One option is to make it illegal to mutate rest arguments.
Currently, in arc.arc, list is defined as
(def list args args)
so this would need to change if rest arguments couldn't be mutated. (Which would be OK with me, I can't think of a single case where I ever mutated a rest argument).
Another is to hook on the fact that rest arguments are not modified often -- ... they can be converted only when needed
How could this possibly work??
((fn args
(= (cadr args) 'X)
args)
'a 'b 'c)
sure, it would be easy for set-cdr! to see that the second pair is immutable and decide to create a new mutable cons. But args is still going to be pointing to the original list of immutable pairs!
As a side benefit it can be used to deal with nil too, which will make that run faster as well.
Can you explain? (I need smaller steps to be able to follow you :-)
(BTW, this will be much faster than using r6rs or r5rs modes.)
Yes, I was getting that impression browsing through the r6rs and r5rs code.
The way I'm leaning right now is to first rewrite the Arc compiler to generate PLT 4 code in the simplest possible way, for example to always convert rest arguments to mutable lists. Then, things like making rest arguments immutable could be done as an optimization if desired.
For the "on-demand" conversion some PLT magic will be needed -- I'm basically talking about doing something at the level of the PLT function that is the result of compiling an arc function. In any case, explaining more through this medium will be hard for me...
