Clojure矢量作为函数参数

Question

Being completely inexperienced in clojure, and without any functional programming practice since college, I'm trying to interpret some example code to figure out the clojure syntax.

I started by coding several versions of Fibonacci ( https://gist.github.com/pcalcao/ea4176719d778ea3ab9e ), but I still can't say I fully understand the more complex forms.

For instance, this:

(defn fib_map [n]
  (last (take (+ n 1)
    (map first (iterate (fn [[a b]] [b (+ a b)]) [0 1])))))   

I'm struggling to really understand the innermost part of this code:

fn [[a b]] [b (+ a b)] 

Now, from my understanding, we're creating an anonymous function that receives one parameter, a vector with two values (this is destructuring, right?), and returns another vector.

Now, what is the reason we would do this, instead of:

fn [a b] [b (+ a b)]

Are these equivalent? Or are we simply making our anonymous function receive a single parameter as a "gimmick" to use in iterate ?

Sorry if this is totally obvious, but like I said, Lisp-like languages aren't my strong point yet.

Answer 1

You already figured it out correctly yourself.

Function of the form (fn [[ab]] ...) is using destructuring. It takes a single parameter that should be a vector or another type of object that supports clojure's nth function. Using destructuring, it "pulls" the first two values out of the vector and assigns them to local variables a and b .

Function of the form (fn [ab] ...) is a function of two parameters. The two are not equivalent.

The reason you have to use the (fn [[ab]] ...) form with iterate is that iterate only works with single-parameter functions.

Answer 2

It's because iterate only takes two parameters, ie one function and one parameter. cf. the docs