(use-modules (ice-9 match)
             (srfi srfi-64))

(define (meaning expr)
  (match expr
    [`(lambda (,var) ,expr)
     (lambda (env)
       (lambda (e)
         ((meaning expr) (rename env e var))))]
    [(exp0 exp1)
     (lambda (env)
       (((meaning exp0) env) ((meaning exp1) env)))]
    [var (lambda (env) (env var))]))

(define (rename env var e)
  (let ([v (env var)])
    (lambda (var)
      (if (eq? e var)
          v
          (env var)))))

;; Okay this isn't really that useful
(define (echo-env)
  (lambda (x) x))
(test-begin "echo-env")
(test-eq 'x ((meaning 'x) (echo-env)))
(test-eq 'y ((meaning '((lambda (x) x) y)) (echo-env)))
(test-end "echo-env")

(define (successor-env)
  (lambda (x)
    (cond
     [(number? x) x]
     [else (lambda (x) (+ x 1))])))
(test-begin "successor-env")
(test-eqv 1 ((meaning '(((lambda (f) (lambda (x) (f x))) succ) 0)) (successor-env)))
(test-eqv 2 ((meaning '(((lambda (f) (lambda (x) (f (f x)))) succ) 0)) (successor-env)))
(test-end "successor-env")