Concurrency

and the Laws of Physics

What is concurrency?

  • Things happen at the same time
  • Things happen in any order

Vending machine

Vending machine

Queuing people

Processor

Threads

Reasoning

  • Tests
  • Looking at the code 🤯
  • Formal verification

Laws

  • Causality: If B happened, it happened after A
  • Determinism: If A happens, B must happen
  • Observation: Events must be observed at the same point in space

Enforcing laws

  • Substitution
  • IO
  • Resource
  • Typeclasses

What happens first?

1 + 2 + 3 + 4
1 + 2 + 3 + 4

(1 + 2) + (3 + 4)

3 + 7

10

It doesn't matter

1 + 2 + 3 + 4

(1 + 2) + 3 + 4

(3 + 3) + 4

6 + 4

10

IO

Stream(1)
.repeat
.debug()
.take(3)
.compile
.toList
Stream.eval(IO.pure(1))
.repeat
.debug()
.take(3)
.compile
.toList
.unsafeRunSync()

IO

Stream(1)
.repeat
.debug()
.take(3)
.compile
.drain
Stream.eval(IO.pure(1))
.repeat
.debug()
.take(3)
.compile
.drain
.unsafeRunSync()
1
1
1
1
1
1
1

What happens first?

IO.println("Hello") >> IO.println("world")

Causality

If B happened, it happened after A

Enforced by IO

Does it always happen?

IO.println("Hello") >> IO.println("world")
val program = IO.println("Hello") >> IO.println("world")

val fiber = program.start
fiber >>= (_.cancel)
val program = Resource.make(IO.println("Hello")) { _ =>
  IO.println("world")
}

resource.use { program =>
  val fiber = program.start
  fiber >>= (_.cancel)
}

Resource

Determinism

 If A happens, B must happen

Enforced with resources

Many other utilities

  • Typeclasses: Sync, Concurrent
  • State management: Ref
  • Concurrency primitives: Queues

Thank you!

Concurrency and the Laws of Physics

By Zainab Ali

Concurrency and the Laws of Physics

  • 243