On 2011-09-05, at 10:02 AM, Steve Graham wrote:

I've noticed that documents which say that Scheme is lexically scoped. Although I've tried reading up on the concept, I'm not sure that I understand it. Can someone help?

Thanks, Steve

If X is declared in some form (e.g. a let), then it is only "visible" in the body of that form (unless it is declared at a deeper level). It is "lexical" because the concept of "nesting" is purely textual (based on the source code only, not the run-time behaviour of the program).

Marc

On 5 Sep 2011, at 15:51, Raffael Cavallaro wrote:

On Sep 5, 2011, at 10:02 AM, Steve Graham wrote:

...

Can someone help?

Marc's reply is, of course, succinct and correct.

If you want a fuller understanding of scope, I've found the wikipedia article to be relatively clear and to the point: http://en.wikipedia.org/wiki/Scope_(computer_science)

It covers both lexical (a.k.a. "static") scope, and dynamic scope, and gives examples.

If you've got a C language kind of background, to a reasonable approximation, dynamic scoping is a lot like a global variable: anyone who reads or writes a given binding sees the same one. It caries similar risks to a global variable, but there are also some handy things you can do with them such as set up exception handlers. Static / lexical scoping is like the normal local variables in a function.

Hope that helps (and that if it's nonsense someone will correct me).

Cheers, B

One thing that helped me grok a little better lexical vs dynamic scope is to make a crude but perhaps useful comparison to C++:

lexical scope <--> tracing C++ method calls on non-virtual methods. You can tell trace the flow of control on paper beforehand; and know what's going to happen before running it, always.

dynamic scope <--> C++ virtual method invocation. You don't know until runtime which code (or variable value in the case of dynamic variables) are being talked about, because method dispatch (and variable access in the case of dynamic variables) depends on runtime current state.

I think dynamic variables (which are available in Common Lisp but not in most Schemes, if I understand right) and dynamic scoping are broader in application (a superset of) the facilities provided by C++ virtual methods, but it's the nearest thing in a language that you more likely familiar with.

Does that help?

Experts, feel free to improve on this comparison.

- Jason

On Mon, Sep 5, 2011 at 9:51 AM, Raffael Cavallaro raffaelcavallaro@mac.comwrote:

On Sep 5, 2011, at 10:02 AM, Steve Graham wrote:

...

Can someone help?

Marc's reply is, of course, succinct and correct.

If you want a fuller understanding of scope, I've found the wikipedia article to be relatively clear and to the point: http://en.wikipedia.org/wiki/Scope_(computer_science)

It covers both lexical (a.k.a. "static") scope, and dynamic scope, and gives examples.

hth

warmest regards,

Ralph

Raffael Cavallaro raffaelcavallaro@me.com

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