https://youtu.be/OyfBQmvr2Hc

the-most-beautiful-program.rkt

#lang racket/base

(require racket/list
         racket/match)

; The call-by-value lambda calculus:
(define (eval-expr expr env)
  (match expr
    [(? symbol?)
     (env expr)]
    [`(lambda (,x) ,body)
     (λ (arg)
       (eval-expr body (λ (y)
                         (if (eq? x y)
                             arg
                             (env y)))))]
    [`(,rator ,rand)
     ((eval-expr rator env)
      (eval-expr rand env))]))