Work in progress on using ghc-mod for flycheck.

ghc-mod.el

(flycheck-define-checker haskell-ghc-mod
  "A Haskell syntax and type checker using ghc-mod."
  :command ("ghc-mod" "check" source)
  :error-patterns
  ((warning line-start (file-name) ":" line ":" column ":"
            "Warning:" (zero-or-more space)
            (message (one-or-more not-newline))
            line-end)
   (error line-start (file-name) ":" line ":" column ":"
          (zero-or-more space)
          (message (one-or-more not-newline))
          line-end))
  :error-filter
  (lambda (errors)
    ;; The commented line below should be the same as the uncommented code below, but doesn't rely on dash.
    ;; (flycheck-sanitize-errors (flycheck-dedent-error-messages (flycheck-replace-nulls errors))))
    (-> errors
      flycheck-replace-nulls
      flycheck-dedent-error-messages
      flycheck-sanitize-errors
      ))
  :modes (haskell-mode literate-haskell-mode)
  )

;; Not ready, but kept around for reference.
;; (flycheck-define-generic-checker 'haskell-ghc-modi
;;   "A Haskell syntax and type checker using ghc-mod."
;;   :start flycheck-haskell-ghc-modi-start
;;   :error-patterns
;;   ((warning line-start (file-name) ":" line ":" column ":"
;;             "Warning:" (zero-or-more space)
;;             (message (one-or-more not-newline))
;;             line-end)
;;    (error line-start (file-name) ":" line ":" column ":"
;;           (zero-or-more space)
;;           (message (one-or-more not-newline))
;;           line-end))
;;   :error-filter
;;   (lambda (errors)
;;     ;; (flycheck-sanitize-errors (flycheck-dedent-error-messages (flycheck-replace-nulls errors))))
;;     (-> errors
;;       flycheck-replace-nulls
;;       flycheck-dedent-error-messages
;;       flycheck-sanitize-errors
;;       ))
;;   :modes (haskell-mode literate-haskell-mode)
;;   )

;; (defun flycheck-haskell-ghc-modi-start (checker callback)
;;   (setq lexical-binding t)
;;   (let ((buffer (current-buffer)))
;;     ;; send command to ghc process with callback
;;     ))

(defun flycheck-replace-nulls (errors)
  (dolist (err errors)
    (let ((msg (flycheck-error-message err)))
      (setq msg (replace-regexp-in-string "\x0" "\n" msg))
      (setf (flycheck-error-message err) msg)))
  errors)