jolt/docs/rfc/0004-type-hints.md

RFC 0004: Type hints and keyword-lookup specialization

Status: accepted (design note)

This note describes how Jolt treats Clojure type hints, and the one place it uses them: a ^:struct or ^Record hint on a local lets a constant-keyword lookup skip its runtime representation guard. It records the rationale, the soundness contract, the checked mode for catching inaccurate hints, and the measured effect, so later work does not relitigate it.

Background: why the lookup carries a guard

A Jolt map value has several runtime representations (see RFC on collections and src/jolt/core.janet): a Janet struct for a small all-scalar-key literal map, a persistent hash map (a table tagged :jolt/type :jolt/phm) when a key is a collection or a value is nil, plus sorted maps, transients, and record/deftype instances. A record instance is a Janet table tagged :jolt/deftype but, like a struct, it carries no :jolt/type, so a raw Janet (get inst :field) reads its fields directly.

A constant-keyword lookup (:k m) compiles to a guarded form:

(if (get m :jolt/type) (core-get m k) (get m k))

The guard is one opcode. A non-nil :jolt/type routes phm/sorted/transient/ lazy-seq values to core-get's full semantics; everything else (structs, records, nil, scalars) takes the bare Janet get, which matches core-get for keyword keys. The guard is correct and cheap, but on a struct it is a second get: profiling the ray tracer (a naive all-maps program) found keyword lookups are about half of a render, and the guard is the only avoidable part of each one. A bare get is roughly 20ns where the guarded form is roughly 36ns.

Dropping the guard is only safe when the subject is known to be a plain struct/record rather than a tagged collection. Jolt does not infer that inter-procedurally (it would be unsound across a dynamic language's call boundaries). A type hint supplies the same fact soundly, as a programmer assertion.

What the hints mean

Two hints on a local resolve to the "plain struct/record" assertion, which we call the :struct hint internally:

• ^:struct — the value is a plain struct or record map. There is no Clojure keyword with this meaning (Clojure's type hints are class names), so this is a Jolt-specific metadata flag, analogous to ^:dynamic.
• ^Name where Name is a defrecord/deftype. Both forms define a ->Name positional constructor, so the analyzer treats a tag whose ->Name resolves as a record type. Record instances are raw-get-safe, so the lookup drops the guard. A ^String, ^long, or any other non-record tag is not a record and is ignored, exactly as before.

Every other hint parses and is inert, matching Clojure (S12b in the reader spec). A hint never changes a program's result; it only permits an optimization.

How it flows

The reader already keeps ^hint metadata on the binding symbol and is otherwise transparent (reader.janet, meta-form->map). The change threads that fact to the lookup site:

1. The analyzer (jolt-core/jolt/analyzer.clj) records a :struct hint per local in its env when a param or let binding carries ^:struct or a record-type tag, and attaches :hint :struct to that local's :local IR node. Resolving a record-type tag uses a new host contract function record-type? (src/jolt/host_iface.janet), which checks for the ->Name constructor.
2. The back end (emit-kw-lookup in src/jolt/backend.janet) emits the bare get when the lookup subject is a :local carrying the hint, and the guarded form otherwise. The unhinted path is byte-identical to before.
3. The inline pass (jolt-core/jolt/passes.clj) propagates the hint: when it binds a non-trivial call argument to a fresh local, it carries the called function's parameter hint onto that local, so lookups inside the spliced body keep the bare path. Without this, inlining a hinted function would erase the benefit, because the hinted parameter is replaced by an unhinted temporary.

The same machinery covers both (:k m) and (get m :k [default]) when the key is a constant keyword. A get with a variable, numeric, or string key falls through to core-get unchanged.

Soundness and the checked mode

An accurate hint is correctness-preserving by construction: for a struct or record the bare get equals the guarded result. An inaccurate hint (asserting ^:struct for a value that is actually a phm) makes the raw get return the wrong thing. This is the same contract as a wrong Clojure ^String, except that a wrong Jolt hint fails silently rather than throwing.

To make a lie visible without taxing the fast path, JOLT_CHECK_HINTS=1 keeps the guard but throws on the tagged arm with a message naming the local and key:

type hint violated on `m`: (:a m) — value carries :jolt/type
(a phm/sorted/transient/lazy-seq), not the plain struct/record the
^:struct/^Record hint asserts

This is a development aid, off by default, with zero cost to normal builds (the flag is read when the lookup is compiled, and the bare get is emitted when it is off). The flag is part of the image-cache fingerprint.

Coverage

Type hints parse in every position Clojure accepts them and are inert except for the optimization above. This matches Clojure's "parse and otherwise do nothing" model, with the difference that Clojure additionally uses hints to avoid reflection and select primitive arithmetic, which do not apply to a Janet host.

Measured effect

On the ray tracer (~/src/examples/ray-tracer, all values are {:r :g :b}-style maps), with inlining on and the hot parameters hinted, a render goes from 13.3s to 10.9s, about 1.22x, taking it to roughly 7.8x the JVM from 9.4x after the inline pass. A seeded render produces an identical pixel checksum hinted and unhinted, confirming the hints are correctness-preserving on the full pipeline.

Status and non-goals

Implemented. Not pursued: inter-procedural shape inference (unsound in a dynamic language without a guard, which costs as much as the one being removed) and a shape-based "hidden class" representation (profiling showed allocation is about 1% of the workload, so a cheaper allocation would not help, and an escaping-map lookup through a runtime shape check costs about the same as the guard it would replace). The hint is the sound, opt-in lever on the part of the cost that can move.