作为我的 previous question 的后续行动,我正在尝试在 Clojure 中实现一个简单的模式匹配。
我想要以下内容:
(match target
[ub] expr1 ; ub should be bound to actual value in expr1
['< ub] expr2 ; match the literal less-than symbol
; and ub should be bound to actual value in expr2
[lb ub] expr3 ; lb and ub should be bound to actual values in expr3
:else expr4 ; default case if none match
)
用法:
(match [< 5.0] ...)
应该安排执行
expr2
在运行时。我想写一个宏,但我不确定扩展。
我正在考虑将每个 case-and-clause 扩展为
let
绑定(bind)到内部变量并检查文字符号 ( '<
) 是否实际匹配模式。也许对于第二种模式( ['< ub]
):(let [[sym1 ub] pattern]
(if (= '< sym1)
expr1)
我需要使用
(gensym)
对于绑定(bind)?如何?大图:
(range-case target
[0.0 < 1.0] :greatly-disagree
[< 2.0] :disagree
[< 3.0] :neutral
[< 4.0] :agree
[5.0] :strongly-agree
42 :the-answer
:else :do-not-care)
我正在尝试匹配
[...]
模式并将它们转换为以下内容:[ub] (if previous-ub `(and (<= ~previous-ub ~target) (<= ~target ~ub))
`(< ~target ~ub))
['< ub] (if previous-ub `(and (<= ~previous-ub ~target) (< ~target ~ub))
`(< ~target ~ub))
[lb ub] `(and (<= ~lb ~target) (<= ~target ~ub))
['< lb ub] `(and (< ~lb ~target) (<= ~target ~ub))
[lb '< ub] `(and (<= ~lb ~target) (< ~target ~ub))
['< lb '< ub] `(and (< ~lb ~target) (< ~target ~ub))
我有一个
cond
检查 case 部分是否为向量。这种模式匹配应该发生在这种情况下。
最佳答案
我的第一个想法基本上是一样的:将东西绑定(bind)到内部本地人并在一个大的 and
中测试它们的内容。 .对于文字,该值绑定(bind)到生成的本地;符号直接在绑定(bind)中使用。
我还添加了检查规范向量是否匹配目标向量的长度。否则你不能拥有 [ub]
以及 [lb ub]
因为两者都不包含可能失败的检查。所以总是会选择第一个。
这是代码:
(defn make-clause
[expr-g [spec expr & more :as clause]]
(when (seq clause)
(let [tests-and-bindings (map (fn [x]
(if-not (symbol? x)
(let [x-g (gensym "x")]
[`(= ~x ~x-g) x-g])
[nil x]))
spec)
tests (keep first tests-and-bindings)
bindings (map second tests-and-bindings)]
`(let [[~@bindings] ~expr-g]
(if (and (= (count ~expr-g) ~(count spec)) ~@tests)
~expr
~(make-clause expr-g more))))))
(defmacro match
[expr & clauses]
(let [expr-g (gensym "expr")]
`(let ~[expr-g expr]
~(make-clause expr-g clauses))))
和一个示例扩展。我没有在示例中使用语法引用来减少扩展中的噪音,但您应该明白这一点。
(let [expr98 [(quote <) 3.0]]
(let [[ub] expr98]
(if (and (= (count expr98) 1))
(if previous-ub
(and (<= previous-ub target) (<= target ub))
(< target ub))
(let [[x99 ub] expr98]
(if (and (= (count expr98) 2) (= (quote <) x99))
(if previous-ub
(and (<= previous-ub target) (< target ub))
(< target ub))
(let [[lb ub] expr98]
(if (and (= (count expr98) 2))
(and (<= lb target) (<= target ub))
nil)))))))
调用是:
(match ['< 3.0]
[ub] (if previous-ub
(and (<= previous-ub target) (<= target ub))
(< target ub))
['< ub] (if previous-ub
(and (<= previous-ub target) (< target ub))
(< target ub))
[lb ub] (and (<= lb target) (<= target ub))))
希望能帮助您入门。
关于macros - Clojure 简单模式匹配,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/6348135/