Concurrency

and the Laws of Physics

What is concurrency?

Things happen at the same time
Things happen in any order

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

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!

Essential Effects, Adam Rosien

Essential Effects (book), Adam Rosien

Concurrency with Cats Effect, Michael Pilquist

Tackling concurrency bugs with TLA+, Hillel Wayne

How we program multicores, Joe Armstrong

Mindmap of this presentation