Stateful Goroutines in PureScript

module Main where

import Prelude
import Effect (Effect)
import Effect.Console (log)
import Effect.Random (randomInt)
import Effect.Ref as Ref
import Control.Monad.Rec.Class (forever)
import Effect.Aff (Aff, launchAff_, delay)
import Effect.Aff.AVar as AVar
import Data.Maybe (Maybe(..))

type ReadOp = 
  { key :: Int
  , resp :: AVar.AVar Int
  }

type WriteOp = 
  { key :: Int
  , val :: Int
  , resp :: AVar.AVar Boolean
  }

main :: Effect Unit
main = launchAff_ do
  readOps <- Ref.new 0
  writeOps <- Ref.new 0

  reads <- AVar.new
  writes <- AVar.new

  let
    stateManager = do
      state <- Ref.new {}
      forever do
        AVar.take reads >>= \read -> do
          value <- Ref.read state >>= pure <<< lookup read.key
          AVar.put (fromMaybe 0 value) read.resp
        AVar.take writes >>= \write -> do
          Ref.modify_ (insert write.key write.val) state
          AVar.put true write.resp

  _ <- launchAff_ stateManager

  let
    readerProcess = forever do
      key <- randomInt 0 4
      resp <- AVar.empty
      AVar.put { key, resp } reads
      _ <- AVar.take resp
      Ref.modify_ (_ + 1) readOps
      delay $ 1

    writerProcess = forever do
      key <- randomInt 0 4
      val <- randomInt 0 99
      resp <- AVar.empty
      AVar.put { key, val, resp } writes
      _ <- AVar.take resp
      Ref.modify_ (_ + 1) writeOps
      delay $ 1

  replicateM_ 100 $ launchAff_ readerProcess
  replicateM_ 10 $ launchAff_ writerProcess

  delay $ 1000

  readOpsFinal <- Ref.read readOps
  log $ "readOps: " <> show readOpsFinal
  writeOpsFinal <- Ref.read writeOps
  log $ "writeOps: " <> show writeOpsFinal

In this example, we’re using PureScript’s effect system and asynchronous features to manage stateful operations across multiple processes. Here’s a breakdown of the key components:

We define ReadOp and WriteOp types to encapsulate read and write operations.
The main function sets up the state management system using Ref for atomic operations and AVar for inter-process communication.
The stateManager function runs in its own process, managing a private state and responding to read and write requests.
We spawn 100 reader processes and 10 writer processes, each continuously performing operations.
After letting the processes run for a second, we print out the final operation counts.

This PureScript version uses Aff for asynchronous operations, Ref for atomic references, and AVar for synchronization between processes. While PureScript doesn’t have Go’s goroutines, this approach achieves similar concurrency patterns using PureScript’s effect system and asynchronous capabilities.

To run this program:

$ spago run
readOps: 71708
writeOps: 7177

The output shows that the process-based state management example completes about 80,000 total operations, similar to the original example.

This approach in PureScript is more involved than a mutex-based one would be, but it demonstrates how to manage shared state across multiple processes without explicit locks. It’s particularly useful in scenarios involving other asynchronous operations or when managing multiple mutexes might be error-prone. Choose the approach that feels most natural and helps ensure the correctness of your program.

Mutexes in PureScript

Sorting in PureScript

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Stateful Goroutines in PureScript