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It is pretty obscure how to get constant-time dispatch for e.g. Enums, even if user knows about case.


The possibility to dispatch to arbitrary constants with case, by wrapper macro, should be documented.


  • Should it warn against doing that with unstable values?
  • Should it mention anything else than java Enums?

Case Techniques

Case is documented for accepting all readable forms as test-constants. However, it can also be made to use any compile-time-known constants as test-constants, by wrapping it in another macro.

Sometimes this is appropriate, e.g. when dispatching on a java Enum.
Other times, less so, e.g. when dispatching on objects whose hash changes when the vm is restarted (breaks AOT).


This technique is an application of a more general technique: Passing non-literals to a macro from another macro.
Are there other macros that have use cases like this?



3 Answers

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Comment made by: bendlas

-This is a duplicate of- http://dev.clojure.org/jira/browse/CLJ-1367

Actually, it's an alternate solution

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Comment made by: petrglad

Probably this ticket and CLJ-1367 linger for so long because there's already 'condp' that can be used as follows:

(condp = test-value JavaClass/CONST1 result1 JavaClass/CONST2 result2)

This is sequential and slower but is about as concise as plain case.

However if this is the form to be used instead of plain 'case' this should be suggested by documentation.

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Reference: https://clojure.atlassian.net/browse/CLJ-1368 (reported by bendlas)