Haskell Scotty 网络服务器发送文本响应

Question

我收到一个 GET 请求并想发送一条文本消息作为对 it.I 的响应有以下代码但我收到以下错误

{-# LANGUAGE OverloadedStrings #-}
import Web.Scotty
import Network.Wai.Middleware.RequestLogger

import Data.Monoid (mconcat) 

main = scotty 4000 $ do
middleware logStdoutDev

get "/" $ do
    beam<- param "q"
    text $ "The message which I want to send"

我尝试运行服务器时遇到的错误是

No instance for (Parsable t0) arising from a use of ‘param’
The type variable ‘t0’ is ambiguous
Note: there are several potential instances:
  instance Parsable () -- Defined in ‘scotty-0.9.1:Web.Scotty.Action’
  instance Parsable Bool
    -- Defined in ‘scotty-0.9.1:Web.Scotty.Action’
  instance Parsable Data.ByteString.Lazy.Internal.ByteString
    -- Defined in ‘scotty-0.9.1:Web.Scotty.Action’
  ...plus 9 others
In a stmt of a 'do' block: beam <- param "q"
In the second argument of ‘($)’, namely
  ‘do { beam <- param "q";
        text $ "The message I want to send" }’
In a stmt of a 'do' block:
  get "/"
  $ do { beam <- param "q";
         text $ "The message I want to send" }

Answer 1

param 的类型为 Parsable a => Text -> ActionM a。如您所见，a 是多态的，只需要 Parsable a 的一个实例。也是 return 类型。但是正如您所看到的 in the documentation Parsable 有几个可用的实例！ GHC 不知道你想要什么，因此会出现模棱两可的错误。您将需要指定一个类型注释，让 GHC 知道您想要什么。但是无论如何，让我们写代码

beam <- param "q" :: ActionM Text

应该可以解决问题。现在 beam 应该是 Text。但也许你希望你的参数是一个整数，比如

beam <- param "q" :: ActionM Integer

也可以。

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