A Haskell mode that uses Tree-sitter.

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

• C-c C-r Open REPL
• C-c C-c Send region to REPL (or reload with :r when no region is active)
• C-c C-l Load the current file into the REPL (starts one if needed)
• C-c C-e Send the current line to the REPL
• C-M-x Send the definition at point to the REPL

Overview of features:

• Syntax highlighting
• Structural navigation
• Comment-aware editing: prose motion, paragraph text objects, and RET continuation inside =–=/Haddock comments
• Imenu support
• REPL (C-c C-r in the mode to run)
• Prettify Symbols mode support
• M-x align support (align the ​=​ signs in a region)

C-c C-r (haskell-ts-run) starts an inferior Haskell process. When the current buffer is inside a cabal project (a cabal.project or *.cabal file is found by walking up the directory tree), the process is started with cabal repl, so the project's dependencies, default language extensions and GHC options are available in the session. Outside a cabal project it falls back to plain ghci. This is controlled by haskell-ts-use-cabal (default auto; set it to nil to always use ghci, or t to always use cabal).

The process is started in the project root when one is found, so relative import's and the module search path resolve from there. When the current file belongs to several cabal components, haskell-ts-run prompts for one and remembers the choice per buffer; a prefix argument (C-u C-c C-r) forces the pick even for an unambiguous file.

• C-c C-c sends the active region to the REPL (or :r reloads when no region is active).
• C-c C-l saves the buffer and loads its file with :load, starting a session if none is running. The REPL is displayed without leaving the current buffer.
• C-c C-e sends the current line to the REPL, verbatim.
• C-M-x sends the definition at point to the REPL – the same definition treesit-defun-name-function and imenu use, so it is either a top-level binding or, from inside a where=/=let block, the enclosing local one.

The inferior buffer runs haskell-ts-inferior-mode (derived from comint-mode): the prompt is read-only and input history is persisted across sessions in haskell-ts-inferior-history-file. Filename completion and history navigation (M-p / M-n, TAB) come from comint.

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 variables 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 accumulated 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 functionality, and leaves other tasks to external packages.

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?

This mode provides structural 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.

Add this into your init.el:

(use-package haskell-ts-mode
 :ensure t
 :custom
 (haskell-ts-font-lock-level 4)
 (haskell-ts-ghci "ghci"))

You also need the Tree-sitter Haskell grammar; see Installing the grammar below.

See CHANGELOG.org for the list of notable changes.