I’ve implemented the `amb`

evaluator fully before starting with section 4.3 – it can be downloaded here

#### Exercise 4.50

I will use the existing `amb`

, passing it a shuffled list of choices:

(defun ramb? (exp) (tagged-list? exp 'ramb)) (defun analyze-ramb (exp) (analyze-amb (cons 'ramb (shuffle-list (amb-choices exp)))))

`shuffle-list`

is this naive^{1} procedure:

(defun shuffle-list (lst) (sort lst #'(lambda (x y) (zerop (random 2)))))

And finally, this has to be added to the `cond`

in `analyze.`

:

((ramb? exp) (analyze-ramb exp))

#### Exercise 4.51

Adding this to the `cond`

in `analyze.`

:

((permanent-set? exp) (analyze-permanent-set exp))

And this is the implementation:

(defun permanent-set? (exp) (tagged-list? exp 'permanent-set!)) (defun analyze-permanent-set (exp) (let ((var (assignment-variable exp)) (vproc (analyze. (assignment-value exp)))) (lambda (env succeed fail) (funcall vproc env (lambda (val fail2) (set-variable-value! var val env) (funcall succeed 'ok (lambda () (funcall fail2)))) fail))))

Note that it’s very similar to analyze-assignment, except taht it doesn’t roll back the old value in the fail continuation passed to `vproc`

.

#### Exercise 4.52

(defun analyze-if-fail (exp) (let ((pproc (analyze. (if-predicate exp))) (cproc (analyze. (if-consequent exp)))) (lambda (env succeed fail) (funcall pproc env (lambda (pred-value fail2) (if (true? pred-value) pred-value (funcall fail))) (lambda () (funcall cproc env succeed fail))))))

With the usual additions to the evaluator:

(defun if-fail? (exp) (tagged-list? exp 'if-fail))

And into `analyze.`

:

((if-fail? exp) (analyze-if-fail exp))

#### Exercise 4.53

It prints:

((8 35) (3 110) (3 20))

Although the `let`

form always fails (it calls `(amb)`

as its last statement), the pairs get added into `pairs`

, because `permanent-set!`

doesn’t roll assignments back from failed paths.

#### Exercise 4.54

(defun analyze-s-require (exp) (let ((pproc (analyze. (s-require-predicate exp)))) (lambda (env succeed fail) (funcall pproc env (lambda (pred-value fail2) (if (not (true? pred-value)) (funcall fail) (funcall succeed 'ok fail2))) fail))))

^{1} It’s naive because it’s inefficient and doesn’t produce a perfect shuffle. Rather, the shuffle depends on the sorting algorithm. However, for our needs here, this shuffle is fine.

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