#+TITLE:   ui/ligatures
#+DATE:    June 16, 2018
#+SINCE:   v2.0.9
#+STARTUP: inlineimages nofold

* Table of Contents :TOC_3:noexport:
- [[#description][Description]]
  - [[#maintainers][Maintainers]]
  - [[#module-flags][Module Flags]]
    - [[#font-ligatures-module-flags][Font ligatures module flags]]
  - [[#plugins][Plugins]]
- [[#prerequisites][Prerequisites]]
  - [[#mutsuharus-emacs-mac-port-or-emacs-28-with-harfbuzz-support][Mutsuharu's emacs-mac port or Emacs 28+ with Harfbuzz support]]
  - [[#not-emacs-mac-and-emacs--27][Not Emacs-mac and Emacs <= 27]]
- [[#features][Features]]
  - [[#mathematical-symbols-replacement][Mathematical symbols replacement]]
  - [[#coding-ligatures][Coding ligatures]]
- [[#configuration][Configuration]]
  - [[#set-ligatures][~set-ligatures!~]]
- [[#troubleshooting][Troubleshooting]]

* Description
This module enables ligatures and arbitrary symbol substitutions with
~mac-auto-operator-composition-mode~ (on supported macOS systems) or composition
tables (harfbuzz on Emacs 28), falling back on ~prettify-symbols-mode~
otherwise.

** Maintainers
This module has no dedicated maintainers.

** Module Flags
+ =+extra= Enables extra symbol substitutions in certain modes, for example
  ~lambda~ in lisps are replaced with ~λ~.

*** Font ligatures module flags
This module provides four flags for enabling fall-back ligature support for a
particular font. They are:

+ =+fira= Enables =Fira Code= ligatures. This requires Fira Code Symbol and a
  patched version of Fira Code (see below).
+ =+hasklig= Enable =Hasklig= ligatures. This requires a patched version of the
  HaskLig font (see below).
+ =+iosevka= Enable =Iosevka= ligatures. This requires a patched version of the
  Iosevka font (see below).
+ =+pragmata-pro= Enable =Pragmata Pro= ligatures. This requires the [[https://www.fsd.it/shop/fonts/pragmatapro/][Pragmata
  Pro font]].

#+begin_quote
All these flags are ignored _if_ you're sporting either a) Emacs 28+ with
Harfbuzz support (which can compose ligatures natively), or b) Mitsuharu's
=emacs-mac= build on macOS (which uses ~mac-auto-operator-composition-mode~).
#+end_quote

** Plugins
This module installs no packages.

* Prerequisites
This module requires one of three setups for ligatures to work:

- A recent enough version of Emacs which will compose ligatures automatically
  (Emacs 28 with Harfbuzz support), or
- Mitsuharu's =emacs-mac= build on macOS (available on homebrew), or
- A patched font for Doom's fallback ligature support.

** Mutsuharu's emacs-mac port or Emacs 28+ with Harfbuzz support
Ligatures should be handled without any additional configuration.

** Not Emacs-mac and Emacs <= 27
1. Enable one of the four ligature font flags: =+fira=, =+hasklig=, =+iosevka=
   or =+pragmata-pro=.
2. Install the patched version of the associated font with ~M-x
   +ligatures/install-patched-font~. Note: Pragmata Pro cannot be installed this
   way because it is a non-free font and must be purchased and installed
   manually.

* TODO Features
** TODO Mathematical symbols replacement
** Coding ligatures
This module includes configuration to compose combinations like =->= or =::=
into prettier glyphs (called a ligature). Depending on the current version of
emacs, this is implemented in two different ways :

- prettify-symbols-mode method :: this is the "legacy" method. It uses a font
  which haves the ligatures as separate unicode symbols, and using
  prettify-symbols-mode, =->=-like combinations are manually listed and replaced
  with the correct symbol. The mapping between =->=-like sequences and unicode
  values in the font are font-specific ; therefore =+fira=, =+iosevka=... files
  and specific fonts are necessary for it to work.
- composition-function-table method :: regexps are used to match all the usual
  sequences which are composed into ligatures. These regexps are passed to emacs
  directly, which asks Harfbuzz to shape it. Ligatures are obtained
  automatically depending on the capabilities of the font, and no font-specific
  configuration is necessary.

Emacs-mac port implements the /composition-function-table/ method in [[https://bitbucket.org/mituharu/emacs-mac/src/26c8fd9920db9d34ae8f78bceaec714230824dac/lisp/term/mac-win.el?at=master#lines-345:805][its code]],
nothing is necessary on Doom side; otherwise, Doom implements the
/composition-function-table/ for emacs 28+ built with Harfbuzz support, and the
/prettify-symbols-mode/ method otherwise.

Even though harfbuzz has been included in emacs 27, there is currently a [[https://lists.gnu.org/archive/html/bug-gnu-emacs/2020-04/msg01121.html][bug
(#40864)]] which prevents a safe usage of /composition-function-table/ method in
emacs 27.

* TODO Configuration
** TODO ~set-ligatures!~

* TODO Troubleshooting
# Common issues and their solution, or places to look for help.