Static vs Dynamic Scope

转自：http://hoolihan.net/blog-tim/2009/02/17/static-vs-dynamic-scope/

// start pseudo-code
var y = "global";

function print-y() {
    print(y);
}

function test-scope() {
    var y = "local";
    print-y();
}

test-scope(); // statically scoped languages print "global"
                   // dynamically languages print "local"

print-y(); // all languages should print "global"

// end pseudo-code

This is the standard type of example used to explain what static scoping is as compared to dynamic scoping. This makes sense to me, but never really sank in.

To anyone who already gets this, this will seem trivial. But the lightbulb went off for me when I thought about static vs dynamic typing…

In a dynamically typed language (like ruby, javascript, etc), types are not checked until execution. If an expression evaluates, then the type-checking worked. If not, it blows up to your error handling or the user. Statically typed languages check types at compile time. The programmer ensures that parameter types are specified and the compiler ensures the programmers wishes will be followed.

Thinking in this fashion, static/dynamic scoping makes sense. For the following explanation, pretend that variables only have one type of storage for simplicity, and that global y is at memory location x01, while local y in test-scope is at x02.

If I’m a compiler in the act of compiling print-y (above code snippet) in a static language, then I know the scope I’m running in (hence static scope). I know that y is bound to the global variable, and I can replace that y with a direct location of x01 in the assembly I’m generating. No lookup tables, etc… fast.

If instead, I’m compling print-y in a dynamic scope, then I can make no such substitution. I’m going to make some calls to print-y that will point to x01 and others that point to x02. What y is bound to is determined by the scope of the call at runtime… which is the definition of dynamic scoping.

So that might help it click. Everything said about a stack in dynamic scoping is true, but I think it’s easier to understand that once you understand the above. Then you realize I could nest 4 or 5 of those calls and the last value of y would win.

So to sum up…

Static Scoping – Variables can be bound at compile time without regards to calling code.

Dynamic Scoping – Variable binding (context) can only be determined at the moment code is executed.

Emacs Lisp (elisp) is one of the few commonly used languages with dynamic scoping. It takes a lot of heat for that, as debugging and understanding the nuances involved place a greater burden on the developer. But I think it’s worth noting that in order to create an extensible system that can completely modify itself at runtime (via loading/reloading custom code) and allow extensions to modify extensions… the combination of a dynamic language and dynamic scoping has proved very successful.