case子句与记录类型不匹配 [英] case clause doesn't match record type

问题描述

为什么 case 子句与记录类型不匹配?

Why doesn't the case clause match the record type?

(defrecord Rec [])
=> fargish.user.Rec

(def rec (->Rec))
=> #'fargish.user/rec

(case (type rec) Rec :YES)
=> IllegalArgumentException No matching clause: class fargish.user.Rec  fargish.user/eval25147 (form-init131856794870899934.clj:1)

如果您想知道，是的，case表达式和test-constant相等:

In case you're wondering, yes, the case expression and the test-constant are equal:

(= (type rec) Rec)
=> true

推荐答案

Rec 不是编译时文字.引用自 https://clojuredocs.org/clojure.core/case :

Rec is not a compile-time literal. Quoting from https://clojuredocs.org/clojure.core/case:

所有情况下的常量表达式都可以接受，包括数字，字符串，符号，关键字及其(Clojure)组合.

All manner of constant expressions are acceptable in case, including numbers, strings, symbols, keywords, and (Clojure) composites thereof.

替代品:

(cond 
  (= (type rec) Rec) :YES)
;;=> :YES
(condp = (type rec) 
  Rec :YES)
;;=> :YES

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