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feat: persistent singly-linked lists with O(1) conj/cons prepend

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Round 3 of the persistent-collections work. Lists were immutable Janet arrays,
so conj/cons-prepend was an O(n) copy (O(n^2) to build a list) — a large perf
gap vs Clojure's PersistentList.

Add src/jolt/plist.janet: an immutable cons-cell list (first/rest/count), same
algorithm as Clojure/CLJS/jank PersistentList. conj/cons onto a list now creates
an O(1) node that shares the existing list as its tail (no copy), with a cached
O(1) count. Repeated conj is O(n) total instead of O(n^2).

Hooked plist through first/rest/next/seq/count/peek/pop/nth/empty/empty?, the
predicates (list?/seq?/coll?/sequential?), realize-for-iteration, =, coll->cells
(concat/lazy), both printers, destructuring, and instance? tags. (list ...) and
quoted lists stay arrays; only conj/cons introduce plist nodes, so the surface
and risk stay small.

Verified: reduce-conj of 200k elements runs in ~0.4s (was effectively O(n^2)).
conformance 206/206, features 78/78 (+7 list regressions), jank 120 (+1).
This commit is contained in:
Yogthos 2026-06-04 19:20:27 -04:00
parent e43f39bc36
commit a0c9696900
9 changed files with 142 additions and 22 deletions
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9
test/features-test.janet
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@ -118,6 +118,15 @@
["for :let" "(quote (1 4 9))" "(for [x [1 2 3] :let [sq (* x x)]] sq)"]
["for :while" "(quote (0 1 2))" "(for [x (range 10) :while (< x 3)] x)"]
### 13b. Persistent lists — O(1) conj-prepend, immutable, value semantics
["list conj prepends" "(quote (0 1 2 3))" "(conj (list 1 2 3) 0)"]
["list conj multi" "(quote (:c :b :a))" "(conj (quote ()) :a :b :c)"]
["list immutable" "true" "(let [l (list 1 2 3) l2 (conj l 9)] (and (= l (quote (1 2 3))) (= l2 (quote (9 1 2 3)))))"]
["list? after conj" "true" "(list? (conj (list 1 2) 0))"]
["list = vector elts" "true" "(= (quote (1 2 3)) [1 2 3])"]
["reduce conj list" "(quote (2 1 0))" "(reduce conj (list) (range 3))"]
["cons onto list" "(quote (0 1 2 3))" "(cons 0 (list 1 2 3))"]
### 14. Janet interop
["interop method" "\"v=41\"" "(. {:value 41 :describe (fn [self] (str \"v=\" (:value self)))} describe)"]
["interop field" "41" "(.-value {:value 41})"]