README.md

Notes on chapter 3

Exercise 3.42

Modify the lexical address translator and interpreter to use the trimmed representation of procedures from exercise 3.26. For this, you will need to translate the body of the procedure not (extend-senv var senv), but in a new static environment that tells exactly where each variable will be kept in the trimmed representation.

Extracting free variables of an expression isn't that hard; the hard part is how to tell value-of which variables need to be reserved after extraction from translation-of, one way is to record the original position of every free variable in nameless-proc-exp.

When extracting free variables, don't forget to consider the case like this:

let x = 3
in let f = proc (y) proc (y) -(y,x)
   in ((f 13) x)

Exercise 3.43

The translator can do more than just keep track of the names of variables. For example, consider the program:

let x = 3
in let f = proc (y) -(y,x)
   in (f 13)

Here we can tell statically that at the procedure call, f will be bound to a procedure whose body is -(y, x), where x has the same value that it had at the procedure creation site. Therefore we could avoid looking up f in the environment entirely. Extend the translator to keep track of "known procedures" and generate code that avoids an environment lookup at the call of such a procedure.

The key is to know what is a known procedure: if we can eliminate all the free variables from the body of a procedure, then that procedure can be put into where it is referred, without the necessity of taking a lengthy environment.

Don't forget to consider the case of recursive function as in exercise 3.23 and 3.25.

Exercise 3.44

In the preceding example, the only use of f is as a known procedure. Therefore the procedure built by the expression proc (y) -(y,x) is never used. Modify the translator so that such a procedure is never constructed.

With the completion of last exercise, this one is much easier. (Maybe this is the so-called inlining?)