* haskell-ts-mode

A haskell mode that uses treesitter.

* Screenshot

[[./ss.png]]

The above screenshot is indented coloured using haskell-ts-mode, with
prettify-symbols-mode enabled.

* Usage

=C-c C-r= to open REPL
=C-c C-c= to send code to repl
=C-M-q=   Indent paragraph

* Features
an overview of the features are:
- Syntax highliting
- Indentation
- Imenu support
- REPL (C-c r in the mode to run)
- Prettify symbols mode support

* Comparasion with haskell-mode
The more interesting features are:
- Logical syntax highlighting:
  - Only arguments that can be used in functions are highlighted, eg
    in `f (_:(a:[])) only 'a' is highlighted, as it is the only
    variable that is captured that can be used in body of function
  - The return type of a function is highlighted
  - All new variabels are(or should be) highlighted, this includes
    generators, lambda args.
  - highlighting the '=' operaotr in guarded matches correctly, this
    would be stupidly hard in regexp based syntax
- Unlike haskell-mode, quasi quotes are understood and do not confuse
  the mode, thanks to treesitter
- Predictable (but less powerful) indentation: haskell-mode's
  indentation works in a cyclical way, it cycles through where you
  might want indentation.  haskell-ts-mode, meanwhile relies on you to
  set the concrete syntax tree changing whitespace.
- More perfomant, this is especially seen in longer files
- Much much less code, haskell mode has accumlated 30,000 lines of
  features to do with all things haskell related, this mode just keeps
  the scope to basic major mode stuff, and leaves other stuff for
  external packages.

* Motivation
  
haskell-mode contains nearly 30k lines of code, and is
about 30 years old.  Therefore, a lot of stuff emacs has gained the
ability to do in those years, haskell-mode already has implemented
them.

In 2018, a mode called haskell-tng-mode was made to solve some of
these problems. However because of haskell's syntax, it too became
very complex and required a web of dependencies.

Both these modes ended up practically parsing haskells syntax to
implement indentation, so I thought why not use tree sitter?

* Installation

The package is avaiable on elpa, you can install it using:
M-x package-install RET haskell-ts-mode RET

#+begin_src elisp
(add-to-list 'load-path "path/to/haskell-ts-mode")
(require 'haskell-ts-mode)
#+end_src

* Customization

If colour is too much or too less for you, adjust
treesit-font-lock-level accordingly.

If you want to highlight signature declarations (disabled by default),
add the following to your init file:
#+begin_src emacs-lisp
(setq haskell-ts-highlight-signature t)
#+end_src

** how to disable haskell-ts-mode indentation

#+begin_src emacs-lisp
(setq haskell-ts-use-indent nil)
#+end_src

** Pretify symbols mode
prettify symbols mode can be used to replace common symbols with
unicode alternatives.

#+begin_src emacs-lisp
(add-hook 'haskell-ts-mode 'prettify-symbols-mode)
#+end_src

** Adjusting font lock level
set haskell-ts-font-lock-level accordingly.

** Language server

haskell-ts-mode is compatiable with lsp-haskell.

To enable eglot support, use the following code in your init.el:
#+begin_src emacs-lisp
(with-eval-after-load 'eglot (haskell-ts-setup-eglot))
#+end_src

* TODO and limitations
- Imenu support for functions with multiple definitions

Limitations: _Proper indenting of multiline signatures_: the
treesitter grammer does not flatten the signautes, rather leaves them
to the standard infix interpretatoin. This makes indentation hard, as
it will mean the only way to check if the the signature node is an
ancestor of node at point is to perfom a recursive ascent, which is
horrible for perfomance.