Interned and unreadable symbols are only weakly held by
the internal symbol table. This weakness can never
affect the result of an eq?, eqv?, or equal? test, but a
symbol may disappear when placed into a weak box (see
Weak Boxes) used as the key in a weak hash table (see
Hash Tables), or used as an ephemeron key (see Ephemerons).
Kinda. It irks me that a value that's still arguably reachable (via 'read) could be observably lost by a weak box, weak hash table, or ephemeron. But it seems feasible to create readable-counts-as-reachable versions of these data structures on top of the built-in ones, so the built-in ones expose strictly better lower-level control.
It could return the list (1 2), but this would mean to cons up a new list every time we use 'pr just to throw it away. Multiple return values would solve the problem. Why not add them to the language?
> How much more expensive is cons compared to multiple return values?
It depends on the GC used and on the implementation. mrvs are usually pushed on the stack, so there is no GC overhead. When I develop applications with Lisp, I notice that most of the time reducing the amount of garbage speeds up the program noticeably. It's not only a problem of consing, it's also a problem of collecting the garbage. More garabage, more collection cycles.
