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Currently the docstring for with-redefs recommends itself for use in testing. However there are a number of reasons why using this macro for testing is suboptimal:

  • with-redefs "bindings" are not transferred to new threads since it's a global mutation
  • users can get runtime errors if they redef a primitive type-hinted function to a function taking only objects
  • If parts of the body of with-redefs is delayed (via a delay, go block, etc.) that code may not see the new root
  • The mutation is global so it "leaks" outside the current scope into other code that may currently be running in another thread
  • Clojure tends to shun global mutation, and yet this macro isn't marked with a ! nor properly warns users about the dangers mentioned here

Due to these reasons I often encounter new users using with-redefs without understanding the ramifications of doing so. All this behavior makes sense if a user understands how Vars work, but that's a lot of knowledge to take on for a new user.

Suggestion:
Remove the suggestion that with-redefs be used in testing
Add a few notes of warning about global mutation, and concurrency issues with the macro.

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Reference: https://clojure.atlassian.net/browse/CLJ-2134 (reported by halgari)
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