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