Andy Wingo <wi...@pobox.com> writes: > On Fri 02 Mar 2012 02:35, David Kastrup <d...@gnu.org> writes: > >> Sure, but things like gensym and make-prompt-tag (and (list '()) for >> creating an eq?-unique object) are artificial hygiene coming at a cost >> in symbol table and symbol generation time rather than "lexical" >> hygiene. > > "Hygiene" is not right the word here. "Hygiene" applies lexically -- > statically. You want a continuation that only works within a certain > dynamic extent -- that's dynamic. Unique objects are well-suited to the > needs of dynamic problems.
The problem is not a unique object (you get one with every (list 0) call), the problem is a unique symbol. Of course an escape continuation works only in a dynamic context. But that does not mean that its identity needs to be represented by a globally persisting symbol. The global symbol space is a different identity space than heap equality, and it never gets garbage collected: the lifetime of a gensym is eternal. > But really, your concerns are entirely misplaced. Choose clean, > clear, optimizable abstractions. Call/ec is a good example. If you > need to implement it, do so using whatever tools are at hand. If you > can't implement it or it's slow, then bring these concerns forward. > But to dismiss Noah's suggestions out of hand is inappropriate at this > time. I am not dismissing his suggestions. I am saying that I find call/ec a nicer primitive than catch/throw exactly because it does not need a name or symbol to work but has its identity and life-time coupled to an object rather than a symbol. And frankly: the manual talks about prompts being composable and gives an example which seems utterly wrong to me since it does not actually abort a computation but rather half-finishes it. It is unclear what part of the computation will finish and what will complete. -- David Kastrup
