My solution to exercise 3.34

[ShorterThanDijkstra]

; empty-env : () → Env
(define empty-env
(lambda ()
(lambda (op)
(cond
[(eqv? op 'envrionment?) #t]
[(eqv? op 'empty?) #t]
[(eqv? op 'search)
(lambda (search-var)
(eopl:error 'empty-env))]
[else (eopl:error 'empty-env)]))))

; extend-env : Var × SchemeVal × Env → Env
(define extend-env
(lambda (saved-var saved-val saved-env)
(lambda (op)
(cond
[(eqv? op 'envrionment?) #t]
[(eqv? op 'empty?) #f]
[(eqv? op 'search)
(lambda (search-var)
(if (eqv? search-var saved-var)
saved-val
(apply-env saved-env search-var)))]
[else (eopl:error 'extend-env)]))))

; extend-env : Var × Var × Expression × Env → Env
(define extend-env-rec
(lambda (proc-name bound-var proc-body saved-env)
(letrec ((env
(lambda (op)
(cond
[(eqv? op 'envrionment?) #t]
[(eqv? op 'empty?) #f]
[(eqv? op 'search)
(lambda (search-var)
(if (eqv? search-var proc-name)
(proc-val (procedure bound-var proc-body env))
(apply-env saved-env search-var)))]
[else (eopl:error 'extend-env-rec)]))))
env)))

; apply-env : Env × Var → SchemeVal
(define apply-env
(lambda (env search-var)
((env 'search) search-var)))

(define environment?
(lambda (env)
(if (procedure? env)
(env 'envrionment?)
#f)))