Org Babel API

Table of Contents

This document aims to explain the Org Babel API with a focus on maintenance and extension. Org Babel is a part of Emacs which allows for embedded code blocks within a text document1.

At the time of writing, the Org Babel API is only described in ob-template.el, the email thread linked to in the Worg docs, and, of course, the source code itself.

The functions that need to be (or even can be), defined aren't really laid out anywhere, nor are the ways they interact explained in one place. This document aims to change that.

NOTE At some point I'll merge this into Worg2. However, this is still a draft and I don't feel it's right to host it there yet. These are my active notes and people probably expect Worg to be somewhat more complete.

Context

The Babel project is no longer maintained by the original author. Instead, the various "ob" (for "org-babel") extensions have individual maintainers. Each extension varies in quality and consistency with the other extensions. Some see regular updates whereas others don't seem to have had much interaction since they were created.

Org Babel is not a single file/package. It's a suite of files. The files are found in the lisp/ directory of the Org project3. The primary file is ob-core.el. 

ls /home/ahab/.emacs.d/straight/repos/org/lisp | grep "ob-" | grep "el$" | head -n 10

The package is called Org Babel, which implies the namespace "org-babel-"4. Note that the various "ob-" extensions also use the "org-babel-" prefix. That is, symbols defined in ob-core.el don't use an "ob-core-" prefix and symbols defined in ob-<lang> use "org-babel-".

That all Babel files use the same prefix makes it difficult to tell which functions are core and which are extensions. It also makes it hard to figure out what the actual API is. Hence, this document.

Overview

The basic idea is to define functions that ob-core.el expects.

In no particular order:

function name ob-core.el ob-template.el
org-babel-execute:template X X
org-babel-template-initiate-session X X
org-babel-expand-body:template X X
org-babel-prep-session:template X X
[BROKEN LINK: =org-babel-template-var-to-template=]   X
[BROKEN LINK: =org-babel-template-table-or-string=]   X
org-babel-variable-assignments:template X  
org-babel-process-params X  
variable name ob-core.el ob-template.el
org-babel-tangle-lang-exts   X
org-babel-default-header-args:template X X
     

Functions from ob-core.el not in ob-template.el, but still used in some ob-template.el languages (e.g. ob-shell.el):

Important functions to understand for any extension that use a comint (all of them?):

  • org-babel-comint-wait-for-output

Terminology

To quote (org) Structure of Code Blocks, 

A source code block conforms to this structure:

    #+NAME: <name>
    #+BEGIN_SRC <language> <switches> <header arguments>
    <body>
    #+END_SRC

TODO info

params

'params' is an alist derived from the src block header. Element keys are referred to as "header args". These are defined in org-babel-common-header-args-w-values, but are not exclusive to that list. Any ":header-arg value" pair defined in the header will appear in the list.

switch

Switches control evaluation, export and tangling of code blocks.

For example, the following block uses the "-n" switch to toggle line numbers in the HTML output: 

#+begin_src elisp -n
(message "hello, world!")
#+end_src

1: (message "hello, world!")

header argument

Header arguments are keywords which control block evaluation or expansion. For instance, the :session <session-name> argument runs a block in a separate shell. Another example is the :var NAME=ASSIGN argument which passes data between blocks such that NAME gets defined as ASSIGN within the block environment.

Header arguments can be more general, however. To quote ob-template.el,

"you are free to define any new header arguments which you feel may be useful – all header arguments specified by the user will be available in the PARAMS variable."

Arguments can be system-wide, using org-babel-default-header-args or language specific with org-babel-default-header-args:<LANG>.

For common header arguments, see org-babel-common-header-args-w-values.

expansion

"Expand" and "expansion" are overloaded terms. Generally, they mean to replace something within a form.

When org-babel "expands" the body of a source block, it resolves any noweb references (noweb allows different source blocks to reference one another)

To expand a variable, in the context of Org babel, means to assign a header variable as a variable within the language runtime.

For example, the named table tbl gets "expanded" and assigned to data within the Python runtime. 

#+name: tbl
| 1 | 2 |
| 3 | 4 |

#+BEGIN_SRC python :var data=tbl :results value
return data
#+END_SRC

#+RESULTS:
| 1 | 2 |
| 3 | 4 |

The :var parameter

The :var parameter has the form 

(:var name . value)

where "value", the cdr, should be a cons cell whose car is the name of the variable "as a symbol" and whose value is the value of the variable.

That is, 

(:var name . (name-symbol . "it's value"))

org-babel-execute:template

Arguments: body, params

Returns: emacs-lisp containing the results of evaluating body in template language.

The function called to evaluate a code block. It is defined in an "ob-" file and gets called by org-babel-execute-src-block.

The org-babel-execute:template function will evaluate the body of the source code and return the results as emacs-lisp depending on the value of the :results header argument:

  • :results output means that the output to STDOUT will be captured and returned
  • :results value means that the value of the last statement in the source code block will be returned

The ob-template.el file claims that "the most common first step in this function is the expansion of the PARAMS argument using org-babel-process-params." This was probably true when that was written. At the time of writing, however, only four of the files in the org-babel suite use it.

Please feel free to not implement options which aren't appropriate for your language (e.g. not all languages support interactive "session" evaluation). Also you are free to define any new header arguments which you feel may be useful – all header arguments specified by the user will be available in the PARAMS variable.

This function is called by

org-babel-variable-assignments:template

Used during tangle (ob-tangle.el) and eval.

It is used as an argument for org-babel-expand-body:generic.

It is used in org-babel-prep-session:template

"Return list of template statements assigning the block's variables."

org-babel-process-params

"Expand variables in PARAMS and add summary parameters."

Undocumented behavior

Don't be surprised if there is undocumented behavior defined within an ob file.

The following were undocumented features of ob-shell:

  • :stdin passes org reference (#+name) as stdin
  • :cmdline with :shebang runs body with given command-line arg(s)
  • :cmdline without :shebang runs body with shell-file-name and command-line arg(s)
  • org-babel-load-session:shell (see org-babel-load-session:template)

org-babel-load-session:template

Called by the ob-core.el function org-babel-load-in-session. That function handles header args, mainly no-web expansion. It then starts the comint process and opens the associated process buffer. The org-babel-load-in-session function is, basically, called by the "org-metaup-hook" which is attached to M-up by default. Otherwise, this function is never called. Also, AFAICT, this behavior is not documented anywhere.

For ob-shell.el, this function is implemented, but only for the "shell" language. For example, press M-up when in the following block. The body will be inserted in a new process buffer "my-shell". 

#+begin_src shell :session my-shell
echo "hello, world!"
#+end_src

#+begin_src sh :session my-sh
echo "hello, world!"
#+end_src

#+begin_src bash :session my-bash
echo "hello, world!"
#+end_src

TODO org-babel-template-table-or-string

This is cruft. It is defined in ob-template.el but not used anywhere, in that file or elsewhere.

TODO org-babel-template-var-to-template

This is cruft. It's only defined in ob-template.el. All it does is this: 

(defun org-babel-template-var-to-template (var)
  "Convert an elisp var into a string of template source code
specifying a var of the same value."
  (format "%S" var))

Surely this doesn't deserve a whole function.

TODO things to do

  • [ ] explain each function
  • [ ] explain terms
  • [ ] remove cruft functions from ob-template.el (or just refactor that example)

References

TODO org-babel-tangle-lang-exts

Gets called by ob-tangle.el in the org-babel-effective-tangled-filename function.

Footnotes:

1

https://orgmode.org/

2

https://orgmode.org/worg/worg-about.html

3

https://git.savannah.gnu.org/cgit/emacs/org-mode.git/tree/lisp

4

Technically, Emacs doesn't have namespaces. Instead, a prefix is given to symbols so that various packages don't conflict.

2021-11-03

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