F# Joinad Samples

This repository contains various examples that use the match! extension for the F# langauge. This is a prototype implementation for research purposes.

Join calculus

The match! extension can be used to encode computations based on join calculus. A buffer that provides two put channels (for storing strings and integers) and a single get channel for retreiving values can be encoded as follows:

let putString = Channel<string>("puts")
let putInt = Channel<int>("puti")
let get = SyncChannel<string>("get")

join {
  match! get, putString, putInt with
  | repl, v, ? -> return react { yield repl.Reply("Echo " + v) }
  | repl, ?, v -> return react { yield repl.Reply("Echo " + (string v)) } 
}

The first three lines define channels. The get value represents a synchronous channel (when we call it, we need to wait for a reply). The putInt and putString channels represent two methods for adding values to the buffer.

The join program is implemented using the join computation builder that defines primitives for merging channels, projecting values in the channel and for non-deterministic choice between channels. These primitives return alias channels that do not actually contain values - they just query the original channels for a value when a value is requested.

The two clauses of the match! expression represent two join patterns. In the first join pattern, we require a value from the get channel and the putString channel. When the values are available, we return a reaction. The reaction is created using react computation builder and it can yield operations to be performed. In this sample, the only operation is to send a reply to the caller of the get channel.

The buffer can be called using F# asynchronous workflows as follows:

// Put 5 values to 'putString' and 5 values to 'putInt'
for i in 1 .. 5 do 
  putString.Call("Hello!")
  putInt.Call(i)

// Repeatedly call 'get' to read the next value. This is a blocking
// operation, so it should be done from asynchronous workflow to 
// avoid blocking physical threads.
async { 
  while true do
    let! repl = get.AsyncCall()
    printfn "got: %s" repl }
|> Async.Start