矢印でメソッドチェイン風に記述する

なんどか似たようなネタでコードを書いている気がします。

矢印シンボルを用いて括弧の数を減らしてみます。矢印の`>'の個数で式を挿入する位置を決定するようにしてみました。

(defun arrow-symbol? (sym)
 (when (symbolp sym)
   (let ((name (symbol-name sym)))
     (and (<= 2 (length name))
    (= (+ (count #\- name)
   (count #\> name))
       (length name))
    (string= (sort (copy-seq name) #'char<)
      name)))))

(defun arrow-count (sym)
 (count #\> (symbol-name sym)))

(defun collect-arrow-clauses (body)
 (do ((rest (reverse body))
      (result nil))
     ((null rest) result)
   (let ((pos (position-if #'arrow-symbol? rest)))
     (when (null pos)
 (error "arrow symbol not found"))
     (push (reverse (subseq rest 0 (1+ pos))) result)
     (setf rest (subseq rest (1+ pos))))))

(defmacro arrow (obj &body body)
   (labels
 ((expand (rest prev)
    (if rest
        (let ((insert-pos (arrow-count (caar rest))))
   (expand
    (cdr rest)
    (append
     (subseq (cdar rest) 0 insert-pos)
     (list prev)
     (subseq (cdar rest) insert-pos))))
        prev)))
     (expand (collect-arrow-clauses body) obj)))

;;; 実効
(arrow "afscd"
 -> copy-seq
 -> sort #'char<
 ->>> format t "sorted:~A~%")

;;; マクロ展開後のコード
(FORMAT T "sorted:~A~%" (SORT (COPY-SEQ "afscd") #'CHAR<))

;;; 出力
sorted:acdfs