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Clojure core.match cannot match class

On dodging this super simple core.match expression, I get:

(match [(class "3.14")]
       [Integer] "Integer"
       [Double] "Doubler")
; => "Integer"

How can this be correct, I am missing something principled about core.match? Doing macro-instance-1 on this form gives me:

=> (clojure.core/let [ocr-2751 (class "3.14")] (clojure.core/let [Integer ocr-2751] "Integer"))

Any pointers appreciated.

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2 answers


Like @Arthur, it core.match

will usually associate values ​​with symbols. However, apparently, he first tries to match with the locals . Who knew?

Anyway, bind the classes as locals in let

before mapping, and you will find it helpful:



(let [Integer java.lang.Integer
      String  java.lang.String]
  (match [(class "3.14")]
         [Integer] "Integer"
         [String] "String"))
+7


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core.match allows you to assign a name to one of the values ​​in a match clause like this (from examples )

(let [x 1 y 2]
  (match [x y]
    [1 b] b
    [a 2] a
    :else nil))

In this example, if the first matching value is one, then in the expression used to generate the result, the second value will be available under the name b

.

Since any character in the match clause is interpreted as an instruction to bind the corresponding value to that name, in your case the name is Integer

bound to the valuejava.lang.String 



user> (match [(class "3.14")]
             [Integer] Integer
             [Double] "Doubler")
java.lang.String

user> (match [(class "3.14")]
             [name-to-bind] name-to-bind
             [Double] "Doubler")
java.lang.String

It is not clear from the documentation that there is a way to use core.match to evaluate the match clause instead of binding to it. It is possible to work around this by matching against a string, although it loses some of the elegance:

user> (match [(str (class (int 3)))]
             ["class java.lang.Integer"] "Integer"
             ["class java.lang.String"] "String"
             ["class java.lang.Double"] "Double")
"Integer"
user> (match [(str (class "3.14"))]
             ["class java.lang.Integer"] "Integer"
             ["class java.lang.String"] "String"
             ["class java.lang.Double"] "Double")
"String"
user> (match [(str (class 3.14))]
             ["class java.lang.Integer"] "Integer"
             ["class java.lang.String"] "String"
             ["class java.lang.Double"] "Double")
"Double"
+7


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