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clojure

What do Clojure symbols do when used as functions?

While trying to solve the 4Clojure problem "Universal Computation Engine" involving reimplementing evaluation, I accidentally ended up calling something like this:

(apply '/ '(16 8))

rather than the intended:

(apply / '(16 8))

This had the confusing side effect of returning 8, which made me think I had messed up my maths.

I later realised my error after some debugging—I was failing to evaluate the / symbol before attempting to call it—and so realised that clojure.lang.Symbol must implement clojure.lang.IFn. But what does that implementation do? All I can get it to do is return nil with one argument, or the second argument if given.

Solution

  • Symbols look themselves up in a map, much as keywords do. See Symbol's implementation:

    …
122 public Object invoke(Object obj) {
123         return RT.get(obj, this);
124 }
125
126 public Object invoke(Object obj, Object notFound) {
127         return RT.get(obj, this, notFound);
128 }
…

    (RT is clojure.lang.RT, which does just about everything. "RunTime"?)

    In the example given, the lookup is failing (because 16 is not a map), and therefore the notFound value (8) is being returned.