11806
nil
arc> (macwalk '(declare name prop a b c))
Expression --> (declare name prop a b c)
macwalk> :s
Macro Expansion ==>
(do1 (do a b c)
(undeclare name nil))
macwalk> :s
Macro Expansion ==>
(let gs2418 (do a b c)
(undeclare name nil)
gs2418)
macwalk> :s
Macro Expansion ==>
(with (gs2418 (do a b c))
(undeclare name nil)
gs2418)
macwalk> :s
Macro Expansion ==>
((fn (gs2418)
(undeclare name nil)
gs2418)
(do a b c))
macwalk> :s
Subexpression -->
(fn (gs2418)
(undeclare name nil)
gs2418)
macwalk> :s
Subexpression --> (undeclare name nil)
macwalk> :s
Value ==> nil
Value ==> gs2418
Value ==> (fn (gs2418) nil gs2418)
Subexpression --> (do a b c)
macwalk> :a
Value ==> (do a b c)
Value ==>
((fn (gs2418) nil gs2418) (do a b c))
((fn (gs2418) nil gs2418) (do a b c))
Note that we reach undeclare before the actual body is expanded!
We can hack it without after or do1 (or mutation, but I avoid that anyway).
This way, declare expands in the right order and we only undeclare once, since it'll expand into nil. The nil is "unnecessary", which seems to be why you want ignore, but it's a terribly pedantic point: ignore is already accomplished by dead code elimination (http://en.wikipedia.org/wiki/Dead_code_elimination). This isn't even a case of "sufficiently smart compilers" for vanilla Arc, since mzscheme already implements the standard optimizations: function inlining, dead code elimination, constant propagation/folding, etc. (see http://download.plt-scheme.org/doc/html/guide/performance.ht...) should all be able to clean up whatever ac.scm generates. E.g.,
(mac foo ()
`(prn ',metadata*!name))
(declare name bar (foo))
Final idea: if expansion-time computation can't be avoided, you can expand the macros manually, if only for the sake of your readers. As a bonus, it does away with the dead code.
