Nix setup for Spacemacs

Edit 2020-06-22: I’ve since found a better setup for this.

When using ghcide and LSP, as I wrote about in my post on Haskell, ghcide, and Spacemacs, I found myself ending up recompiling a little too often. This pushed me to finally start looking at Nix. After a bit of a fight I managed to get ghcide from Nix, which brought me the issue of setting up Spacemacs. Inspired by a gist from Samuel Evans-Powell and a guide to setting up an environment for Reflex by Thales Macedo Garitezi I ended up with the following setup:

(defun dotspacemacs/layers ()
  (setq-default
   ...
   dotspacemacs-additional-packages
   '(
     nix-sandbox
     nix-haskell-mode
     ...
     )
   ...
   ))
(defun dotspacemacs/user-config ()
  ...
  (add-hook 'haskell-mode-hook #'lsp)
  (add-hook 'haskell-mode-hook 'nix-haskell-mode)
  (add-hook 'haskell-mode-hook
            (lambda ()
              (setq-local flycheck-executable-find
                          (lambda (cmd)
                            (nix-executable-find (nix-current-sandbox) cmd)))
              (setq-local flycheck-command-wrapper-function
                          (lambda (argv)
                            (apply 'nix-shell-command (nix-current-sandbox) argv)))
              (setq-local haskell-process-wrapper-function
                          (lambda (argv)
                            (apply 'nix-shell-command (nix-current-sandbox) argv)))
              (setq-local lsp-haskell-process-wrapper-function
                          (lambda (argv)
                            `("nix-shell" "-I" "." "--command" "ghcide --lsp" ,(nix-current-sandbox))))))
  (add-hook 'haskell-mode-hook
            (lambda ()
              (flycheck-add-next-checker 'lsp-ui '(warning . haskell-stack-ghc))))
  ...
  )

It seems to work, but please let me know if you have suggestions for improvements.

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