WAI tutorial - no instance for (Show Response)

Question

Again the n00b here: trying Warp and WAI with the following code as in the documentation.

{-# 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:

<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?

Answer 1

Running plainIndex in GHCi, GHCi tries to compute the Response and then print it in the terminal. A Show instance defines how a given type should be represented as a String. The library authors have chosen not to provide a Show instance for Response, probably to separate its representation from its interface.

Various parts of a response have Show instances, so you can use accessors provided by Wai:

> responseStatus plainIndex
> responseHeaders plainIndex

More documentation for Response.