haskell-mode setup:

(add-hook 'after-init-hook 'global-company-mode)

(add-hook 'haskell-mode-hook
          (lambda ()
            (set (make-local-variable 'company-backends)
                 (append '((company-capf company-dabbrev-code))
                         company-backends))))

(require 'haskell-interactive-mode)
(require 'haskell-process)
(add-hook 'haskell-mode-hook 'interactive-haskell-mode)

(custom-set-variables
  '(haskell-process-suggest-remove-import-lines t)
  '(haskell-process-auto-import-loaded-modules t)
  '(haskell-process-log t))

(require 'company-ghci)
(push 'company-ghci company-backends)
(add-hook 'haskell-mode-hook 'company-mode)
;;; To get completions in the REPL
(add-hook 'haskell-interactive-mode-hook 'company-mode)