Happstack.lite教程无法使用"Illegal type signature: `String'"编译

Question

我一直在关注这个Happstack。life tutorial: http://www.happstack.com/page/view-page-slug/9/happstack-lite-tutorial非常成功，但我似乎无法克服这个编译错误:

$ ghc crashcourse.hs -o crashcourse
[1 of 1] Compiling Main             ( crashcourse.hs, crashcourse.o )
crashcourse.hs:52:21:
    Illegal type signature: `String'
      Perhaps you intended to use -XScopedTypeVariables
    In a pattern type-signature

所讨论的代码:

echo :: ServerPart Response
echo =
    path $ (msg :: String) ->
        ok $ template "echo" $ do
          p $ "echo says: " >> toHtml msg
          p "Change the url to echo something else."

上下文中的代码:({-# LANGUAGE OverloadedStrings #-} pragma对这个问题没有影响，它是用来解决其他问题的)

{-# LANGUAGE OverloadedStrings #-}
module Main where
import Control.Applicative ((<$>), optional)
import Data.Maybe (fromMaybe)
import Data.Text (Text)
import Data.Text.Lazy (unpack)
import Happstack.Lite
import Text.Blaze.Html5 (Html, (!), a, form, input, p, toHtml, label)
import Text.Blaze.Html5.Attributes (action, enctype, href, name, size, type_, value)
import qualified Text.Blaze.Html5 as H
import qualified Text.Blaze.Html5.Attributes as A
main :: IO ()
main = serve Nothing myApp
myApp :: ServerPart Response
myApp = msum
  [ dir "echo"    $ echo
   ,dir "query"   $ queryParams
--  , dir "form"    $ formPage
--  , dir "fortune" $ fortune
--  , dir "files"   $ fileServing
--  , dir "upload"  $ upload
  , homePage
  ]
template :: Text -> Html -> Response
template title body = toResponse $
  H.html $ do
    H.head $ do
      H.title (toHtml title)
    H.body $ do
      body
      p $ a ! href "/" $ "back home"
homePage :: ServerPart Response
homePage =
    ok $ template "home page" $ do
           H.h1 "Hello!"
           H.p "Writing applications with happstack-lite is fast and simple!"
           H.p "Check out these killer apps."
           H.p $ a ! href "/echo/secret%20message" $ "echo"
           H.p $ a ! href "/query?foo=bar" $ "query parameters"
           H.p $ a ! href "/form" $ "form processing"
           H.p $ a ! href "/fortune" $ "(fortune) cookies"
           H.p $ a ! href "/files" $ "file serving"
           H.p $ a ! href "/upload" $ "file uploads"
echo :: ServerPart Response
echo =
    path $ (msg :: String) ->
        ok $ template "echo" $ do
          p $ "echo says: " >> toHtml msg
          p "Change the url to echo something else."
queryParams :: ServerPart Response
queryParams =
    do mFoo <- optional $ lookText "foo"
       ok $ template "query params" $ do
         p $ "foo is set to: " >> toHtml (show mFoo)
         p $ "change the url to set it to something else."

我已经成功地从这里执行了这个:

module Main where
import Control.Monad    (msum)
import Happstack.Server (nullConf, simpleHTTP, ok, dir, path, seeOther)
main :: IO ()
main = simpleHTTP nullConf $
  msum [ dir "hello" $ path $ s -> ok $ "Hello, " ++ s
       , seeOther "/hello/Haskell" "/hello/Haskell"
       ]

非常感谢您的帮助!

Answer 1

正如在happstack-lite教程中提到的，您需要以下pragmas:

{-# LANGUAGE OverloadedStrings, ScopedTypeVariables #-}

如果我添加ScopedTypeVariables，它为我工作