Nilable record types + flow-sensitive nil narrowing (#235)

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A record-or-nil (a protocol method whose impls return a record in one branch and
nil in another, or an `if` over a ctor and nil) now types as a NILABLE record
instead of widening to :any. A nilable record still bare-indexes its field reads
(jrec-field-at falls back to jolt-get on nil), but some?/nil? do NOT fold on it, so
a runtime guard is preserved — and inside (if (some? x) ..) / (if x ..) the then-
branch narrows x to the non-nil record, so its reads bare-index AND unbox there.

This is what lets the bounced ray type without a hint: scatter returns
ScatterResult-or-nil (Metal absorbs some rays), and the consumer reads
(:ray scattered) only under (if (some? scattered) ..). The narrowing proves
scattered non-nil there.

lattice: :nil type; :nil ∨ struct -> nilable struct, ∨ anything else -> :any;
nilability is contagious through a struct join, which also now preserves :type when
both sides agree (needed so a record ∨ its nilable self stays that record).
truthy-type?/field-type/pred-on treat a nilable struct as maybe-nil. types: nil
literal -> :nil; an `if` whose test is (some? x)/(nil? x)/x narrows the nilable
local x in the proven branch.

Ray tracer with NO hints: 38.4s -> 23.9s (~1.6x) — hit-sphere now types fully
(0 jolt-get, 57 jrec-field-at, 38 fl-ops), identical to the hand-hinted build.

run-narrow.ss gate, incl. the load-bearing check that the nil case still takes the
else branch (the guard is not folded away). make test / shakesmoke green, selfhost
holds, 0 new divergences.

Co-authored-by: Yogthos <yogthos@gmail.com>

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Dmitri Sotnikov 2026-06-26 17:16:16 +00:00 • committed by GitHub

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