1. Effect-driven CPS transformation in interpreters
Dries Harnie

2. Table assignment struct { EXP tab,idx,val; } TAB;
VAL tbl_assign(TAB tAB) {
VAL tab = eval(tAB->tab);
VAL idx = eval(tAB->idx);
VAL val = eval(tAB->val);
tab[idx] = val; }

3. Tpico: thunks

4. Table assignment in Tpico
tab_thunk(TAB tAB) {
TMP tmp = new tmp;
tmp->tAB = tAB;
thr_poke(ta1_thunk, tmp);
eval(tAB->tab); }
ta1_thunk(TMP_TAB tmp, VAL tab) {
tmp->tab = tab;
thr_poke(ta2_thunk, tmp);
eval(tmp->tAB->idx);
ta2_thunk(TMP_TAB tmp, VAL idx) {
tmp->idx = idx;
thr_poke(ta3_thunk, tmp);
eval(tmp->tAB->val); }
ta3_thunk(TMP_TAB tmp, VAL val) {
tmp->val = val;
thr_zap();
(tmp->tab)[tmp->idx] = tmp->val;

5. The perils of garbage collection
struct {
EXP tab,idx,val;
} TAB;
VAL tbl_assign(TAB tAB) {
VAL tab = eval(tAB->tab);
VAL idx = eval(tAB->idx);
VAL val = eval(tAB->val);
tab[idx] = val; }
GC can happen at
any of these points!

6. Thunkmaster Looks like Scheme: (define (map f l) (case l
((cons x xs) (cons (f x) (map f xs)))
((nil) (nil))))
(define (fac x)
(if (< x 1) 1
(* x (fac (- x 1)))))
(define (eval-tbl-assign TAB)
(let* ((tab (eval (get-tab TAB)))
(idx (eval (get-idx TAB)))
(val (eval (get-val TAB))))
(set-tab! tab idx val)))
(define (thr-loop val)
(let ((thk (current-thunk)))
(thr-loop (thk val))))

7. Thunkmaster Has effects:
(define (map f l)
(case l
((cons x xs) (cons (f x) (map f xs)))
((nil) (nil))))
Effects associate computations with side effects.
If f ! print, then (map f l) ! print.

8. Thunkifying code (define (eval-tbl-assign TAB)
(let* ((tab (eval (get-tab TAB)))
(idx (eval (get-idx TAB)))
(val (eval (get-val TAB))))
(set-tab! tab idx val)))
(define (eval-tbl-assign TAB)
(eval (get-tab TAB) (lambda (tab)
(eval (get-idx TAB) (lambda (idx)
(eval (get-val TAB) (lambda (val)
(set-tab! tab idx val))))))))

9. Thunkifying code (define (eval-tbl-assign TAB)
(eval (get-tab TAB) (lambda (tab)
(eval (get-idx TAB) (lambda (idx)
(eval (get-val TAB) (lambda (val)
(set-tab! tab idx val))))))))
(define (eval-tbl-assign TAB)
(defstruct (tmp tab idx))
(define (ta1 tmp tab)
(thr-poke! ta2 (tmp-set-tab! Tmp tab))
(eval (get-idx TAB)))
(define (ta2 tmp idx)
(thr-poke! ta3 (tmp-set-idx! Tmp idx))
(eval (get-val TAB)))
(define (ta3 tmp val)
(thr-zap!)
(set-tab! (tmp-get-tab tmp) (tmp-get-idx tmp) val)))
(thr-poke! ta1 (new tmp))
(eval (get-tab TAB)))

10. Thunkifying code (defstruct (tmp TAB tab idx)) (define (ta1 tmp tab)
(thr-poke! ta2 (tmp-set-tab! Tmp tab))
(eval (get-idx (tmp-get-TAB tmp))))
(define (ta2 tmp idx)
(thr-poke! ta3 (tmp-set-idx! Tmp idx))
(eval (get-val (tmp-get-TAB tmp))))
(define (ta3 tmp val)
(thr-zap!)
(set-tab! (tmp-get-tab tmp) (tmp-get-idx tmp) val)))
(define (eval-tbl-assign TAB)
(thr-poke! ta1 (tmp-set-TAB! (new tmp) TAB))
(eval (get-tab TAB)))

11. Thesis: goals Use effect system to determine where CPS is needed.
Find other places
where CPS could help Tpico.