Álvaro Ramírez

27 October 2020 Emacs: chaining org babel blocks

Recently wanted to chain org babel blocks. That is, aggregate separate source blocks and execute as one combined block.

I wanted the chaining primarily driven through header arguments as follows:

#+name: block-0
#+begin_src swift
  print("hello 0")
#+end_src

#+name: block-1
#+begin_src swift :include block-0
  print("hello 1")
#+end_src

#+RESULTS: block-1
: hello 0
: hello 1

I didn't find the above syntax and behaviour supported out of the box (or didn't search hard enough?). Fortunately, this is our beloved and malleable editor, so we can always bend it our way! Wasn't quite sure how to go about it, so I looked at other babel packages for inspiration. ob-async was great for that.

Turns out, advicing org-babel-execute-src-block did the job:

(defun adviced:org-babel-execute-src-block (&optional orig-fun arg info params)
  (let ((body (nth 1 info))
        (include (assoc :include (nth 2 info)))
        (named-blocks (org-element-map (org-element-parse-buffer)
                          'src-block (lambda (item)
                                       (when (org-element-property :name item)
                                         (cons (org-element-property :name item)
                                               item))))))
    (while include
      (unless (cdr include)
        (user-error ":include without value" (cdr include)))
      (unless (assoc (cdr include) named-blocks)
        (user-error "source block \"%s\" not found" (cdr include)))
      (setq body (concat (org-element-property :value (cdr (assoc (cdr include) named-blocks)))
                         body))
      (setf (nth 1 info) body)
      (setq include (assoc :include
                           (org-babel-parse-header-arguments
                            (org-element-property :parameters (cdr (assoc (cdr include) named-blocks)))))))
    (funcall orig-fun arg info params)))

(advice-add 'org-babel-execute-src-block :around 'adviced:org-babel-execute-src-block)

Before I built my own support, I did find that noweb got me most of what I needed, but required sprinkling blocks with placeholder references.

Combining :noweb and :prologue would have been a great match, if only prologue did expand the noweb reference. I'm sure there's an alternative I'm missing. Either way, it was fun to poke at babel blocks and build my own chaining support.

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