Module place 

Stage 7 — placement: inferred from effects, solved against a cost model, verified.

docs/03-type-and-effect-system.md §3.3: “Placement is not a primitive annotation; it is a constraint solution — with explicit @on(...) always available and always winning. Purity means unplaced: legal on every tier, compiled to each tier that needs it.”

Phase 1 shipped the verification half. Phase 2 adds the other two thirds of §3.3 and all of §3.4: candidates come from the inferred effect row, the choice among them comes from a cost model (crate::cost), and every guardrail §3.4 calls non-negotiable is here —

• determinism — integer costs, a fixed enumeration order and a total tie-break, so the same program always yields the same solution;
• stability — the previous solution is persisted in beck.lock and preferred on a tie, and churn against it is reported rather than absorbed;
• explainability — every candidate’s cost survives the solve, so beck explain place can print the derivation rather than the conclusion;
• assertability — beck check --assert-place page=client fails the build on a change.

What is actually being decided

Less than it first appears, and that is the design working rather than the solver shirking. Three rules pin most of a program before the solver runs, and each is a statement about meaning rather than about cost:

1. @on(...) wins (§3.3).
2. A pure definition is unplaced — Tier::Any — and is compiled into every tier that calls it. “That duplication is the payoff, not waste.”
3. A Signal[Html] is the browser’s subscription (§4.3), so it is pinned to the client. Where the view function runs — Mode A server-side rendering versus Mode B in the browser — is the 05 §5.1 decision that §3.4 also mentions, and it is Phase 3’s: Mode B does not exist, so the cost model has nothing to choose between and should not pretend otherwise.

What is left is genuinely open, and the corpus exercises it: which tier holds a durable fold, where decide sits, and where an effectful definition that more than one tier could discharge ends up — net.out(origin) being the sharp case, since a browser and a server can both make that call and only the callers decide which is cheaper.

Structs

Explanation

Why a node’s placement is what it is — the content of beck explain place.

Lock

The persisted previous solution — §3.4’s stability guardrail.

Solution

The solved placement of a whole program.

Enums

Key

A placeable thing: a top-level definition, or a signal.

Method

Functions

apply

Write the solved tiers back into the program.

check_placement

Verify every placement in a checked program, annotated or solved.

mentions

Every top-level name an expression mentions — the placement graph’s edges.

solve

Solve the placement of every definition and signal in a checked program.