What is a 'Closure'?

Question

I asked a question about Currying and closures were mentioned. What is a closure? How does it relate to currying?

Answer 1

A closure is a function that can reference state in another function. For example, in Python, this uses the closure "inner":

def outer (a):
    b = "variable in outer()"
    def inner (c):
        print a, b, c
    return inner

# Now the return value from outer() can be saved for later
func = outer ("test")
func (1) # prints "test variable in outer() 1