NonGNU ELPA - haskell-ts-mode

A haskell mode that uses treesitter.

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

an overview of the features are:

• Syntax highliting
• Indentation
• Imenu support
• REPL
• Prettify symbols mode support

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.

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?

#+BEGIN_SRC: elisp (add-to-list 'load-path "path/to/haskell-ts-mode") (require 'haskell-ts-mode) #+END_SRC

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

If that is not enough, you can customize haskell-ts-font-lock-feature-list

• support for customization UI
• 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.