I think I spotted an error

[SpLauT]

Hello! I really think what you are doing here is grand!
I am in the mutable state area

counterHandler :: Int -> Server.Request -> (Int, Server.Response)
counterHandler currentCount
= (newCount, response)
where
newCount
= currentCount + 1
response
= Server.simpleHandler ("Current count: " ++ show newCount)

Where the counterHandler is, I think you wanted to use Server.stringResponse?
Also you only have the currentCount, and not the req as a parameter

Only want to help, so if I am stupid just tell me to shut up! :)

[SpLauT]

main :: IO ()
main
= Server.startServer
[ Server.handlersWithState initialState [counterHandler]
]

I made this work with: Server.handlersWithState initialState [Server.statefulHandler Server.GET "/count" counterHandler]

[alpacaaa]

You're absolutely right, thanks for spotting this! :)