6.001 recitation 18 4/25/07interpretationour evaluatorDr. Kimberle Koiletrue*interpretation key ideas"(average 40 (+ 5 5))"25"25"input to each stagestages of an interpretereval: dispatch on expression typeapply: eval args then apply operatorEvalproc & argsexp &envApplyour evaluator; the initial global environment(define GE(extend-env-with-new-frame(list 'plus* 'greater*)(list (make-primitive +) (make-primitive >))'( )))(define (eval exp env)(cond((number? exp) exp)((symbol? exp) (lookup exp env))((define? exp) (eval-define exp env))((if? exp) (eval-if exp env))((lambda? exp) (eval-lambda exp env))((let? exp) (eval-let exp env))((application? exp)(apply* (eval (car exp) env)(map (lambda (e) (eval e env))(cdr exp))))(else(error "unknown expression " exp))))Evalproc & argsexp &envApplyour evaluator(define (eval exp env)(cond…((if? exp) (eval-if exp env))…))our evaluator(define (eval exp env)(cond…((if? exp) (eval-if exp env))…))(define (if? exp) (tag-check exp 'if*))(define (eval-if exp env)(let ((predicate (cadr exp))(consequent (caddr exp))(alternative (cadddr exp)))(let ((test (eval predicate env)))(cond((eq? test #t) (eval consequent env))((eq? test #f) (eval alternative env))(else (error "predicate not a conditional: "predicate))))))our evaluatorwhen1. (quote* expr) returns expr without evaluating it. Assume eval calls eval-quote if the procedure quote? is true for a given quote* statement. Write eval-quote, which takes one argument.quote*2. (eval-sequence exps env) evaluates each expression is a list of expressions, and returns the value of the last one. Assume eval calls eval-sequence if the procedure sequence? is true for a given expression. Write eval-sequence, which takes two arguments, expr and env.eval-sequence3. (case* expr((val1 val2 …) consequent)((vali valj …) consequent) …(else* alternative))Case* evaluates expr and compares its value (using eqv?) against each of the listed values, which are not evaluated. When a match is found, the corresponding consequent expression is evaluated and returned as the result of the case*. If no matches are found, the alternate expression is evaluated and returned instead. You can assume the else* clause is required if you like.Assume eval calls eval-case if the procedure case? is true for a given case* statement.(define (eval-case exp env)(let ((target-value (eval (second exp) env)))(eval-case-clauses target-value (cddr exp) env)))On the next slide, write eval-case-clauses, which takes three arguments: a target-value, a list of clauses, and env.case*3. (case* expr((val1 val2 …) consequent)((vali valj …) consequent) …(else* alternative))Write eval-case-clauses.(define (eval-case exp env)(let ((target-value (eval (second exp) env)))(eval-case-clauses target-value (cddr exp) env)))case* (cont'd)4. (begin* expr1 expr2 … exprn) evaluates each expression in the sequence, returning the value of exprn as its final result. Assume eval calls eval-begin if the procedure begin? returns true for a given begin* statement.(define (begin? exp) (tag-check exp 'begin*))(define (eval-begin exp env) (eval-begin-body (cdr exp) env))Write eval-begin-body, which takes two arguments, body and
View Full Document
Unlocking...