CS 5510 Homework 4

Due: Tuesday, September 24th, 2013 11:59pm

Part 1 — Improving Assignment

Start with store-with.rkt. In the starting program, the representation of the store grows every time that a box’s content is modified with set-box!. Change the implementation of set-box! so that the old value of the box is dropped (i.e., replaced with the new value) instead of merely hidden by the outside-in search order of fetch.

Example:

  (test (interp (parse '{let {[b {box 1}]}
                          {begin
                           {set-box! b 2}
                           {unbox b}}})
                mt-env
                mt-store)
        (v*s (numV 2)
             (override-store (cell 1 (numV 2))
                             mt-store)))

Part 2 — Sequences

Generalize begin to allow one or more sub-expressions, instead of exactly two sub-expressions.

  <Expr> = ...
         | {begin <Expr> <Expr>*}

Example:

  (test (interp (parse '{let {[b {box 1}]}
                          {begin
                           {set-box! b {+ 2 {unbox b}}}
                           {set-box! b {+ 3 {unbox b}}}
                           {set-box! b {+ 4 {unbox b}}}
                           {unbox b}}})
                mt-env
                mt-store)
        (v*s (numV 10)
             (override-store (cell 1 (numV 10))
                             mt-store)))

Midterm Practice

See also the practice mid-term (but it’s not to hand in).