Save state in memory [Haskell Server]

Question

I am attempting to create a server that returns two different values from a route depending if a user has visited it before. I have the following code:

{-# LANGUAGE OverloadedStrings #-}
module Main where

import Web.Scotty

main = do
    putStrLn "Starting Server..."
    scotty 3000 $ do
        get "/" $ do
            -- if first time 
            text "hello!"
            -- if second time
            text "hello, again!"

I have two questions: 1. How can I check if a user has requested the route before? 2. Where and how can I persist application state?

Answer 1

You can use STM to keep a mutable variable in memory:

import Control.Concurrent.STM.TVar

main = do
    putStrLn "Starting Server..."
    state <- newTVarIO :: IO VisitorsData
    scotty 3000 $ do
        get "/" $ do
            visitorsData <- readTVarIO state
            -- if the visitor's ID/cookie is in visitorsData
            text "hello!"
            -- if new visitor, add them to visitorsData
            atomically $ modifyTVar state $ insertVisitor visitorId visitorsData
            -- if second time
            text "hello, again!"

(If you expect to scale this to a complex server, you'll want to pass the TVar around in the form of a ReaderT pattern)