Numeric return-type hints: ^double/^long on a defn (round 3)

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A ^double/^long return hint on a fn's name now (a) coerces the fn's value on the
way out — exact->inexact / jolt->fx, like a JVM primitive return — and (b) types a
call to it, so an accumulator over the result specializes:

  (defn ^double work [^double x ^double y] (+ (* x x) (* y y)))
  (loop [acc 0.0] (recur (+ acc (work a b))))   ; (+ acc (work ..)) -> fl+

The analyzer pushes the name's numeric tag onto each arity (:ret-nhint) for the
back-end coercion, and resolve-global surfaces the callee's declared return
(:num-ret, read from var meta) onto the :var node so jolt.passes.numeric types the
call. defn carries the name hint through.

This unblocks the accumulator-over-fn-result pattern that round 2 had to demote.
The win is bounded by call overhead in an open/dispatched build (~1.15x on a hot
loop whose body is a helper call); it compounds with direct-linking and, later,
inlining. A numeric return hint is a contract, like ^long — redefining the var to
return another type in an open build breaks it.

Not yet: per-arity arglist return hints, (defn f (^double [x] ..)). Gate:
test/chez/numeric-test.ss 39/39; full make test green, 0 new corpus divergences.

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Yogthos 2026-06-23 17:21:53 -04:00

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