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A Note on Environments, frames, bindings, etc.
A frame is just a table. A variable is just an ENTRY (or place) in the
table. E.g., if we have a frame like this one
x: 1
y: 2
we can think of x and y as labelling the entries in the table, in which
are written 1 and 2. The places where 1 and 2 live are the variables.
When we say that we bind or rebind the variable x, we mean that we
create an entry for it in new frame we are using to extend the
environment. "set!" simply puts a new value in the table entry
corresponding to a given name.
All this terminology may be confusing because, in the book, they tend to
confuse variables and names. That is, they say ``create a new binding
for the variable x'', while it would be more accurate to say ``create a
new place (i.e., variable) named x and give it a value''. (Note that if
you just do a "set!", no one (including A&S) says that we have created a
new binding for x; rather, everyone would say that the binding has
changed. What's really changed is simply the value of the variable
(i.e., the value in the place) corresponding to x in the frame; the
association between x and this place remains unchanged.
Similarly, when they say, ``the variable x is bound to 4 in this
frame'', it would be more accurate to say ``the new variable
corresponding to x in this frame has the value 4''. The difference is
that the first expression gives that impression that there is only one
variable x, and that it is rebound each time a function is called that
uses x as a formal parameter, whereas it is more accurate to say that
there are many variables corresponding to x, each one created when such
a function is called.
It's important to emphasize that most of the time, the extensions to
environments that are created are thrown away after their use, i.e.,
when the call to the function that created them finishes. This is
emphatically NOT the case when the call to a function returns a newly
created function. That is the one instance in which a frame will
persist, even though the function call that created it has ceased to
exist.
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