模式匹配

使用 match 表达式优雅地解构和处理数据。

match 基础

#lang racket

(require racket/match)

(define (describe x)
  (match x
    [0         "zero"]
    [1         "one"]
    [(? string?) "a string"]
    [(? number?) "a number"]
    [_          "something else"]))

(describe 0)        ; => "zero"
(describe "hello")  ; => "a string"
(describe '(1 2))   ; => "something else"

解构列表

(define (head-tail lst)
  (match lst
    ['()         "empty"]
    [(list x)    (format "one element: ~a" x)]
    [(list x y)  (format "two elements: ~a and ~a" x y)]
    [(list x xs ...) (format "first: ~a, rest: ~a" x xs)]))

(head-tail '())       ; => "empty"
(head-tail '(1))      ; => "one element: 1"
(head-tail '(1 2))    ; => "two elements: 1 and 2"
(head-tail '(1 2 3))  ; => "first: 1, rest: (2 3)"

解构结构体

(struct point (x y) #:transparent)

(define (quadrant p)
  (match p
    [(point 0 0) "origin"]
    [(point x 0) (format "on x-axis at ~a" x)]
    [(point 0 y) (format "on y-axis at ~a" y)]
    [(point x y) (format "(~a, ~a)" x y)]))

守卫条件

(define (classify n)
  (match n
    [(? (lambda (x) (> x 0))) "positive"]
    [(? (lambda (x) (< x 0))) "negative"]
    [_ "zero"]))

小结

模式匹配让数据解构变得直观和安全。它是替代嵌套 if 和 cond 的利器。