How to unload a function from another namespace?

Question

I load a function say-hi from namespace learning.greeting

(use 'learning.greeting)

When I try to re-defn the say-hi function under the current (user) namespace, I got the error:

CompilerException java.lang.IllegalStateException: say-hi already refers to: #'learning.greeting/say-hi in namespace: user, compiling:(NO_SOURCE_PATH:1:1) 

So how to unload the function from other namespaces?

Answer 1

If you want to get rid of a direct mapping to a Var from another namespace at the REPL, say

(ns-unmap 'current-namespace 'local-alias)

Example:

user=> (ns-unmap *ns* 'reduce)
nil
user=> (reduce + 0 [1 2 3])
CompilerException java.lang.RuntimeException: Unable to resolve symbol: reduce in this context, compiling:(NO_SOURCE_PATH:2:1)

Local alias will differ from the actual name of the Var if :rename was used:

(use '[clojure.walk
       :only [keywordize-keys]
       :rename {keywordize-keys keywordize}])

To remove all mappings pointing at Vars in clojure.walk:

(doseq [[sym v] (ns-map *ns*)]
  (if (and (var? v)
           (= (.. v -ns -name) 'clojure.walk))
    (ns-unmap *ns* sym)))

Answer 2

Do you really want to remove say-hi from learning.greeting? If not, it might be better to use require in this situation. Instead of (use 'learning.greeting), execute:

(require `[learning.greeting :as lg])

Then you can refer to the original definition as lg/say-hi, and you can define a new version in the current namespace, e.g. as

 (def say-hi [x] (lg/say-hi (list x x))

(I don't know whether that makes sense for the say-hi function, but the general point is the same regardless.)