is this partial function a closure?

Question

I am discussing closure with a friend and he thinks (partial + 5) is a closure. But I think a closure is a function closing over a free variable, for example

(let [a 10]
  (defn func1 [x] (+ x a))
)

then func1 is a closure. But in this case 5 is not a free variable. So which is the right answer?

While the accepted answer is correct in terms of implementation, I'd argue that it shouldn't be considered a closure, but rather "just" a partial application. The use of a closure is an internal implementation detail that is not directly exposed to the caller. — user2864740, Nov 30 '13 at 20:28

score 8 · Accepted Answer · edited Jul 19 '12 at 10:22

8

partial uses a closure to make the partial function. Check out the code of partial by using (source partial) in repl and you will see that it uses closures.

(defn partial
  "Takes a function f and fewer than the normal arguments to f, and
  returns a fn that takes a variable number of additional args. When
  called, the returned function calls f with args + additional args."
  {:added "1.0"}
  ([f arg1]
   (fn [& args] (apply f arg1 args)))
  ([f arg1 arg2]
   (fn [& args] (apply f arg1 arg2 args)))
  ([f arg1 arg2 arg3]
   (fn [& args] (apply f arg1 arg2 arg3 args)))
  ([f arg1 arg2 arg3 & more]
   (fn [& args] (apply f arg1 arg2 arg3 (concat more args)))))

edited Jul 19 '12 at 10:22

mikera

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answered Jul 19 '12 at 10:11

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Why is it written like that? I'm just learning right now and [f arg1] seems redundant since there is a "varargs" implementation (the one with &more) – themistoklik Nov 20 '16 at 16:57
1

A safe bet would be it's optimizing the most common cases – Sushisource Aug 17 '17 at 20:55

score 0 · Answer 2 · edited May 23 '17 at 11:53

(partial + 5) is an anonymous function or "lambda".

Anonymous functions are often¹ called "closures" but it's an abuse of the term ; see the discussion in "What is the difference between a 'closure' and a 'lambda'?"

[¹] Maybe because in most popular languages that support them, closures and anonymous functions are created using the same language features - which renders them undistinguishable at first glance.

is this partial function a closure?

2 Answers2