Replace completing-read-function with custom for haskell-mode

This should be backwards compatible with emacs23, which does not have
completing-read-function

ghc-core.el

@@ -29,7 +29,6 @@
 ;; and removal of commonly ignored annotations.
 
 ;;; Code:
-
 (require 'haskell-mode)
 (require 'haskell-font-lock)
 

haskell-mode.el

@@ -133,8 +133,6 @@
 (require 'haskell-string)
 (with-no-warnings (require 'cl))
 
-(defvar completing-read-function)
-
 ;; FIXME: code-smell: too many forward decls for haskell-session are required here
 (defvar haskell-session)
 (declare-function haskell-process-do-try-info "haskell-process" (sym))
@@ -641,6 +639,15 @@ If nil, use the Hoogle web-site."
                  (const "ghc -fno-code")
                  (string :tag "Other command")))
 
+(defcustom haskell-completing-read-function 'ido-completing-read
+  "Default function to use for completion."
+  :group 'haskell
+  :type '(choice
+          (function-item :tag "ido" :value ido-completing-read)
+          (function-item :tag "helm" :value helm--completing-read-default)
+          (function-item :tag "completing-read" :value completing-read)
+          (function :tag "Custom function")))
+
 (defcustom haskell-stylish-on-save nil
   "Whether to run stylish-haskell on the buffer before saving."
   :group 'haskell
@@ -717,7 +724,7 @@ Brings up the documentation for haskell-mode-hook."
     (cond ((save-excursion (forward-word -1)
                            (looking-at "^import$"))
            (insert " ")
-           (let ((module (funcall completing-read-function "Module: " (haskell-session-all-modules))))
+           (let ((module (funcall haskell-completing-read-function "Module: " (haskell-session-all-modules))))
              (insert module)
              (haskell-mode-format-imports)))
           ((not (string= "" (save-excursion (forward-char -1) (haskell-ident-at-point))))

haskell-process.el

@@ -36,8 +36,6 @@
 (require 'haskell-navigate-imports)
 (with-no-warnings (require 'cl))
 
-(defvar completing-read-function)
-
 ;; FIXME: haskell-process shouldn't depend on haskell-interactive-mode to avoid module-dep cycles
 (declare-function haskell-interactive-mode-echo "haskell-interactive-mode" (session message &optional mode))
 (declare-function haskell-interactive-mode-compile-error "haskell-interactive-mode" (session message))
@@ -434,7 +432,7 @@ for various things, but is optional."
   "Prompts for a Cabal command to run."
   (interactive)
   (haskell-process-do-cabal
-   (funcall completing-read-function "Cabal command: "
+   (funcall haskell-completing-read-function "Cabal command: "
                         haskell-cabal-commands)))
 
 (defun haskell-process-add-cabal-autogen ()
@@ -757,7 +755,7 @@ now."
            ((> (length modules) 1)
             (when (y-or-n-p (format "Identifier `%s' not in scope, choose module to import?"
                                     ident))
-              (funcall completing-read-function "Module: " modules)))
+              (funcall haskell-completing-read-function "Module: " modules)))
            ((= (length modules) 1)
             (when (y-or-n-p (format "Identifier `%s' not in scope, import `%s'?"
                                     ident

haskell-session.el

@@ -29,10 +29,9 @@
 
 (require 'haskell-cabal)
 (require 'haskell-string)
+(require 'haskell-mode)
 (with-no-warnings (require 'cl))
 
-(defvar completing-read-function)
-
 (declare-function haskell-interactive-mode "haskell-interactive-mode" ())
 (declare-function haskell-kill-session-process "haskell-process" (&optional session))
 (declare-function haskell-process-start "haskell-process" (session))
@@ -202,7 +201,7 @@ If DONTCREATE is non-nil don't create a new session."
 (defun haskell-session-choose ()
   "Find a session by choosing from a list of the current sessions."
   (when haskell-sessions
-    (let* ((session-name (funcall completing-read-function
+    (let* ((session-name (funcall haskell-completing-read-function
                           "Choose Haskell session: "
                           (mapcar 'haskell-session-name haskell-sessions)))
            (session (find-if (lambda (session)