On Wed, Dec 01, 2010 at 05:00:51PM -0800, YC wrote: > On Wed, Dec 1, 2010 at 4:13 PM, Hendrik Boom <hend...@topoi.pooq.com> wrote: > > > > > I'm not sure I understand the question, though I suspect I may know the > > answer. Could you be a bit more specific? > > > > Yes - definitely. See below. > > > > I'm not sure what you mean by "optimized away so they won't show up on a > > call to eval". And what, spoecifically, are "lexical variables"? > > Variables that have scope determined lexically? > > > > Yes I mean variables with scopes determined lexically - I thought that's > what they are called - is there another term for them? lexically-scoped > variable/identifier, local variable?
Your terminology is OK. It's just that it's occasionally misused. > > So what I am trying to do is to build an interpreter, hopefully a > metacircular one so I can leverage racket as much as possible, but without > using the built-in eval, i.e. I need to construct my own lambda, etc. > > In this interpreter, identifiers are resolved against a custom environment > made of an immutable hash table. This works well for toplevel calls, but > when coming to lexically-scoped variables the choice isn't as certain. The immutable hash table doesn't work well for lexically scoped variables. > > The simplest approach I can think of is to put it into the same environment. > This also work nicely w.r.t. shadowing since they effective overwrite the > local copy of the environment (it's an immutable hash table so other > references are not affected). You can end up with multiple simultaneous bindings of the same variable in different contexts. Look up the Knuth Man-or-boy problem, for example, in http://en.wikipedia.org/wiki/Man_or_boy_test > > But what I've read about lexical scoping is that the variables do not really > exist since they are substituted away - they do not leave trace on the > namespace: > > (let ((a 2)) > (eval 'a)) > ;; => Error: reference to undefined identifier: a Subsituting them away is a correct, but slow way of implementing lexical scoping. It was invented in the days when mathematicians were inventing formal logic, and computers weren't around yet -- more as a way of preciely defining concepts tha a way of being practical. It had a clear meaning before side-effects became commonplace. > > > So I am trying to figure out what is a better representation. > > What I can think of so far all involve some sort of activation frame for > holding the values. Yes. > That basically means I will need to maintain my own > stack, and I was trying to see if there are simpler ways to approach the > problem. But the activation frame doesn't have to be on the stack. It could, say, be om the heap. > > Please let me know if this is still being unclear. Thanks. > yc YOu've become quite clear. I've given more details in another post in this thread. -- hendrik _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
