1 

     2 * haskell-ts-mode

     3 

     4 A haskell mode that uses treesitter.

     5 

     6 * Screenshot

     7 

     8 [[./ss.png]]

     9 

    10 The above screenshot is indented coloured using haskell-ts-mode, with

    11 prettify-symbols-mode enabled.

    12 

    13 * Usage

    14 

    15 =C-c C-r= to open REPL

    16 =C-c C-c= to send code to repl

    17 =C-M-q=   Indent paragraph

    18 

    19 * Features

    20 an overview of the features are:

    21 - Syntax highliting

    22 - Indentation

    23 - Imenu support

    24 - REPL (C-c r in the mode to run)

    25 - Prettify symbols mode support

    26 

    27 * Comparasion with haskell-mode

    28 The more interesting features are:

    29 - Logical syntax highlighting:

    30   - Only arguments that can be used in functions are highlighted, eg

    31     in `f (_:(a:[])) only 'a' is highlighted, as it is the only

    32     variable that is captured that can be used in body of function

    33   - The return type of a function is highlighted

    34   - All new variabels are(or should be) highlighted, this includes

    35     generators, lambda args.

    36   - highlighting the '=' operaotr in guarded matches correctly, this

    37     would be stupidly hard in regexp based syntax

    38 - Unlike haskell-mode, quasi quotes are understood and do not confuse

    39   the mode, thanks to treesitter

    40 - Predictable (but less powerful) indentation: haskell-mode's

    41   indentation works in a cyclical way, it cycles through where you

    42   might want indentation.  haskell-ts-mode, meanwhile relies on you to

    43   set the concrete syntax tree changing whitespace.

    44 - More perfomant, this is especially seen in longer files

    45 - Much much less code, haskell mode has accumlated 30,000 lines of

    46   features to do with all things haskell related, this mode just keeps

    47   the scope to basic major mode stuff, and leaves other stuff for

    48   external packages.

    49 

    50 * Motivation

    51   

    52 haskell-mode contains nearly 30k lines of code, and is

    53 about 30 years old.  Therefore, a lot of stuff emacs has gained the

    54 ability to do in those years, haskell-mode already has implemented

    55 them.

    56 

    57 In 2018, a mode called haskell-tng-mode was made to solve some of

    58 these problems. However because of haskell's syntax, it too became

    59 very complex and required a web of dependencies.

    60 

    61 Both these modes ended up practically parsing haskells syntax to

    62 implement indentation, so I thought why not use tree sitter?

    63 

    64 * Installation

    65 

    66 The package is avaiable on elpa, you can install it using:

    67 M-x package-install RET haskell-ts-mode RET

    68 

    69 #+begin_src elisp

    70 (add-to-list 'load-path "path/to/haskell-ts-mode")

    71 (require 'haskell-ts-mode)

    72 #+end_src

    73 

    74 * Customization

    75 

    76 If colour is too much or too less for you, adjust

    77 treesit-font-lock-level accordingly.

    78 

    79 If you want to highlight signature declarations (disabled by default),

    80 add the following to your init file:

    81 #+begin_src emacs-lisp

    82 (setq haskell-ts-highlight-signature t)

    83 #+end_src

    84 

    85 ** how to disable haskell-ts-mode indentation

    86 

    87 #+begin_src emacs-lisp

    88 (setq haskell-ts-use-indent nil)

    89 #+end_src

    90 

    91 ** Pretify symbols mode

    92 prettify symbols mode can be used to replace common symbols with

    93 unicode alternatives.

    94 

    95 #+begin_src emacs-lisp

    96 (add-hook 'haskell-ts-mode 'prettify-symbols-mode)

    97 #+end_src

    98 

    99 ** Adjusting font lock level

   100 set haskell-ts-font-lock-level accordingly.

   101 

   102 ** Language server

   103 

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

   105 

   106 To enable eglot support, use the following code in your init.el:

   107 #+begin_src emacs-lisp

   108 (with-eval-after-load 'eglot (haskell-ts-setup-eglot))

   109 #+end_src

   110 

   111 * TODO and limitations

   112 - Imenu support for functions with multiple definitions

   113 

   114 Limitations: _Proper indenting of multiline signatures_: the

   115 treesitter grammer does not flatten the signautes, rather leaves them

   116 to the standard infix interpretatoin. This makes indentation hard, as

   117 it will mean the only way to check if the the signature node is an

   118 ancestor of node at point is to perfom a recursive ascent, which is

   119 horrible for perfomance.