clojure reduced not terminating reduce function

Question

In the clojure documentation it says:

Usage: (reduced x)

Wraps x in a way such that a reduce will terminate with the value x

I am trying to return from a reduce function with a boolean and a vector values.

(def bp (reduce (fn [[balanced stack] singlenum]
    (def stack2 (conj stack singlenum))
    (println stack2)
    (if (= 2 singlenum)
      (reduced [false stack2])
    )
    [balanced stack2]
  )
  [true (vector)] [1 2 3 4]
))

bp evaluates as [true [1 2 3 4]], I was expecting [false [1 2]]. The reduced did not terminate the reduce function. I was attempting to terminate the reduce function with a specific values.

rmcv rmcv · Accepted Answer · 2019-11-30T07:56:12

You have the correct logic there. I just revised your usage of if and def.

if - I moved [balanced stack2] to the else part. Otherwise reduced will never be detected.
def - the def inside fn should be replaced with let

(def bp (reduce (fn [[balanced stack] singlenum]
                  (let [stack2 (conj stack singlenum)]
                       (println stack2)
                       (if (= 2 singlenum)
                         (reduced [false stack2])
                         [balanced stack2])))
                [true (vector)]
                [1 2 3 4]))

| | | | | stack=> []
| | | | | singlenum=> 1
| | | | (conj stack singlenum)=> [1]
| | | | stack2=> [1]
[1]
| | | (println stack2)=> nil
| | | | | singlenum=> 1
| | | | (= 2 singlenum)=> false
| | | | | balanced=> true
| | | | | stack2=> [1]
| | | (if (= 2 singlenum) (reduced #) [balanced stack2])=> [true [1]]
| | (let [stack2 #] (println stack2) (if # # #))=> [true [1]]
| | | | | stack=> [1]
| | | | | singlenum=> 2
| | | | (conj stack singlenum)=> [1 2]
| | | | stack2=> [1 2]
[1 2]
| | | (println stack2)=> nil
| | | | | singlenum=> 2
| | | | (= 2 singlenum)=> true
| | | | | | stack2=> [1 2]
| | | | (reduced [false stack2])=> #reduced[{:status :ready, :val [false [1 2]]} 0x5fbdbb78]
| | | (if (= 2 singlenum) (reduced #) [balanced stack2])=> #reduced[{:status :ready, :val [false [1 2]]} 0x5fbdbb78]
| | (let [stack2 #] (println stack2) (if # # #))=> #reduced[{:status :ready, :val [false [1 2]]} 0x5fbdbb78]
(reduce (fn # #) [true #] [1 2 3 4])=> [false [1 2]]

clojure reduced not terminating reduce function

1 Answers