NOTICE: Indentation has temporarily been disabled by default. SEE THE INDENTATION SECTION for why this was needed and what the plan is. TLDR indentation is going through a full rewrite.

Contributing

Please contribute ideas on how the new indentation system would work. See rewriting the indentation section.

Haskell mode based on treesitter

A Haskell mode that uses Tree-sitter.

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

Usage

• C-c C-r Open REPL
• C-c C-c Send code to REPL
• M-q Indent the function

Features

Overview of features:

• Syntax highlighting
• Structural navigation
• Indentation (now disabled)
• Imenu support
• REPL (C-c C-r in the mode to run)
• Prettify Symbols mode support

Comparison with haskell-mode

The more interesting features are:

• Logical syntax highlighting:
  • Only arguments that can be used in functions are highlighted, e.g., in f (_:(a:[])) only a is highlighted, as it is the only variable that is captured, and that can be used in the body of the function.
  • The return type of a function is highlighted.
  • All new variabels are (or should be) highlighted, this includes generators, lambda arguments.
  • Highlighting the === operator in guarded matches correctly, this would be stupidly hard in regexp based syntax.
• More performant, this is especially seen in longer files.
• Much, much less code, haskell-mode has accumlated 30,000 lines of code and features to do with all things Haskell related. haskell-ts-mode just keeps the scope to basic major mode stuff, and leaves other stuff to external packages.

Motivation

haskell-mode contains nearly 30k lines of code, and is about 30 years old. A lot of features implemented by haskell-mode are now also available in standard Emacs, and have thus become obsolete.

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 Haskell’s syntax to implement indentation, so I thought why not use Tree-sitter?

Structural navigation

This mode provides strucural navigation, for Emacs 30+.

combs (x:xs) = map (x:) c ++ c
  where c = combs xs

In the above code, if the pointer is right in front of the function definition combs, and you press C-M-f (forward-sexp), it will take you to the end of the second line.

Installation

Add this into your init.el:

(use-package haskell-ts-mode
  :ensure t
  :custom
  (haskell-ts-font-lock-level 4)
  (haskell-ts-use-indent t)
  (haskell-ts-ghci "ghci")
  (haskell-ts-use-indent t)
  :config
  (add-to-list 'treesit-language-source-alist
   '(haskell . ("https://github.com/tree-sitter/tree-sitter-haskell" "v0.23.1")))
  (unless (treesit-grammar-location 'haskell)
   (treesit-install-language-grammar 'haskell)))

That is all. This will install the grammars if not already installed. However, you might need to update the grammar version in the future.

Other recommended packages

Unlike haskell-mode, this mode has a limited scope to just worrying about haskell. There are other packages that I find help a lot with development:

• consult-hoogle great interface for hoogle.
• dante
• apheleia I suggest using this with ormolu to provide formatting for haskell.
• hcel Codebase navigator, if you want a lighter alternaitve to a full blown LSP.

Customization

Indentation <p:indent>

Indentation has been disabled by default. To enable it, use the following code.

(setq haskell-ts-use-indent t)

Why indentation has been disabled temporarily

Simply because the indention code became a monstrosity. Don’t belive me? check the haskell-ts-indent-rules variable. Bugs are rampent, fixing one bug created another. Its a torturous game of wack-a-mole with no end in sight.

Indentation rewrite <p:rewrite>

Check out the newindent branch to see the repo to see the progress.

There are some options to rewriting indentation:

1. Do the same approach of having strict indentation that doesn’t modify the syntax tree, just impliment it better, potentilly using a style guide, like this one.
2. Rely on a external package like hi2 or hyai, at the compromise they don’t use treesitter, so just end up inefficialty reparsing haskell.
3. My prefered: try to impliment haskell-mode type indentation. I have no idea how we would do this, since each indentation attempt could change the parse tree, changing the treesitter concrete syntax tree.
4. Adaptive indentation: Like python-mode, we can try memorise the user’s indentation prefrences.

Pretify Symbols mode

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

Turning on prettify-symbols-mode does stuff like turn -> to →. If you want to prettify words, set haskell-ts-prettify-words to non-nil. This will do stuff like prettify forall into ∀ and elem to ∈.

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

Adjusting font lock level

Set haskell-ts-font-lock-level accordingly. The default and highest value is 4. You are against vibrancy, you can lower it to match your dreariness.

Language server

haskell-ts-mode works with lsp-mode and, since Emacs 30, with eglot.

To add eglot support on Emacs 29 and earlier, add the following code to your init.el:

(with-eval-after-load 'eglot
  (defvar eglot-server-programs)
  (add-to-list 'eglot-server-programs
               '(haskell-ts-mode . ("haskell-language-server-wrapper" "--lsp"))))

list

• Support for M-x align, so that calling it will align all the ‘=’ signs in a region.
• Imenu support for functions with multiple definitions.
• Merge the indent branch