The IR is pure call-by-push-value

The intermediate representation is structurally pure call-by-push-value: values, computations, force and thunk, and the dedicated nodes for the five control operators. A surface convenience whose unfolding is purely syntactic does not survive into the IR — it unfolds at elaboration into the existing primitives:

• currying sugar { |x y| M } lowers to nested lambdas;
• a control operator lowers to its structural IR node, not a name-keyed builtin;
• the byte/value pipeline forms lower to one pipeline node, the byte-vs-value distinction recovered by mode inference.

Two conveniences are resolved by runtime values and therefore stay in the IR as their own constructors, not as unfoldings:

• a spread is a value-level element constructor (ValListElem::Spread, ValMapEntry::Spread, spread Args) — how many slots it fills depends on the length of the spread value;
• string interpolation is a computation node (CompKind::Interpolation) whose part-computations evaluate and concatenate.

The elaborator is the one place that knows about surface forms; everything downstream — typechecker, evaluator — sees only the IR’s fixed constructor set.

This keeps the core small and the semantics regular. When adding a surface convenience, lower it during elaboration into what the IR already has; the IR grows a constructor only when the convenience’s meaning depends on runtime values.

See also fixed-arity. Cite: docs/SPEC.md §1 (grammar / scope-stage), §IR.