Selection and Choice in Weaver

In this tutorial, you’ll learn how to control which operations execute and when using Weaver’s selection and choice operators. We’ll write executable programs demonstrating if, await, or, and xor.

What You’ll Learn

• Deterministic (or) vs non-deterministic (xor) selection
• Using await and if for conditional execution
• Combining selection with parallel composition (and)
• Compiling and simulating selection behaviors

Step 1: Write a Deterministic Selection Program

Create select.wv:

func MergeOr(chan InA, InB, Out) {
	while {
		await InA {
			Out.send(InA.recv())
		} or await InB {
			Out.send(InB.recv())
		}
	}
}

Explanation:

• await InA { ... } or await InB { ... } Executes exactly one branch. If both channels are ready simultaneously, Loom signals an error.
• Out.send(...) sends the processed value to the output channel.

Step 3: Compile and Simulate Deterministic Selection

lm build
lm sim "select:MergeOr(chan)"

In the simulator, send values on InA and InB to see how the process reacts.

<chpsim> InA.send(5)
<chpsim> Out.recv()
5
<chpsim> InB.send(10)
<chpsim> Out.recv()
10
<chpsim> InA.send(1)
<chpsim> InB.send(1)
<chpsim> Out.recv()
error: mutual exclusion violation

• Only one branch executes per iteration.

Step 4: Write a Non-Deterministic Choice Program

Create non_deterministic.wv:

func NonDeterministic(chan ReqA, ReqB) {
	while {
		await ReqA {
			ReqA.recv()
		} xor await ReqB {
			ReqB.recv()
		}
	}
}

• xor allows the hardware to pick arbitrarily if both requests are ready.
• Useful for modeling arbiters or asynchronous contention.

Step 5: Compile and Simulate Non-Deterministic Choice

lm build
lm sim "selection-demo:NonDeterministic(chan,chan)"

Send requests on ReqA and ReqB:

> ReqA.send()
> ReqB.send()

Step 6: Combine await with if and Parallel Composition

Create combined.wv:

func Combined(chan In, Out, Flag) {
	var fixed x = 0
	
	while {
		await In and if Flag {
			x = In.recv()
			Out.send(x * 3)
		}
	}
}

• Waits for both data and a condition to be true before executing.
• Demonstrates parallel composition with selection.

Step 7: Build and Run the Combined Example

lm build
lm sim "selection-demo:Combined(chan,chan,chan)"

> send In 4
> send Flag 0
# Nothing happens because Flag is false
> send Flag 1
> recv Out
12

Step 8: Inspect Internal Representation

lm show "selection-demo:Combined(chan,chan,chan)"

Generates a Petri-net visualization of guards and choices.

What’s Next?

You’ve now learned:

• Deterministic (or) vs non-deterministic (xor) branching
• How await waits for data validity and if checks conditions
• Combining selection with parallel execution (and)
• Running simulations and inspecting internal representations

The next tutorial will cover communicating processes, showing how multiple processes interact through channels.