Re: class and case

[Timothy Baldridge]

Using a protocol is probably the optimal choice, since it also opens up the
dispatch allowing for extension without modifying exiting code. Downstream
users of your library can add extensions to your code without having to
create a patch.



On Sat, May 13, 2017 at 7:43 AM, Alex Miller  wrote:

> Or case on (.getName the-class).
>
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Re: class and case

[Alex Miller]

Or case on (.getName the-class).

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Re: class and case

[John Mastro]

Kevin Kleinfelter <kleinfelter.gro...@gmail.com> wrote:
> Can someone help me get un-stuck with a problem of using class in a case?
>
> This returns true, as I expect:
> (= (type "x") java.lang.String)
>
> This returns an error, and I'd hoped it would print STRING:
> (case (type "x")
>  class java.lang.String (println "STRING")
>  class java.lang.Long (println "NUM"))
>
> Because the error said "No matching clause: class java.lang.String", I
> thought maybe this would be better, but if gave the same error:
> (case (type "x")
>  class java.lang.String (println "STRING")
>  class java.lang.Long (println "NUM"))
>
> Is in not possible to match on class in a case?

Correct, because the test expressions have to be compile-time constants.
Also, "class java.lang.String" isn't valid Clojure syntax for an
expression. Just "java.lang.String" evaluates to the class with that
name, but the test expressions aren't evaluated.

I think the most common approach for this scenario is to use condp:

(condp instance? "x"
 java.lang.String "STRING"
 java.lang.Long "LONG")

Hope that helps

John

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class and case

[Kevin Kleinfelter]

Can someone help me get un-stuck with a problem of using class in a case?

This returns true, as I expect:
(= (type "x") java.lang.String)

This returns an error, and I'd hoped it would print STRING:
(case (type "x")
 class java.lang.String (println "STRING")
 class java.lang.Long (println "NUM"))

Because the error said "No matching clause: class java.lang.String", I 
thought maybe this would be better, but if gave the same error:
(case (type "x")
 class java.lang.String (println "STRING")
 class java.lang.Long (println "NUM"))

Is in not possible to match on class in a case?

Tnx.

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