Why an unprovided module is refused

// The same source against two targets.
//
// One provides `drift/animation` and one does not, and the difference is a refusal with the
// module and the target named in it. Nothing about this file changes between the two.
//
// **A script drives a blend tree; it does not build one.** The tree is built where the skeleton
// and the clips already are, and handed to a system through `uses` — so what a script does with
// it is set its parameters and evaluate it. Both kinds of parameter are set the same way: a blend
// weight, and a clock, which is the time a `clip` node was told to read instead of the frame's.

import { blendSet, blendAt, pose } from "drift/animation"

data GaitState {
    // Seconds, for anything that carries on while the character stands still.
    phase: f32 = 0
    // The distance the character has covered, on the gait clips' own axis.
    stride: f32 = 0
}

fn step(state: mut GaitState, dt: f32, speed: f32) {
    state.phase += dt
    state.stride += dt * speed
}

fn drive(tree: Blend, state: GaitState, joints: u32) -> Pose {
    let out = animation.pose(joints)
    animation.blendSet(tree, "speed", 1.0)
    animation.blendSet(tree, "gait", state.stride)
    animation.blendAt(tree, state.phase, out)
    return out
}

// Written before this host provided the capability. It links now, and not a character of it moved.
//
// This is the property the whole language is arranged around, and this file is the receipt. It was
// written when `drift/ai` had no provider here: it parsed, it type-checked, and only linking
// declined it, naming the module and the target and whether the capability existed at all, which is
// a different answer from "no". Then the engine bound the module, and the file linked that day,
// unedited.
//
// So press compile and watch it link. The point is not that it works; it is that nothing about the
// file had to change for it to start working.
//
// What the script does and does not do is worth reading twice. It tells an agent that something
// happened and asks what the agent is doing. It does not receive a plan, and it never runs one:
// a model's output is not an execution path, and a tool resolves by id to something registered
// long before the model was asked.

import { agent, wake, degraded, intentId } from "drift/ai"

data Sentry {
    alarmed: bool = false
    lastIntent: String = ""
}

fn alarm(sentry: mut Sentry, id: String, priority: i32) {
    if let found = ai.agent(id) {
        ai.wake(found, "heard something", priority)
        sentry.alarmed = true
    }
}

// `degraded` is how a script asks whether the agent is over budget and running on its policy floor
// alone. A floor is deterministic and synchronous, so an agent with no provider still behaves, and
// the answer here is a quality signal rather than a failure.
fn observe(sentry: mut Sentry, id: String) -> bool {
    if let found = ai.agent(id) {
        sentry.lastIntent = ai.intentId(found)
        return ai.degraded(found)
    } else {
        return true
    }
}