haskellconduitnetwork-conduit

Missing data constructor HostAny in Haskell

I am experimenting with Conduit Network and I cannot compile this code because it is cannot find the data constructor: HostAny

conduit-extra is installed so I am very puzzled why it cannot find it?

{-# LANGUAGE ScopedTypeVariables #-}
import Data.Conduit
import Data.Conduit.Network


serverApp :: AppData -> IO ()
serverApp d = do appSource d $$ appSink d

main :: IO()
main = runTCPServer (serverSettings 8900 HostAny) serverApp

Here's the GHC error:

pez@devbox:~/dev$ runhaskell server.hs 

server.hs:10:42: Not in scope: data constructor `HostAny'
Solution

  • If you look at the documentation for conduit-extras, you'll see

    data HostPreference

    Which host to bind.

    Note: The IsString instance recognizes the following special values:

    * means HostAny
*4 means HostIPv4
!4 means HostIPv4Only
*6 means HostIPv6
!6 means HostIPv6Only

    Which tells me that you should be using the extension OverloadedStrings and then you can just write your code as

    main = runTCPServer (serverSettings 8900 "*") serverApp

    Although I have to say that is a strange API this library has chosen. I personally would much rather have the ability to use IsString or an explicit constructor in cases where I don't want to use OverloadedStrings for whatever reason.