Agents, L1 — Thermostat#

Level 1 · Novice

Source: src/MLambda.AI.Agent/Thermostat.ha

What makes this an agent and not an `if`#

Here is a thermostat as ordinary code:

if (temperature < 18) { heater.On(); }

And here is one as an agent:

belief temperature(degrees: Int)

desire Comfortable reachable by [Warm, Cool]

intend Warm for Comfortable when B(self) temperature(t) ∧ t < 18
achieved Comfortable when B(self) temperature(t) ∧ t ≥ 18 ∧ t ≤ 24

Three differences, and they are the whole subject.

It holds a goal. Comfortable is a thing the agent wants. The if version has no goal anywhere — the goal lives in the head of whoever wrote it.
An intention is licensed only while the goal is unmet. achieved says what being comfortable is, and once that holds the agent proposes nothing. Nothing had to remember to stop it.
It does not act. intend Warm means I intend Warm, toward Comfortable. The heater is never switched on by this file.

The agent decides; the host acts#

plan Warm ↦ Room.Heat

Room.Heat is a label. There is no Room.Heat anywhere in this repository and there does not need to be. What the engine reports is:

I intend Warm, toward Comfortable.

and Program.cs reads that and decides what it means. This seems like a small thing and is not: it means the deliberation can be tested, printed, and reasoned about without anything happening to a real heater.

The five keywords#

Keyword	What it says	In `Thermostat.ha`
`belief`	what the agent can hold true	`belief temperature(degrees: Int)`
`desire … reachable by`	a goal, and the plans that could reach it	`desire Comfortable reachable by [Warm, Cool]`
`intend … for … when`	adopt a plan toward a goal when a condition holds	`intend Warm for Comfortable when …`
`achieved … when`	the goal is met — stop	`achieved Comfortable when …`
`plan … ↦`	the label a host acts on	`plan Warm ↦ Room.Heat`

And one the compiler insists on#

commit open_minded

You cannot write an agent without this. The compiler refuses one that does not say how long its intentions survive contact with the world, and the three answers are:

Strategy	The intention survives until
`blind`	you believe the goal achieved — come what may
`single_minded`	achieved, or believed impossible
`open_minded`	achieved, or you stop holding the goal — reconsidered as beliefs change

A thermostat is open-minded, because its world will not hold still. Somebody opens a window and the right thing to do changes. An agent that blindly finished heating would be a worse thermostat than a bimetallic strip.

Collector is single-minded, and the pair is the comparison worth making once you have met both.

One thing to know now and understand later#

B(self) temperature(t) does not mean look up this agent's temperature field. It means temperature(t) is true in every world this agent takes to be possible — which is what belief means in modal logic, and why an agent can be wrong without being broken.

The engine actually runs something slightly different, for a reason that has its own page at L2. You do not need that yet. You do need to know that B(self) is a claim about possibility and not a field lookup, because everything above is built on it.

Run it#

dotnet run --project src/MLambda.AI.Agent -- Thermostat

  degrees   intends      goal met
  ───────   ──────────   ────────
       10   Warm         —
       18   —            Comfortable
       24   —            Comfortable
       30   Cool         —

One agent, one goal, four temperatures. The goal never changes and what it intends does.

What the tests assert#

ThermostatTests.cs

Cold means it intends to warm; hot, to cool.
The intention names the goal it serves, not just the plan — which is what lets a host ask why, not only what.
Comfortable means it intends nothing at all, and the goal is met.
The band is closed at both ends: 18 and 24 are comfortable, 17 and 25 are not. An off-by-one here would be invisible in every other test in the file.
One thermostat's beliefs never become another's.
A belief that changes changes what it intends.

Try it yourself#

Widen the comfortable band to 16–26 and re-run. Which rows change?
Add a third plan, Vent, for hot weather, and give it a condition of its own.
Change commit open_minded to commit blind and read Cleaner's page to see what you gave up.