:PROPERTIES:
:ID:       994be8e1-6bf2-4621-adf2-1c9a5ec46521
:END:
#+title: Anki: Emacs
#+date: "2021-05-02 19:18:32 +08:00"
#+date_modified: "2021-05-10 08:40:00 +08:00"
#+language: en
#+property: anki_deck Emacs

* The overview of buffers
:PROPERTIES:
:ANKI_NOTE_TYPE: Styled cards
:ANKI_NOTE_ID: 1620039512243
:END:
** Front
What is a buffer?
** Back
A buffer is anything that Emacs displays.
It usually display file contents among other examples with ~butterfly~, ~doctor~, or the starting buffer when you first open Emacs.

While buffers usually have an associated file path, a buffer doesn't need one.
This is one of the concepts that is applied to other text editors (Vim, Atom, Visual Studio Code).

* The basics of modes
:PROPERTIES:
:ANKI_NOTE_TYPE: Styled cards
:ANKI_NOTE_ID: 1620039513634
:END:
** Front
What is a mode?
** Back
A mode is set of behavior quite similar to Vim modes.

Emacs further divides modes into two.

*Major modes are Emacs' way of supporting programming languages and file formats.*
Programming language support usually comes in major mode — e.g., ~R-mode~ for R files, ~python-mode~ for Python scripts, ~org-mode~ for Org mode documents.
Think of them as an equivalent to Vim's filetype.
Only one major mode can be activated in one buffer at a time and all buffers have a major mode.

*Minor modes usually contain little behavioral changes that improve the editing experience.*
When enabled, some of them are global modes — affecting every buffer in your session.
Others are only buffer-local — affecting only the buffer when you activated the mode.
Unlike major modes, multiple minor modes can be enabled at any given time.

* Eagle's eye view of a window
:PROPERTIES:
:ANKI_DECK: Emacs
:ANKI_NOTE_TYPE: Styled cards
:ANKI_NOTE_ID: 1620039514055
:END:
** Front
What is a window?
** Back
A window is where the buffers are being displayed.
One window can display all buffers but only one at a time.
To display two buffers at a single time, just add another window.

All windows display the same buffer;
if the buffer is modified in one of the window, it will show the changes in all windows.

* Point and marker
:PROPERTIES:
  :ANKI_NOTE_TYPE: Styled cards
  :ANKI_NOTE_ID: 1620040989788
  :END:
** Front
   What is a point and a marker?
** Back
   A point is the current location of the cursor in the buffer.
   You can get the point with ~point~ function.
   Often helpful for interacting with buffers.

   A marker is another point in the buffer.
   It is usually found when interacting with regions when asked for the two points (i.e., the beginning and the ending position).
   Furthermore, a marker can be used to save locations and jump back to that marker when asked.

* Interacting with buffers in Elisp
:PROPERTIES:
:ANKI_NOTE_TYPE: Styled cards
:ANKI_NOTE_ID: 1620387028893
:END:
** Front
Give a rough example on how to do the following:

Create a temporary buffer named "hello" containing an Org mode document with a "Hello world" entry.
** Back
#+begin_src elisp  :exports code
(with-temp-buffer
  (insert "* Hello world")
  (rename-buffer "hello")
  (org-mode))
#+end_src

* String comparison in Emacs Lisp
:PROPERTIES:
:ANKI_NOTE_TYPE: Styled cards
:ANKI_NOTE_ID: 1620387016294
:END:
** Front
How to compare two strings?
** Back
~(string= STR1 STR2)~

#+begin_src elisp  :exports both
(print (string= "WHOA" "whoa"))
(print (string= "WHOA" (upcase "whoa")))
(print (string= "Hello world" "HeLL0 World"))
#+end_src

#+results:
:
: nil
:
: t
:
: nil

* Using the help system
:PROPERTIES:
:ANKI_NOTE_TYPE: Styled cards
:ANKI_NOTE_ID: 1620386877252
:END:
** Front
Name different ways to use the help system inside Emacs.
** Back
- ~help-for-help~ is the most comprehensive help section (in my opinion).
- ~describe-*~ series of functions are the next.
  Among the list of describe functions, you have: ~describe-key~, ~describe-function~, ~describe-variable~, and ~describe-package~.
  You can just open up the minibuffer and see what else is there.
- ~apropos~ is similar to Unix apropos command which searches for every symbol in Emacs.

* Word manipulation
:PROPERTIES:
:ANKI_NOTE_TYPE: Styled cards
:ANKI_NOTE_ID: 1620607164221
:END:
** Front
Give some functions on word manipulations.
** Back
The following functions have multiple variations each for a character (~$F-char~), word (~$F-word~), region (~$F-region~), and region or point (~$F-dwim~).

- ~capitalize-*~ for making the first of the word in uppercase.
- ~downcase-*~ for making a region all lowercase.
- ~upcase-*~ for making a region all uppercase.

evil-mode has a keybinding associated with uppercase and downcase a certain region with ~evil-upcase~ and ~evil-downcase~, respectively.

* Line manipulation
:PROPERTIES:
:ANKI_NOTE_TYPE: Styled cards
:ANKI_NOTE_ID: 1620607164494
:END:
** Front
Give some functions on manipulating lines.
** Back
- evil-mode has ~evil-join~ which works the same way Vim's join complete with smart spacing and everything.

- ~fill-region~ is useful for formatting requirements/preferences like in the Linux kernel where the maximum width of 80 characters.
  evil-mode has an associated keybinding function with ~evil-fill~.

- ~sort-lines~ is pretty useful for the common task of sorting lines.
  Though, not useful for items that consist of multiple lines.