nix-haskell-mode is an easy way to get Haskell projects set up in Emacs. It uses haskell-mode’s interactive-haskell-mode to provide integration with Emacs. Nix is used to pull in dependencies of each Cabal file.

To use with use-package, just add this to your configuration,

(use-package nix-haskell-mode
  :hook (haskell-mode . nix-haskell-mode))

nix-haskell-mode is available in MELPA. You can install it yourself with M-x package-install nix-haskell-mode. Alternative configuration for vanilla Emacs is available below,

(require 'haskell-mode)
(require 'nix-haskell-mode)
(add-hook 'haskell-mode-hook 'nix-haskell-mode)

This configuration should work out of the box with Cabal projects. If you have custom dependencies that need to be available, you should create override in Haskell packages in Nix. More documentation on this is available in the Nixpkgs manual available here:

https://nixos.org/nixpkgs/manual/#users-guide-to-the-haskell-infrastructure

These modes are affected by nix-haskell-mode. They are listed below along with what variables are set by nix-haskell-mode.

• Flycheck - sets flycheck-ghc-package-databases and flycheck-haskell-ghc-executable.
• Haskell Mode - sets haskell-process-type to cabal-new-repl, haskell-process-path-cabal, haskell-process-args-cabal-new-repl.

This project is available at http://github.com/matthewbauer/nix-haskell-mode. Pull requests and issues are accepted.

Occasionally you will want to disable nix-haskell-mode. This might occur in very large projects where interactive-haskell-mode fails at or when some dependency cannot be fetched succesfully. In these cases, you should run the following in the root of your project:

=M-x add-dir-local-variable<RET>haskell-mode<RET>eval<RET>(nix-haskell-mode -1)<RET>=