Simple questions about the scotty Haskell web framework

Question

Consider the simplest scotty app:

{-# LANGUAGE OverloadedStrings #-}
import Web.Scotty

import Data.Monoid (mconcat)

main = scotty 3000 $ do
    get "/:word" $ do
        beam <- param "word"
        html $ mconcat ["<h1>Scotty, ", beam, " me up!</h1>"]

I put this code into app.hs and compile it with GHC. I run it with ./app. Simple.

1. What really happens when people visit the site? It's only one ./app that's running. Does a new thread get created within this same app whenever each user triggers a get "/:word" $ do line? How many such threads can exist? Thousand? Ten thousand?

2. After running ./app it shows the message Setting phasers to stun... (port 3000) (ctrl-c to quit). But it shows nothing more. It doesn't output the incoming web requests. How can I make it do that? This would be useful for logging purposes.

Answer 1

Assuming you are using GHC, each request to the scotty server essentially creates a "green thread" that is scheduled by the GHC runtime. You can easily have thousands of those running at a time.

Scotty itself doesn't do any request logging but since it is built on top of WAI, you can use any middleware component that exists for it, for example RequestLogger.

{-# LANGUAGE OverloadedStrings #-}
import Web.Scotty
import Network.Wai.Middleware.RequestLogger

import Data.Monoid (mconcat)

main = scotty 3000 $ do
    middleware logStdoutDev

    get "/:word" $ do
        beam <- param "word"
        html $ mconcat ["<h1>Scotty, ", beam, " me up!</h1>"]