# SICP Solutions

### Chapter 4, Metalinguistic Abstraction

#### Exercise 4.23

This is interesting.

Eva already explained it better than I can explain :)

The main difference is that loop in Alyssa’s version is executed at the time of execution but in the book’s version loop gets executed during analyze.

To understand it simply, let’s say we have 2 expressions e1, e2.

What book’s version will give after analyze:

What if we have 3 expression’s e1, e2, e3, then book’s version will give:

Now, note what will happen during analyze in Alyssa’s code:

In both cases(two expressions or 3 expressions), it will return a lambda procdure that will loop through the sequence/expressions. This looping does not happen in analysis phase! This looping will happen in the execution phase instead.

But…

During execution, even if there is no looping, there are still extra invocations of the lambda which were not there in Alyssa’s version. Does these extra ‘lambda’ will incur less cost than the looping?

Initially, I thought that number of lambda invocations in book’s version is small because we are combining two in sequentially but if we look closely(try 4, 5, or more expressions) it becomes clear that number of these lambda is always = number of expressions - 1.

So, the other possible reason is in the book’s version there are no extra expressions like null?, cond, car, cdr etc. In Alyssa’s version we have these extra expressions in every invocation of the loop.