bontaq / purview

Purview

A work in progress implementation of Phoenix Liveview for Haskell

The main library, that doesn't really do anything fancy yet, is in src/Lib.hs. It's just replacing the entirety of the html each time it receives an event from the front end.

Performance

Tested it serving the below component with artillery with 1000 users doing two requests (the increment & decrement below) and it didn't freak out, but 5000 certainly killed it. It got about 200 req/s, which while not terrible for doing very little besides slapping some things together, I'd like to see how high it can get. That only got the cheapest digital ocean vps to 7% CPU and used "fuck-all" ram so I'm not sure what's killing it. I might have to do actual programming. The response time is about 30 ms which is acceptable for an FPS counter.

After realizing it was hitting the files-open limit on the server and switching to Text, it hits up to 1,500 req/s with about 800 req/s sustained and properly uses 98% of the CPU. This is an adequate baseline and I don't have to throw the whole thing in the trash.

What it looks like

Here's what a component looks like (see app/Main.hs):

module Main where

import Prelude hiding (div)
import Lib

newtype State = State
  { count :: Int } deriving Show

defaultCounterState = State { count = 0 }

handlers' :: State -> String -> State
handlers' state message = case message of
  "increment" -> state { count = count state + 1 }
  "decrement" -> state { count = count state - 1 }

render' :: State -> Html
render' state =
  div [] 
    [ div [ onClick "increment" ] [ text "increment" ]
    , text ("count: " <> show (count state))
    , div [ onClick "decrement" ] [ text "decrement" ]
    ]

counter = Component
  { state    = defaultCounterState
  , handlers = handlers'
  , render   = render'
  }

main = run counter

It's not awful, but stringly typed messages are gross and a sin against SPJ.

Todo

• Actually diff the html (though who knows, sending the entire html every time might be smaller than your API request currently)
• Development environment (hot reloading, printing messages to/from the server)
• Allowing CSS + JS snippets in components
• Tests
• Performance
• Type the messages or die trying
• Collect ideas
• Hey how do I do side effects?

Since it's still tiny it could go anywhere. CSS styling the components? Allow adding JS to optimistically update things that might fail? The sky's the limit.

Installation

1. Install stack
2. stack build
3. stack exec bridge-exe for just running the example above
4. stack exec bridge for the ~ experimental ~ and not-currently working repl