以包名称和版本为前缀的类型名称

Question

我是 Haskell 的新手，我正在尝试跟进 Happstack Crash Course. I've done some of the examples, but when I tried the happstack-heist example，我遇到了一个奇怪的编译错误。我正在编译的文件如下所示：

module Main where

import Control.Applicative    ((<$>))
import Control.Monad          (msum)
import qualified Data.Text    as T
import Happstack.Server       ( dir, nullConf, nullDir, simpleHTTP
                              , seeOther, toResponse
                              )
import Happstack.Server.Heist (heistServe, initHeistCompiled)
import Heist                  (Splices, (##), getParamNode, noSplices)
import Heist.Compiled         (Splice, yieldRuntimeText)
import qualified Text.XmlHtml as X

-- | factorial splice
factSplice :: (Monad m) => Splice m
factSplice = do
  intStr <- T.unpack . X.nodeText <$> getParamNode
  let res = yieldRuntimeText $ do
        case reads intStr of
          [(n,[])] ->
            return (T.pack $ show $ product [1..(n :: Integer)])
          _ ->
            return (T.pack $ "Unable to parse " ++
                    intStr ++ " as an Integer.")
  return $ res

main :: IO ()
main = do
  heistState <- do
    r <- initHeistCompiled (T.pack "fact" ## factSplice) noSplices "."
    case r of
      (Left e) -> error $ unlines e
      (Right heistState) -> return $ heistState
  simpleHTTP nullConf $ msum
    [ dir "heist" $ heistServe heistState
    , nullDir >>
      seeOther "/heist/factorial" (toResponse "/heist/factorial")
    ]

错误是：

test.hs:37:36:
    Couldn't match expected type `happstack-server-7.3.9:Happstack.Server.Internal.Types.Response'
                with actual type `Happstack.Server.Internal.Types.Response'
    In the return type of a call of `toResponse'
    In the second argument of `seeOther', namely
      `(toResponse "/heist/factorial")'
    In the second argument of `(>>)', namely
      `seeOther "/heist/factorial" (toResponse "/heist/factorial")'

好像有些东西想要以包名和版本号为前缀的类型，我不明白。 happstack-server 和 happstack-heist 都安装了 cabal install.

Answer 1

欢迎来到阴谋地狱！发生的事情是，当您为此示例安装两个包 happstack-server 和 happstack-heist 时，其中一个引入了与您系统上已安装版本不同的另一个版本。当您尝试编译该示例时，编译器无法确定要使用哪一个。解决方案是 sandboxes。只需 cd 到您有此示例的目录，运行 cabal sandbox init，然后 cabal install --dependencies-only。这将使用 .cabal 文件获取项目的所有依赖项并将它们安装在本地 .cabal-sandbox/ 目录中。当您运行 cabal build 或 cabal install 从该本地文件夹中提取依赖项时，任何可执行文件都将安装在 .cabal-sandbox/bin.

中

以包名称和版本为前缀的类型名称

Type names prefixed with package name and version

haskell

happstack

heist