Yesod wai-eventsource example. Package dependency issue? - haskell

Question

I'm trying to get this example code to compile on my system.
When I try to compile the Chat module with ghc Chat.hs, ghc gives me this:
Chat.hs:76:39:
Couldn't match expected type `Network.Wai.Request'
with actual type `wai-0.4.3:Network.Wai.Request'
In the second argument of `eventSourceApp', namely `req'
In the second argument of `($)', namely `eventSourceApp chan req'
In a stmt of a 'do' expression:
res <- lift $ eventSourceApp chan req
I am on OS X Snow Leopard and cleared up (everything?) except the Haskell Platform like this:
rm -r ~/.cabal
rm -r ~/.ghc
rm -r ~/Library/Haskell
and installed yesod and wai-eventsource anew from hackage.
As far as I understand the error comes from a dependency problem.
wai
Synopsis: Web Application Interface.
Default available version: 1.0.0
Installed versions: 0.4.3, 1.0.0
Homepage: https://github.com/yesodweb/wai
License: BSD3
Where yesod-0.9.4.1 requires wai == 0.4.* and wai-eventsource-1.0.0 requires wai >= 1.0.
So, my question would be: Is it possible to get this example (with the official releases of yesod) to work right now? With all the change the yesod project is going through atm?
Do I have to be more precise on the versions I try to install and, if so, how?
Edit:
I wiped out ~/.ghc (or actually followed a more rigorous approach given here, just in case) and tried to install the packages with a single
cabal install yesod wai-eventsource resulting in (incomplete):
Resolving dependencies...
cabal: cannot configure yesod-0.9.4.1. It requires wai ==0.4.* and warp ==0.4.*
For the dependency on wai ==0.4.* there are these packages: wai-0.4.0,
wai-0.4.1, wai-0.4.2 and wai-0.4.3. However none of them are available.
wai-0.4.0 was excluded because wai-eventsource-1.0.0 requires wai >=1.0
...
wai-0.4.3 was excluded because wai-eventsource-1.0.0 requires wai >=1.0
For the dependency on warp ==0.4.* there are these packages: warp-0.4.0,
warp-0.4.0.1, warp-0.4.1, warp-0.4.1.1, warp-0.4.1.2, warp-0.4.2, warp-0.4.3,
warp-0.4.3.1, warp-0.4.4, warp-0.4.5, warp-0.4.6, warp-0.4.6.1, warp-0.4.6.2
and warp-0.4.6.3. However none of them are available.
warp-0.4.0 was excluded because wai-eventsource-1.0.0 requires warp >=1.0
...
warp-0.4.6.3 was excluded because wai-eventsource-1.0.0 requires warp >=1.0
Before that (with yesod and wai-eventsource installed separately) I tried ghc -hide-package wai-1.0.0 Chat.hs resulting in,
Chat.hs:77:39:
Couldn't match expected type `wai-1.0.0:Network.Wai.Request'
with actual type `Network.Wai.Request'
In the second argument of `eventSourceApp', namely `req'
In the second argument of `($)', namely `eventSourceApp chan req'
In a stmt of a 'do' expression:
res <- lift $ eventSourceApp chan req

Answer 1

I think you need to unregister (or hide) wai 1.0.0. The current Yesod is using wai 0.4, which is where the mismatch is coming from. (Once a newer Yesod is released, this problem will disappear.)
Alternatively, you could wipe out your ~/.ghc folder again and run cabal install yesod wai-eventsource, which should automatically install only the compatible versions.
Edit: You need to hide wai-eventsource as well, and possibly a few others. And simplest approach is to run ghc-pkg unregister wai-eventsource-1.0.0 --force.