5 Completion support

haskell-mode can complete symbols, pragma directives, language extensions, and language keywords out-of-box. haskell-mode completes identifiers (symbols) using tags (see “Tags”), however you can get more precise completions with haskell-interactive-mode. In interactive mode completion candidates are produced by querying GHCi REPL.

If haskell-interactive-mode is enabled and working Haskell mode provides completions for import statements taking into account currently loaded and available packages. Also it completes symbols querying REPL with :complete command, hence completion candidate list also includes symbols from imported modules.

Unfortunately, it is not possible to provide candidates for identifiers defined locally in let and where blocks even in interactive mode. But if you’re using company-mode you can override company-backends variable for Haskell buffers to combine completion candidates from completion-at-point function (company-capf backend) and dynamic abbrevs. company-mode provides special backend for dabbrev code completions, namely company-dabbrev-code. To combine completions from diffrent backends you can create grouped backends, it is very easy — a grouped backend is just a list of backends, for example:

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

If you use a GHCi version prior to 8.0.1 you might want to set haskell-completions-complete-operators to nil, if you experience major slowdown while trying to complete after an Haskell operator (See GHC-Bug 10576).