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A closure is a powerful programming concept that allows a function to maintain access to its enclosing lexical scope, even after the outer function has finished executing. In simpler terms, a closure "closes over" variables from its surrounding environment, making those variables accessible and modifiable from within the closure's body.

Define a closure

In Argon, you can create a closure simply by defining a function within another function. This allows the inner function(nested function), to have visibility and access to all the variables defined in the outer function, see example below:

func prepended_print(prepend) {
return (string) => {
io.print("%s%s" % (prepend, string))

pprint := prepended_print("$$ ")

pprint("Hi!") # Output: $$ Hi!

We have just seen how a closure can capture the variables of the scope in which it is defined, now let's see another example of how a closure can capture and modify its internal state:

func incrementer(amount=1) {
count := 0

func _incrementer {
count += amount
return count

return _incrementer

inc := incrementer(amount=2)

inc() # Output: 2
inc() # Output: 4
inc() # Output: 6