06_function.clj contains this question, I can't figure out why there is an extra pair of () in position 1 and 2, since position 3 already has brackets wrapped up.
"One function can beget another"
(= 9 (
( <---- 1
(fn [] (fn [a b] (+ a b))) <----3
) <-----2
4 5))
(fn [a b] (+ a b))
is a function that takes 2 arguments and returns their sum, let's substitute if with name fun1
(fn [] fun1)
is a function that takes nothing and returns function object fun1. Let's call this new function fun2
(
fun2
)
here we call fun2, which, as we previously discussed, returns function fun1
(
fun1
4 5)
here we call fun1 (returned from (fun2)) with 2 arguments - 4 and 5. This gives us 9
(= 9
9)
and finally we check equality of 2 numbers. They are actually equal.
The main thing you should understand here is that functions in Clojure are also first-class citizens. You may produce them (like fun1), pass them to other functions and return from them (like we returned fun1 from fun2). So each layer of ( and ) is just another call to a function (possibly returned from some other function).
It's there to evaluate the function created by outer fn.
So, in turn:
(fn [a b] (+ a b)
creates the inner function that sums it's arguments
(fn [] (fn [a b] (+ a b))
creates the outer function with taking zero arguments and returning a function that sums it's arguments.
(
(fn [] (fn [a b] (+ a b)))
)
forces evaluation of the outer function (and returns it's result - a function that sums two values).
Remember that when you see parentheses in lisps the first thing that should pop in your mind is that it's an application of the function/form/macro to it's arguments.
