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

Question

What's the difference between using "def" to update a var and using "alter-var-root"? e.g.

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

vs

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

Answer 1

I find alter-var-root very rarely comes up in idiomatic Clojure code; not that there is anything wrong with it, it's just intended for corner cases. If you find yourself using it to build loops and such it's a sign something needs a different approach. I mostly see it in initialization routines for setting access credentials or loggers and such.

alter-var-root uses a function to mechanically change the value of a var while def just sets it to a new value. In your example they are equivalent.

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

alter-var-root is also unwilling to create a new var:

hello.exp> (alter-var-root #'foo1 inc) 
CompilerException java.lang.RuntimeException: Unable to resolve var: foo1 in this context, compiling:(NO_SOURCE_PATH:1) 

alter-var-root can work on other namespaces as 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 is the only one I have ever needed in practice. For instance forcing clojure.logging to use the correct slf4j logger as an example from the Pallet project:

(defn force-slf4j
  "The repl task brings in commons-logging, which messes up our logging
   configuration. This is an attempt to 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 is just using alter-var-root to reset a var in another namespace regardless of its content on initialization. I suppose it's a bit of a hack ...