なぞのエラー

gistfile1.hs

{-# LANGUAGE TypeFamilies, QuasiQuotes, MultiParamTypeClasses,
             TemplateHaskell, OverloadedStrings #-}

import Yesod
import Yesod.Auth
import Yesod.Auth.OAuth
import Data.Text.Internal

data HelloWorld = HelloWorld

mkYesod "HelloWorld" [parseRoutes|
/ HomeR GET
/user/#String/ UserHomeR GET
|]

instance Yesod HelloWorld where
    approot _ = ""

getHomeR :: Handler RepHtml
getHomeR = defaultLayout $ do html_header

html_header = [whamlet|<h1>Simple Code Book</h1>|]
html_breadload userid = [whamlet|<a href=@{HomeR}>Simple Code Book</a> -> #{userid} |]

getUserHomeR :: String -> Handler RepHtml
getUserHomeR userid = defaultLayout $
        do html_header
           html_breadload userid


consumerKey :: String
consumerKey = "hogehoge"

consumerSecret :: String
consumerSecret = "fugafuga"

twittername :: AuthPlugin m -> String
twittername (AuthPlugin name dispatch login) = showText $ name

twitter :: YesodAuth a => AuthPlugin a
twitter = authTwitter consumerKey consumerSecret
{-
    YesodAuth a を抜くと下のようなエラー。エラーの意図がよくわからない。

    No instance for (YesodAuth m0)
      arising from a use of `authTwitter'
    In the expression: authTwitter consumerKey consumerSecret
    In an equation for `twitter':
        twitter = authTwitter consumerKey consumerSecret
-}

-- html_login = [whamlet|#{get_name $ twitter}|]

{-
　　上を追加すると、下のようなエラー
    Ambiguous type variable `m1' in the constraint:
      (YesodAuth m1) arising from a use of `twitter'
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `get_name', namely `twitter'
    In the first argument of `toHtml', namely `get_name twitter'
    In the first argument of `addHtml', namely
      `toHtml (get_name twitter)'
-}

main :: IO ()
main = warpDebug 3000 HelloWorld