clojure - Difference between using "def" to update a var and "alter-var-root" -

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what's difference between using "def" update var , using "alter-var-root"? e.g.

(def x 3) (def x (inc x))

vs

(def x 3) (alter-var-root #'x inc)

i find alter-var-root comes in idiomatic clojure code; not there wrong it, it's intended corner cases. if find using build loops , such it's sign needs different approach. see in initialization routines setting access credentials or loggers , such.

alter-var-root uses function mechanically change value of var while def sets new value. in example equivalent.

hello.exp> (def foo 4) #'hello.exp/foo hello.exp> (alter-var-root #'foo inc) 5 hello.exp> foo 5

alter-var-root unwilling create new var:

hello.exp> (alter-var-root #'foo1 inc)  compilerexception java.lang.runtimeexception: unable resolve var: foo1 in context, compiling:(no_source_path:1)

alter-var-root can work on other namespaces well:

hello.exp> (in-ns 'user) #<namespace user>  user> (alter-var-root #'hello.exp/foo inc)   6 user> (def hello.exp/foo 4) compilerexception java.lang.runtimeexception: can't create defs outside of current ns, compiling:(no_source_path:1) user>

this last use case 1 have ever needed in practice. instance forcing clojure.logging use correct slf4j logger example pallet project:

(defn force-slf4j   "the repl task brings in commons-logging, messes our logging    configuration. attempt restore sanity."    []   (binding [*ns* (the-ns 'clojure.tools.logging.slf4j)]     (alter-var-root      #'clojure.tools.logging/*logger-factory*      (constantly (clojure.tools.logging.slf4j/load-factory)))))

which using alter-var-root reset var in namespace regardless of content on initialization. suppose it's bit of hack ...


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