functional-programming - Maybe monad in elisp

Question

Here is my attempt to implement Maybe monad in elisp. It it function composition that breaks on first nil. However value is always 13. Where is my error?

(defun .compose-maybe (&rest funcs)
  "Monad Maybe function composition with nil as Nothing."
  (lambda (arg)
    (if funcs
        (let ((value (funcall (apply #'.compose-maybe (cdr funcs)) arg)))
          (message "%s" value)
          (when value (funcall (car funcs) value))))
        arg))

(funcall (.compose-maybe (lambda (x) (* x 5)) (lambda (x) (+ 100 x))) 13)

Answer 1

您arg不在 的范围内(if funcs ...)，因此 in 的内部 lambda.compose-maybe始终返回arg，而不是仅在funcsis时返回nil。

(defun .compose-maybe (&rest funcs)
  "Monad Maybe function composition with nil as Nothing."
  (lambda (arg)
    (if funcs
        (let ((value (funcall (apply #'.compose-maybe (cdr funcs)) arg)))
          (message "%s" value)
          (when value (funcall (car funcs) value)))
      arg)))