I don’t. That’s just something I pulled out of the oven, Julia Child-style. The real point of this article is just how to use the stack command. The bot is really simple. Here’s the entire source (slightly modified):

{-# LANGUAGE OverloadedStrings #-}
module Main where

import Data.Monoid ((<>))
import System.Environment (getEnvironment)
import Web.Spock.Safe

main :: IO ()
main = do
    env <- getEnvironment
    let port = maybe 8080 read $ lookup "PORT" env

    runSpock port $ spockT id $
        post "spock" $ do
            comment <- param "text"
            text $ case comment of
                Just t  -> "\"" <> t <> "\"" <> "highly illogical, captain"
                Nothing -> "I didn't understand that."

That just gets the environment variable, PORT, to run the server on and then runs the app. The app responds to a POST at http://some.url.here/spock with a param of text and then builds a response based on that. That’s enough to handle a slash command in Slack. Like I said, really simple.

Sorry for coming off as linkbait. :(