Clojure - recur applies to loop or let statement?

Question

I have a question in Clojure about recur. If I have a let-statement inside the loop, can the recur call be applying to the let statement instead of the values of the loop? For example, in this scenario:

(defn someFunction [listA listB]
  ("do something here...." 
      [new-listA new-listB]))

(defn anotherFunction [listA listB]
  ("do something here...." 
      [new-listA new-listB]))

(defn myFunction [firstList secondList]
  (loop [list1 (someMutation firstList)
         list2 (someMutation secondList)]
    (if (= "true" (someCondition))
      (let [[newlist1 newlist2]
           (someFunction list1 list2)] 
        (recur newlist1 newlist2))
      (anotherFunction list1 list2) )))

is (recur newlist1 newlist2) applying to the loop or to the let? And is there a way of skipping this let statement and calling recur directly with the two values returned by "someFunction", assuming that I can't change the fact that "someFunction" returns a vector with two arguments?

Arthur Ulfeldt Arthur Ulfeldt · Accepted Answer · 2017-01-19T19:00:31

recur always recurs to the closest loop or function, whichever is closer to the recur form. It does this by compiling to essentially a loop/jump statement that changes the values of the loop variables and then jumps back to the loop/fn point. This way it only uses one stack frame and can run full speed. This design has some intentional trade-offs:

you cannot interleave nested loops
recur can only be the last thing in an expression.
there must be the same number of args to recur as loop/the-function

This last one requires that you keep the let expression or something equivalent such as a destructuring form. Loops do allow destructuring of the arguments just like functions:

 (loop [[a b] [1 2]]
    (println a b)
    (if (< a 10)
      (recur [(inc a) (inc b)])))
1 2
2 3
3 4
4 5
5 6
6 7
7 8
8 9
9 10
10 11

So you could write your loop expression to take the result of someFunction directly

(loop [[list1 list2] [(someMutation firstList) (someMutation secondList)]]
  ...
  (recur (someFunction ...))

Clojure - recur applies to loop or let statement?

2 Answers