diff --git a/elisp/ghc-comp.el b/elisp/ghc-comp.el
index 423cc64..71159b3 100644
--- a/elisp/ghc-comp.el
+++ b/elisp/ghc-comp.el
@@ -15,13 +15,20 @@
 ;;; Customize Variables
 ;;;
 
-(defvar ghc-idle-timer-interval 30)
+(defvar ghc-idle-timer-interval 30
+ "*Period of idele timer in second. When timeout, the names of
+unloaded modules are loaded")
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;
+;;; Constants
+;;;
 
 ;; must be sorted
-(defvar ghc-reserved-keyword-for-bol '("class" "data" "default" "import" "infix" "infixl" "infixr" "instance" "main" "module" "newtype" "type"))
+(defconst ghc-reserved-keyword-for-bol '("class" "data" "default" "import" "infix" "infixl" "infixr" "instance" "main" "module" "newtype" "type"))
 
 ;; must be sorted
-(defvar ghc-reserved-keyword '("case" "deriving" "do" "else" "if" "in" "let" "module" "of" "then" "where"))
+(defconst ghc-reserved-keyword '("case" "deriving" "do" "else" "if" "in" "let" "module" "of" "then" "where"))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;
@@ -42,7 +49,7 @@
 (defvar ghc-merged-keyword nil) ;; completion for type/func/...
 (defvar ghc-language-extensions nil)
 
-(defvar ghc-keyword-prefix "ghc-keyword-")
+(defconst ghc-keyword-prefix "ghc-keyword-")
 (defvar ghc-keyword-Prelude nil)
 (defvar ghc-loaded-module nil)
 
diff --git a/elisp/ghc-doc.el b/elisp/ghc-doc.el
index f50a6e9..69a1a75 100644
--- a/elisp/ghc-doc.el
+++ b/elisp/ghc-doc.el
@@ -34,8 +34,8 @@
     (when (looking-at "^haddock-html: \\([^ \n]+\\)$")
       (match-string-no-properties 1))))
 
-(defvar ghc-doc-local-format "file://%s/%s.html")
-(defvar ghc-doc-hackage-format
+(defconst ghc-doc-local-format "file://%s/%s.html")
+(defconst ghc-doc-hackage-format
   "http://hackage.haskell.org/packages/archive/%s/latest/doc/html/%s.html")
 
 (defun ghc-display-document (pkg mod haskell-org)
diff --git a/elisp/ghc-flymake.el b/elisp/ghc-flymake.el
index 96840f1..03e9959 100644
--- a/elisp/ghc-flymake.el
+++ b/elisp/ghc-flymake.el
@@ -16,12 +16,12 @@
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-(defvar ghc-error-buffer-name "*GHC Errors*")
+(defconst ghc-error-buffer-name "*GHC Errors*")
 
-(defvar ghc-flymake-allowed-file-name-masks
+(defconst ghc-flymake-allowed-file-name-masks
   '("\\.l?hs$" ghc-flymake-init flymake-simple-cleanup flymake-get-real-file-name))
 
-(defvar ghc-flymake-err-line-patterns
+(defconst ghc-flymake-err-line-patterns
   '("^\\(.*\\.l?hs\\):\\([0-9]+\\):\\([0-9]+\\):[ ]*\\(.+\\)" 1 2 3 4))
 
 (add-to-list 'flymake-allowed-file-name-masks
diff --git a/elisp/ghc-func.el b/elisp/ghc-func.el
index 09c58b9..07dd2cb 100644
--- a/elisp/ghc-func.el
+++ b/elisp/ghc-func.el
@@ -8,7 +8,8 @@
 
 ;;; Code:
 
-(defvar ghc-module-command "ghc-mod")
+(defvar ghc-module-command "ghc-mod"
+"*The command name of \"ghc-mod\"")
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
diff --git a/elisp/ghc.el b/elisp/ghc.el
index ccc7d81..833c0ed 100644
--- a/elisp/ghc.el
+++ b/elisp/ghc.el
@@ -16,7 +16,7 @@
 
 ;;; Code:
 
-(defvar ghc-version "0.4.0")
+(defconst ghc-version "0.4.0")
 
 ;; (eval-when-compile
 ;;  (require 'haskell-mode))