[英]WAI tutorial - no instance for (Show Response)
Again the n00b here: trying Warp and WAI with the following code as in the documentation. 这里再次是n00b:使用文档中的以下代码尝试Warp和WAI。
{-# LANGUAGE OverloadedStrings, DeriveGeneric #-}
import Network.Wai
import Network.HTTP.Types
import Network.Wai.Handler.Warp (run)
app3 :: Application
app3 request respond = respond $ case rawPathInfo request of
"/" -> index
"/raw/" -> plainIndex
_ -> notFound
plainIndex :: Response
plainIndex = responseFile
status200
[("Content-Type", "text/plain")]
"index.html"
Nothing
notFound :: Response
notFound = responseLBS
status404
[("Content-Type", "text/plain")]
"404 - Not Found"
Running plainIndex in GHCi returns: 在GHCi中运行plainIndex返回:
<interactive>:12:1: error:
• No instance for (Show Response) arising from a use of ‘print’
• In a stmt of an interactive GHCi command: print it
*Main> :r
[1 of 1] Compiling Main ( WAI.hs, interpreted )
Two questions in one: can you help me out fixing this, and in exension to that: I am the only one frequently encountering issues like this when following documentation examples? 两个问题合二为一:您能帮助我解决此问题吗,以及除此之外:在遵循文档示例时,我是唯一经常遇到此类问题的人吗?
Running plainIndex
in GHCi, GHCi tries to compute the Response
and then print it in the terminal. 在GHCi中运行
plainIndex
,GHCi尝试计算Response
,然后在终端中将其打印出来。 A Show
instance defines how a given type should be represented as a String
. Show
实例定义如何将给定类型表示为String
。 The library authors have chosen not to provide a Show
instance for Response
, probably to separate its representation from its interface. 库作者选择不为
Response
提供Show
实例,可能是为了将其表示与其界面分开。
Various parts of a response have Show
instances, so you can use accessors provided by Wai : 响应的各个部分都有
Show
实例,因此您可以使用Wai提供的访问器 :
> responseStatus plainIndex
> responseHeaders plainIndex
More documentation for Response . 有关响应的更多文档 。
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