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How to get AST with tail recursion in Clojure

I have an ANTLR3 AST that I need to go through post-order, the first pass, which I implemented like this: Clojure:

(defn walk-tree [^CommonTree node]
  (if (zero? (.getChildCount node))
    (read-string (.getText node))
    (execute-node
      (map-action node)
      (map walk-tree (.getChildren node)))))))

I would like to convert this to tail recursion using a ... recur loop, but I could not figure out how to use the explicit stack efficiently to do this, since I need a traversal after a traversal.

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3 answers

, node, tree-seq, . , , . , , loop recur .

, , . " " ""

(map #(.getText %) ;; Has Children?      List of Children    Input Tree
     (tree-seq #(> (.getChildCount #) 0) #(.getChildren %) my-antlr-ast))

tree-seq , . .

+8

, - . , , ( ). .

, , , Java postorder .

(def op-map {'+ +, '- -, '* *, '/ /})

;; Convert the tree to a linear, postfix notation stack
(defn build-traversal [tree]
  (loop [stack [tree] traversal []]
    (if (empty? stack)
      traversal
      (let [e (peek stack)
            s (pop stack)]
        (if (seq? e)
          (recur (into s (rest e)) 
                 (conj traversal {:op (first e) :count (count (rest e))}))
          (recur s (conj traversal {:arg e})))))))

;; Pop the last n items off the stack, returning a vector with the remaining
;; stack and a list of the last n items in the order they were added to
;; the stack
(defn pop-n [stack n]
  (loop [i n s stack t '()]
    (if (= i 0)
      [s t]
      (recur (dec i) (pop s) (conj t (peek s))))))

;; Evaluate the operations in a depth-first manner, using a temporary stack
;; to hold intermediate results.
(defn eval-traversal [traversal]
  (loop [op-stack traversal arg-stack []]
    (if (empty? op-stack)
      (peek arg-stack)
      (let [o (peek op-stack)
            s (pop op-stack)]
        (if-let [a (:arg o)]
          (recur s (conj arg-stack a))
          (let [[args op-args] (pop-n arg-stack (:count o))]
            (recur s (conj args (apply (op-map (:op o)) op-args)))))))))

(defn eval-tree [tree] (-> tree build-traversal eval-traversal))

:

user> (def t '(* (+ 1 2) (- 4 1 2) (/ 6 3)))
#'user/t
user> (eval-tree t)
6

, Antlr AST;)

+4

I don’t know how to work on clojure, but I think I understand what you are looking for.

Here is some pseudo code. The stack in my pseudo-code looks like a stateful object, but it's possible to use an immutable one. It uses something like O (tree depth * max children per bunch of node).

walk_tree(TreeNode node) {
    stack = new Stack<Pair<TreeNode, Boolean>>();
    push(Pair(node, True), stack)
    walk_tree_aux(stack);
}
walk_tree_aux(Stack<Pair<TreeNode, Boolean>> stack) { -- this should be tail-recursive
    if stack is empty, return;
    let (topnode, topflag) = pop(stack);
    if (topflag is true) {
        push Pair(topnode, False) onto stack);
        for each child of topnode, in reverse order:
            push(Pair(child, True)) onto stack
        walk_tree_aux(stack);
    } else { -- topflag is false
        process(topnode)
        walk_tree_aux(stack);
    }
}
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