Verification and Tests

Althread allows you to formally verify the behavior of your programs. There are two approaches: simple invariants and Linear Temporal Logic (LTL).

Simple Invariants

The always block allows you to define simple properties on the global state of the program, which must always be true, in all reachable states.

Example:

shared {
    let X: int = 0;
}

program A() {
    X++;
}

always {
    X >= 0;
}

info

Here, the always block verifies that the shared variable X is always greater than or equal to 0. It is not possible to access local variables of processes.

Linear Temporal Logic (LTL)

For more complex properties involving time and causality (e.g., "if I make a request, I always get a response later"), Althread offers the check block.

The check block contains an LTL (Linear Temporal Logic) formula.

LTL Operators

Operator	Althread Syntax	Description
Always	always ( P )	P must be true now and for the entire future.
Eventually	eventually ( P )	P must be true at some point (now or later).
Next	next ( P )	P must be true in the next state.
Until	( P ) until ( Q )	P must be true until Q becomes true (Q must happen).
Implication	if P { Q }	If P is true, then Q must be true.

Formula Examples

Safety: "Two lights are never green at the same time"

check {
    always (Light1_red || Light2_red);
}

Liveness: "If the light is red, it will eventually turn green"

check {
    always ( if (Light == RED) { eventually (Light == GREEN) } );
}

Response: "Every request receives a response"

check {
    always ( if Request { eventually Response } );
}

Structure of a Verification Project

It is recommended to group your properties into multiple check blocks to isolate issues.

check {
    // Critical property
    always ( X > 0 );
}

check {
    // Fairness property
    always ( if Request { eventually Response } );
}

Assert Function

For imperative checks within process code, see the documentation for the assert() function.