Haskell Scotty网络服务器发送文本响应

Question

我收到一个get请求，并希望发送一条短信作为对它的响应。我有以下代码，但正在收到以下错误

{-# 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"

当我试图运行服务器时，我遇到的错误是

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，只需要一个Parsable a实例。这也是返回类型。但是正如您所看到的，在文件中可解析有几个实例可用！GHC不知道您想要什么，因此出现了不明确的错误。您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，让我们来写代码

beam <- param "q" :: ActionM Text

应该能起作用。现在，beam应该是一个Text。但也许您希望参数是一个整数，所以类似于

beam <- param "q" :: ActionM Integer

也能起作用。