add twitterId function

yesod-auth-oauth/Yesod/Auth/OAuth.hs

@@ -5,6 +5,7 @@ module Yesod.Auth.OAuth
     , oauthUrl
     , authTwitter
     , twitterUrl
+    , twitterId
     , authTumblr
     , tumblrUrl
     , module Web.Authenticate.OAuth
@@ -108,6 +109,25 @@ authTwitter key secret = authOAuth
 twitterUrl :: AuthRoute
 twitterUrl = oauthUrl "twitter"
 
+-- | Gets Twitter ID (/user_id/) from @Creds@.
+--
+-- Never use @credsIdent@ for Twitter OAuth. @credsIdent@ returns /screen_name/, which shouldn't be used for authentication.
+-- /screen_name/ is text like /foo/ of /\@foo/ which is unique but __mutable__. So /screen_name/ cannot authenticate users.
+-- /user_id/ is integer which is unique and __immutable__. So you should use this for authentication.
+--
+-- Because of compatibility, @credsIdent@ returns /screen_name/ yet.
+--
+-- Since 1.4.x.x
+twitterId :: Yesod m => Creds m -> Text
+twitterId creds =
+    let
+        key   = "user_id"
+        extra = credsExtra creds
+    in
+        case lookup key extra of
+            Just uId -> uId
+            Nothing  -> throw $ CredentialError ("key not found: " ++ (T.unpack key)) (Credential $ map (encodeUtf8 *** encodeUtf8) extra)
+
 authTumblr :: YesodAuth m
             => ByteString -- ^ Consumer Key
             -> ByteString -- ^ Consumer Secret