# Emacs Lisp Best Practices for Package Development

This document covers best practices for writing Emacs Lisp packages, specifically targeting org-babel language implementations.

## Table of Contents

- [File Structure and Headers](#file-structure-and-headers)
- [Naming Conventions](#naming-conventions)
- [Variables and Customization](#variables-and-customization)
- [Function Definitions](#function-definitions)
- [Error Handling](#error-handling)
- [Documentation](#documentation)
- [Dependencies and Loading](#dependencies-and-loading)
- [References](#references)

---

## File Structure and Headers

Every Emacs Lisp package file should follow a standard structure:

```elisp
;;; ob-elixir.el --- Org Babel functions for Elixir evaluation -*- lexical-binding: t; -*-

;; Copyright (C) 2024 Your Name

;; Author: Your Name <your.email@example.com>
;; URL: https://github.com/username/ob-elixir
;; Keywords: literate programming, reproducible research, elixir
;; Version: 1.0.0
;; Package-Requires: ((emacs "27.1") (org "9.4"))

;; This file is not part of GNU Emacs.

;; This program is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; This program is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with this program.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:

;; This package provides Org Babel support for evaluating Elixir code blocks.
;;
;; Features:
;; - Execute Elixir code in org-mode source blocks
;; - Support for Mix project context
;; - Session support via IEx
;; - Variable passing between code blocks
;;
;; Usage:
;; Add (elixir . t) to `org-babel-load-languages':
;;
;;   (org-babel-do-load-languages
;;    'org-babel-load-languages
;;    '((elixir . t)))

;;; Code:

(require 'ob)
(require 'ob-ref)
(require 'ob-comint)
(require 'ob-eval)

;; ... your code here ...

(provide 'ob-elixir)
;;; ob-elixir.el ends here
```

### Key Header Elements

| Element              | Purpose                                    | Example               |
|----------------------|--------------------------------------------|-----------------------|
| `lexical-binding: t` | Enable lexical scoping (always use this)   | First line comment    |
| `Package-Requires`   | Declare dependencies with minimum versions | `((emacs "27.1"))`    |
| `Keywords`           | Help with package discovery                | Standard keyword list |
| `Version`            | Semantic versioning                        | `1.0.0`               |

---

## Naming Conventions

### Package Prefix

All symbols (functions, variables, constants) must be prefixed with the package name to avoid namespace collisions:

```elisp
;; GOOD - properly prefixed
(defvar ob-elixir-command "elixir")
(defun ob-elixir-evaluate (body params) ...)
(defconst ob-elixir-eoe-indicator "---ob-elixir-eoe---")

;; BAD - no prefix, will collide
(defvar elixir-command "elixir")
(defun evaluate-elixir (body) ...)
```

### Naming Patterns for org-babel

Org-babel uses specific naming conventions that must be followed:

```elisp
;; Required function - note the colon syntax
(defun org-babel-execute:elixir (body params)
  "Execute a block of Elixir code with Babel.")

;; Optional session function
(defun org-babel-elixir-initiate-session (&optional session params)
  "Create or return an Elixir session buffer.")

;; Variable assignments function
(defun org-babel-variable-assignments:elixir (params)
  "Return Elixir code to assign variables.")

;; Default header arguments variable
(defvar org-babel-default-header-args:elixir '()
  "Default header arguments for Elixir code blocks.")

;; Prep session function
(defun org-babel-prep-session:elixir (session params)
  "Prepare SESSION with PARAMS.")
```

### Private Functions

Use double hyphens for internal/private functions:

```elisp
;; Public API
(defun ob-elixir-evaluate (body params)
  "Evaluate BODY with PARAMS.")

;; Internal helper - note the double hyphen
(defun ob-elixir--format-value (value)
  "Internal: Format VALUE for Elixir.")

(defun ob-elixir--clean-output (output)
  "Internal: Remove ANSI codes from OUTPUT.")
```

---

## Variables and Customization

### User-Customizable Variables

Use `defcustom` for variables users should configure:

```elisp
(defgroup ob-elixir nil
  "Org Babel support for Elixir."
  :group 'org-babel
  :prefix "ob-elixir-")

(defcustom ob-elixir-command "elixir"
  "Command to invoke Elixir.
Can be a full path or just the command name if in PATH."
  :type 'string
  :group 'ob-elixir)

(defcustom ob-elixir-iex-command "iex"
  "Command to invoke IEx for session mode."
  :type 'string
  :group 'ob-elixir)

(defcustom ob-elixir-mix-command "mix"
  "Command to invoke Mix."
  :type 'string
  :group 'ob-elixir)

(defcustom ob-elixir-default-session-timeout 30
  "Default timeout in seconds for session operations."
  :type 'integer
  :group 'ob-elixir)

;; For options with specific choices
(defcustom ob-elixir-output-format 'value
  "Default output format for results."
  :type '(choice (const :tag "Return value" value)
                 (const :tag "Standard output" output))
  :group 'ob-elixir)
```

### Internal Variables

Use `defvar` for internal state:

```elisp
(defvar ob-elixir--session-buffer nil
  "Current active session buffer.
This is internal state and should not be modified directly.")

(defvar ob-elixir--pending-output ""
  "Accumulated output waiting to be processed.")
```

### Constants

Use `defconst` for values that never change:

```elisp
(defconst ob-elixir-eoe-indicator "__ob_elixir_eoe__"
  "String used to indicate end of evaluation output.")

(defconst ob-elixir-error-regexp "\\*\\* (\\w+Error)"
  "Regexp to match Elixir error messages.")
```

---

## Function Definitions

### Function Documentation

Always include comprehensive docstrings:

```elisp
(defun ob-elixir-evaluate (body params)
  "Evaluate BODY as Elixir code according to PARAMS.

BODY is the source code string to execute.
PARAMS is an alist of header arguments.

Supported header arguments:
  :session  - Name of IEx session to use, or \"none\"
  :results  - How to handle results (value, output)
  :var      - Variables to inject into the code

Returns the result as a string, or nil if execution failed.

Example:
  (ob-elixir-evaluate \"1 + 1\" \\='((:results . \"value\")))"
  ...)
```

### Interactive Commands

Mark user-facing commands as interactive:

```elisp
(defun ob-elixir-check-installation ()
  "Check if Elixir is properly installed and accessible.
Display version information in the minibuffer."
  (interactive)
  (let ((version (shell-command-to-string
                  (format "%s --version" ob-elixir-command))))
    (message "Elixir: %s" (string-trim version))))
```

### Using `cl-lib` Properly

Prefer `cl-lib` functions over deprecated `cl` package:

```elisp
(require 'cl-lib)

;; Use cl- prefixed versions
(cl-defun ob-elixir-parse-result (output &key (format 'value))
  "Parse OUTPUT according to FORMAT."
  ...)

(cl-loop for pair in params
         when (eq (car pair) :var)
         collect (cdr pair))

(cl-destructuring-bind (name . value) variable
  ...)
```

---

## Error Handling

### Signaling Errors

Use appropriate error functions:

```elisp
;; For user errors (incorrect usage)
(user-error "No Elixir session found for %s" session-name)

;; For programming errors
(error "Invalid parameter: %S" param)

;; Custom error types
(define-error 'ob-elixir-error "Ob-elixir error")
(define-error 'ob-elixir-execution-error "Elixir execution error" 'ob-elixir-error)
(define-error 'ob-elixir-session-error "Elixir session error" 'ob-elixir-error)

;; Signaling custom errors
(signal 'ob-elixir-execution-error (list "Compilation failed" output))
```

### Handling Errors Gracefully

```elisp
(defun ob-elixir-safe-evaluate (body params)
  "Safely evaluate BODY, handling errors gracefully."
  (condition-case err
      (ob-elixir-evaluate body params)
    (ob-elixir-execution-error
     (message "Elixir error: %s" (cadr err))
     nil)
    (file-error
     (user-error "Cannot access Elixir command: %s" (error-message-string err)))
    (error
     (message "Unexpected error: %s" (error-message-string err))
     nil)))
```

### Input Validation

```elisp
(defun ob-elixir-evaluate (body params)
  "Evaluate BODY with PARAMS."
  ;; Validate inputs early
  (unless (stringp body)
    (error "BODY must be a string, got %S" (type-of body)))
  (unless (listp params)
    (error "PARAMS must be an alist"))
  (when (string-empty-p body)
    (user-error "Cannot evaluate empty code block"))
  ...)
```

---

## Documentation

### Commentary Section

Write helpful commentary:

```elisp
;;; Commentary:

;; ob-elixir provides Org Babel support for the Elixir programming language.
;;
;; Installation:
;;
;; 1. Ensure Elixir is installed and in your PATH
;; 2. Add to your init.el:
;;
;;    (require 'ob-elixir)
;;    (org-babel-do-load-languages
;;     'org-babel-load-languages
;;     '((elixir . t)))
;;
;; Usage:
;;
;; In an org file, create a source block:
;;
;;    #+BEGIN_SRC elixir
;;    IO.puts("Hello from Elixir!")
;;    1 + 1
;;    #+END_SRC
;;
;; Press C-c C-c to evaluate.
;;
;; Header Arguments:
;;
;; - :session NAME  Use a persistent IEx session
;; - :results value Return the last expression's value
;; - :results output  Capture stdout
;; - :mix PROJECT  Execute in Mix project context
;;
;; See the README for complete documentation.
```

### Info Manual Integration (Optional)

For larger packages, consider an info manual:

```elisp
(defun ob-elixir-info ()
  "Open the ob-elixir info manual."
  (interactive)
  (info "(ob-elixir)"))
```

---

## Dependencies and Loading

### Requiring Dependencies

```elisp
;;; Code:

;; Required dependencies
(require 'ob)
(require 'ob-ref)
(require 'ob-comint)
(require 'ob-eval)

;; Soft dependencies (optional features)
(require 'elixir-mode nil t)  ; t = noerror

;; Check for optional dependency
(defvar ob-elixir-has-elixir-mode (featurep 'elixir-mode)
  "Non-nil if elixir-mode is available.")
```

### Autoloads

For functions that should be available before the package loads:

```elisp
;;;###autoload
(defun org-babel-execute:elixir (body params)
  "Execute a block of Elixir code with Babel."
  ...)

;;;###autoload
(eval-after-load 'org
  '(add-to-list 'org-src-lang-modes '("elixir" . elixir)))
```

### Provide Statement

Always end with provide:

```elisp
(provide 'ob-elixir)
;;; ob-elixir.el ends here
```

---

## References

- [GNU Emacs Lisp Reference Manual](https://www.gnu.org/software/emacs/manual/elisp.html)
- [Emacs Lisp Style Guide](https://github.com/bbatsov/emacs-lisp-style-guide)
- [Packaging Guidelines (MELPA)](https://github.com/melpa/melpa/blob/master/CONTRIBUTING.org)
- [Writing GNU Emacs Extensions](https://www.gnu.org/software/emacs/manual/html_node/eintr/)
- [Org Mode Source Code](https://git.savannah.gnu.org/cgit/emacs/org-mode.git/)
- [Elisp Conventions (Emacs Wiki)](https://www.emacswiki.org/emacs/ElispConventions)