Subscript design notes (maintainers)

Why the user-facing behavior in reference/subscripts.md is sound. Users don't need this page.

Lowering

A subscript is a compile-time place-macro: at a call site g.edge(i) is replaced by the yielded place with the arguments substituted (g.nodes[i]), and the surrounding read or write flows through the ordinary lvalue machinery. There is no runtime object. It generalizes the built-in &m[k] place to user code: a scoped, transient projection into part of a value — never a stored reference, never crossing the deep-copy thread boundary, needing no lifetime analysis. That is what fits it inside the arena + deep-copy model.

Why each rule exists

• Rooted in a parameter. The projection must reach into an argument, so it cannot dangle. A place rooted in a fresh local is rejected. This is the same scoping argument as the limited-references spike (limited-references): the one limited-reference construct that stays sound is the one that cannot escape.
• Each parameter used at most once. Substitution duplicates the argument expression at each mention, so a double mention would double-evaluate it. Multi-use with argument hoisting is the natural future extension. The receiver counts, which is why 2-D indexing into a flat array (yield &g.cells[r * g.w + c]) is refused; storing rows nested (&g.rows[r][c]) mentions the receiver once. tools/tycho-grid/ works exactly that way.
• Concrete receiver. Selection is by the receiver's exact monomorphized type, so a Pool($T) receiver could never match the Pool(int) a call site presents. Rejected at the declaration. The remedy is a generic function — which is what Go and Odin do, neither having user-defined indexing at all.
• Scoped and transient. Never stored in a field, never sent across a spawn/channel boundary. Storing a reference (a "remote part") is a non-goal: it has no sound deep-copy at the thread boundary.

Value semantics

Writing through a projection into a by-value parameter mutates that parameter's private copy, exactly like the spelled-out place. The usual mutability rules apply unchanged; the projection adds no new storage shape, only ergonomic traversal of the index-pool idiom.

Decided 2026-10-18 (update probe)

• No inout callbacks. A function with an inout parameter is not a value and lambdas cannot declare one, so the update(g, id, fn(p)) shape is a copy-back transformer plus a writeback through the projection. Kept deliberately: the idiom is proven, and real inout captures would need escape analysis the tree does not have. Revisit on a real program that needs it, not on principle.
• Index-assign and the mutating builtins refuse subscript calls. push(g.fr(0), v) and g.fr(0)[0] = v die with the variable-or-field rule even though the projection is a place. This is parity with the reference compiler, kept by default: the owner states no preference for or against subscript targets here, so the normative behavior stands. Details in FRICTION 206.