6.001, Fall 2007—Recitation 18 — 11/7/2007 1MASSACHVSETTS INSTITVTE OF TECHNOLOGYDepartment of Electrical Engineering and Computer Science6.001—Structure and Interpretation of Computer ProgramsFall 2007Recitation 18 — 11/7/2007InterpretationInterpreter code from lecture, with only a few minor modifications. This version still uses almostno abstractions.(define (eval exp env)(cond ((number? exp) exp)((symbol? exp) (lookup exp env))((eq? (car exp) ’quote) (cadr exp))((eq? (car exp) ’cond)(eval-clauses (cdr exp) env))((eq? (car exp) ’lambda)(eval-lambda exp env))(else(apply (eval (car exp) env)(values (cdr exp) env)))))(define (apply proc args)(cond ((primitive? proc) (papply proc args))((eq? (car proc) ’procedure)(let ((parameters (cadr proc))(body (caddr proc))(env (cadddr proc)))(eval body(extend envparametersargs))))(else (error "not a proc"))))(define (eval-lambda exp env)(let ((parameters (cadr exp))(body (caddr exp)))(list ’procedure parameters body env)))(define (values exps env)(cond ((null? exps) ’())(else (cons (eval (car exps) env)(values (cdr exps) env)))))(define (eval-clauses clauses env)(cond ((null? clauses) ’())((eq? (caar clauses) ’else)(eval (cadar clauses) env))((not (eq? (eval (caar clauses) env) #f))(eval (cadar clauses) env))(else (eval-clauses (cdr clauses) env))))6.001, Fall 2007—Recitation 18 — 11/7/2007 2(define (extend base-env vars values)(cons (make-frame vars values) base-env))(define (make-frame vars vals)(cond ((null? vars) (cond ((null? vals) ’())(else (error "too many args"))))((null? vals) (error "too few args"))(else (cons (cons (car vars) (car vals))(make-frame (cdr vars) (cdr vals))))))(define (lookup var env)(cond ((null? env) (error "unbound variable" var))(else (let ((binding (assq var (car env))))(cond (binding (cdr binding))(else (lookup var (cdr env))))))))(define (assq var bindings)(cond ((null? bindings) #f)((eq? (caar bindings) var) (car bindings))(else (assq var (cdr bindings)))))(define (primitive? proc)(memq proc (list car cdr cons eq? + = > <)))(define (papply proc args)(cond ((eq? proc car) (car (car args)))((eq? proc cdr) (cdr (car args)))((eq? proc cons) (cons (car args) (cadr args)))((eq? proc eq?) (eq? (car args)))((eq? proc +) (+ (car args) (cadr args)))((eq? proc >) (> (car args) (cadr args)))))ProblemsSuppose the global environment is defined as such:(define *GE*(extend ’()’(car cdr cons eq? + >)(list car cdr cons eq? + >)))Evaluate the following, in order1. (eval ’x *GE*)6.001, Fall 2007—Recitation 18 — 11/7/2007 32. (eval ’(eval ’(> x 4)(extend *GE*’(x)(list 3)))3. (eval ’((lambda (x) (+ x 2)) 5) *GE*)4. (eval ’(cond ((> x 4) (quote a))((> 1 x) (quote b))(else (quote c)))(extend *GE*’(x)(list 2)))5. Add a new special form: begin(define (eval-begin exp env)6.001, Fall 2007—Recitation 18 — 11/7/2007 46. What other standard special forms are missing? How would you add
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