Racket[1]

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1.生成可执行文件
DrRacket->Racket->Creat Executable…
2.λ符号通过Insert插入
3.Identifiers 标志符
( ) [ ] { } ” , ’ ` ; # | \ 除这几个字符以外都可以；
4.函数调用

#lang racket
(require racket/base)
(define(double x)
  ((if(string? x)string-append +) x x))
  //(if(string? x)string-append +)  evaluates to a function

> (1 2 3 4)
application: not a procedure;
 expected a procedure that can be applied to arguments
  given: 1
  arguments...:
   2
   3
   4
   //Syntactically, the first expression in a function call could even be a number—but that leads to an error, since a number is not a function.

5.局部绑定

(define (converse s)
  (define (starts? s2) ; local to converse
    (define len2 (string-length s2))  ; local to starts?
    (and (>= (string-length s) len2)
         (equal? s2 (substring s 0 len2))))
  (cond
   [(starts? "hello") "hi!"]
   [(starts? "goodbye") "bye!"]
   [else "huh?"]))

> (converse "hello!")
"hi!"
> (converse "urp")
"huh?"
> starts? ; outside of converse, so...
starts?: undefined;
 cannot reference an identifier before its definition
  in module: top-level
  internal name: starts?

编程：比较两个随机数的大小

(let ([x (random 4)]
        [o (random 4)])
    (cond
     [(> x o) "X wins"]
     [(> o x) "O wins"]
     [else "cat's game"]))