Subject: Re: [CSG111] CSG111: MP4 Extend-Env-Rec?
From: Therapon Skotiniotis (skotthe@ccs.neu.edu)
Date: Sun Feb 26 2006 - 19:55:13 EST
On Sun, Feb 26, 2006 at 05:23:20PM -0500, Wendy wrote:
> Hi,
>
> I'm having an issue understanding the difference between extend-env and
> extend-env-recursively. I'm looking at the code from lecture three, where
> both these are not procedures, but types of environments. The only
> difference I'm seeing is that extend-env-recursivily is adding a proc body,
> and extend-env does not. Is that the only difference, or does the use of
> "extend-env-recursivly" imply that recursion is occuring? i
That is one difference sure. What is important is also how apply-env deals
with extend-env-recursivily. Have a look at that.
> I'm assuming that
> the actual recursion is handled by a call to value-of on a procedure,
> thus creating a nesting of such calls to value-of. So the addition of a
> proc in an extend-env-recursivly is more or less a place holder in the
> call stack. Is this correct, or am I missing something here?
I find it easier not to think about call stacks for this. Evaluation of a
recursive call is handled just as any other call, that is we look up the
environment to find out what the operator and the operands of a call are.
The difference in a recursive call is that the operator we are looking for
is the closure that represents the function we are currently evaluating. So
we need to keep that around in the environment.
Your description seems to be on the right path.
HTH,
-- Theo
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