Question

我正在关注Snap示例项目，但是当我复制示例代码时，Stack给了我错误抱怨使用writeBS以及它如何期望ByteString但是给了[Char] 1}}。所以我导入了一个ByteString库并尝试使用'fromString'方法将所有字符串转换为ByteStrings，但Stack给了我同样的错误，声称Srings仍然是[Char]。这是经过修改的示例代码：

module Main where

import           Snap
import           Snap.Types
import           Snap.Util.FileServe
import           Control.Applicative
import           Heist
import qualified Data.ByteString.UTF8 as BU

main :: IO ()
main = quickHttpServe site

site :: Snap ()
site =
    ifTop (writeBS $ BU.fromString "hello world") <|>
    route [ ("foo", writeBS $ BU.fromString "bar")
          , ("echo/:echoparam", echoHandler)
          ] <|>
    dir "static" (serveDirectory ".")

echoHandler :: Snap ()
echoHandler = do
    param <- getParam "echoparam"
    maybe (writeBS $ BU.fromString "must specify echo/param in URL")
          writeBS $ BU.fromString param

尝试使用fromString进行修复后出现

错误消息：

/home/jeshaitan/Personal/yapp/src/Main.hs:16:14:
    Couldn't match expected type ‘BU.ByteString’
                with actual type ‘[Char]’
    In the expression: "foo"
    In the expression: ("foo", writeBS $ BU.fromString "bar")
    In the first argument of ‘route’, namely
      ‘[("foo", writeBS $ BU.fromString "bar"),
        ("echo/:echoparam", echoHandler)]’

/home/jeshaitan/Personal/yapp/src/Main.hs:17:14:
    Couldn't match expected type ‘BU.ByteString’
                with actual type ‘[Char]’
    In the expression: "echo/:echoparam"
    In the expression: ("echo/:echoparam", echoHandler)
    In the first argument of ‘route’, namely
      ‘[("foo", writeBS $ BU.fromString "bar"),
        ("echo/:echoparam", echoHandler)]’

/home/jeshaitan/Personal/yapp/src/Main.hs:19:9:
    Couldn't match expected type ‘BU.ByteString’
                with actual type ‘[Char]’
    In the first argument of ‘dir’, namely ‘"static"’
    In the second argument of ‘(<|>)’, namely
      ‘dir "static" (serveDirectory ".")’
    In the expression:
      ifTop (writeBS $ BU.fromString "hello world")
      <|>
        route
          [("foo", writeBS $ BU.fromString "bar"),
           ("echo/:echoparam", echoHandler)]
      <|> dir "static" (serveDirectory ".")

/home/jeshaitan/Personal/yapp/src/Main.hs:23:23:
    Couldn't match expected type ‘BU.ByteString’
                with actual type ‘[Char]’
    In the first argument of ‘getParam’, namely ‘"echoparam"’
    In a stmt of a 'do' block: param <- getParam "echoparam"
    In the expression:
      do { param <- getParam "echoparam";
           maybe
             (writeBS $ BU.fromString "must specify echo/param in URL") writeBS
           $ BU.fromString param }

/home/jeshaitan/Personal/yapp/src/Main.hs:25:21:
    Couldn't match expected type ‘Maybe BU.ByteString’
                with actual type ‘BU.ByteString’
    In the second argument of ‘($)’, namely ‘BU.fromString param’
    In a stmt of a 'do' block:
      maybe
        (writeBS $ BU.fromString "must specify echo/param in URL") writeBS
      $ BU.fromString param

/home/jeshaitan/Personal/yapp/src/Main.hs:25:35:
    Couldn't match type ‘Maybe BU.ByteString’ with ‘[Char]’
    Expected type: String
      Actual type: Maybe BU.ByteString
    In the first argument of ‘BU.fromString’, namely ‘param’
    In the second argument of ‘($)’, namely ‘BU.fromString param’

--  While building package yapp-0.1.0.0 using:
      /home/jeshaitan/.stack/setup-exe-cache/x86_64-linux/setup-Simple-Cabal-1.22.4.0-ghc-7.10.2 --builddir=.stack-work/dist/x86_64-linux/Cabal-1.22.4.0 build exe:yapp --ghc-options " -ddump-hi -ddump-to-file"
    Process exited with code: ExitFailure 1

Answer 1

我需要将{-# LANGUAGE OverloadedStrings #-}添加到文件的顶部，以便字符串文字可以读作ByteStrings。

为什么Snap示例使用Bytestring？

1 个答案: