Closures

Aug 10 2016

Prereqs

Before jumping into what a closures, we need to examine a supporting topic: free variables.

A free variable is a variable that is available to the function but is not defined locally nor is a parameter. It is probably best to lean on an example to explain this.

def f(x):
    local_to_f = "local_to_f"

    def g(y):
        local_to_g = "local_to_g"
        print(local_to_g)
        print(local_to_f)

The function f has two bound variables, the parameter x and the local variable local_to_f. Similarly, h has the parameter y and the local variable local_to_g.

g also has a free variable, local_to_f. This is free variable because it is not defined locally (i.e. within the definition of g) and it is not a parameter to the function.

The name is a bit unfortunate. I imagine the term derives from the converse of a bound variable.

On to closures

A closure is a function that references free variables. That is it. The implementation of closures in a language is surely not as simple, but from the outside, this is all we really need to know.

Let’s look at another example.

(defn f [x]
  (fn [] (+ 10 x)))

(def a-closure (f 20))

(a-closure)
; => 30

The anonymous function created in f is a closure because it has a free variable x.

On the other hand, this would not be a closure because the function returned by f does not have any free variables.

(defn f []
  (fn [x] (+ 10 x)))

(def not-a-closure (f))

(not-a-closure 20)
; => 30

Conclusion

Once you understand lexical scope, and free variables, closures become relatively simple. The term free variables isn’t the most intuitive name, which can cause some issues, especially when the definition of a closure relies on that confusing terminology. (This is the reason I covered it before going into to closures.)

Sources

tags:closureslexical closuresfree variables

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