-- A simple distributed computation ability
ability Remote where

  -- Spawn a new node
  spawn : {Remote} Node

  -- Sequentially evaluate the given thunk on another node
  -- then return to the current node when it completes
  at : n -> '{Remote} a -> {Remote} a

  -- Start a computation running, returning an `r` that can be forced to
  -- await the result of the computation
  fork : '{Remote} a -> {Remote} ('{Remote} a)

type Node = Node Nat -- more realistic would be perhaps a (Hostname, PublicKey) pair

replicate : Nat -> a -> [a]
replicate n a = toSequence (take n (constant a))

-- here's a simple usage of it - this ships the program `replicate n a`
-- to another node and evaluates it there before returning to the current node

ex1 : Nat -> a -> {Remote} [a]
ex1 n a =
  node = Remote.spawn -- conjures up a new node!
  Remote.at node '(replicate n a) -- and transports a computation to it!

-- Let's test out this beast! do we need to deploy our code to some EC2 instances??
-- Gak, no not yet, we just want to test locally, let's write a handler
-- for the `Remote` ability that simulates everything locally!

Remote.runLocal : '{Remote} a -> a
Remote.runLocal r =
  step nid = cases
    {a} -> a
    {Remote.fork t -> k} -> handle k t with step nid
    {Remote.spawn -> k} -> handle k nid with step (Node.increment nid)
    {Remote.at _ t -> k} -> handle k !t with step nid
  handle !r with step (Node.Node 0)

-- Q: where do these nodes come from? that depends on the handler -
-- you might have a handler like this, or a handler backed by an autoscaling EC2 pool...

-- easy peasy, let's give it a go...

> Remote.runLocal '(ex1 10 "hi")

-- let's do some stuff in parallel on multiple nodes

ex2 n =
  -- spin up two remote computations on fresh nodes, in parallel, then combine their results
  r1 = Remote.forkAt Remote.spawn '(replicate n "hi") -- returns a 'future'
  r2 = Remote.forkAt Remote.spawn '(replicate n "there")
  !r1 ++ !r2

> Remote.runLocal '(ex2 5)

-- little helper functions used above

Remote.forkAt : Node -> '{Remote} a -> {Remote} ('{Remote} a)
Remote.forkAt node r = Remote.fork '(Remote.at node r)

Node.increment : Node -> Node
Node.increment n =
  use Node.Node -- the constructor
  match n with Node n -> Node (n + 1)