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Step 4: lazy mapcat overlay — eliminate apply forcing on infinite inputs

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mapcat in 10-seq.clj now uses a defn- mapcat-step helper that walks
lazy map results element-by-element (cons + lazy-seq), with explicit
arities for 1/2/3/4+ colls. The 4+-coll arity still uses apply to
spread colls to map, but apply no longer forces the inner map result.

Previously (apply concat (apply map f colls)) forced the lazy map
result through core-apply realize-for-iteration, which hung on
infinite inputs. The new step-based impl walks one element at a time
via first/rest/seq primitives.

Re-enabled deferred mapcat infinite-input harness case. 19/19 pass.

Gates: lazy-infinite 19/19, conformance 229/229 interpret+self-host
(compile 228/229 — 1 pre-existing protocol error), specs 32/32.
This commit is contained in:
Yogthos 2026-06-08 08:14:59 -04:00
parent e2e189acfe
commit d16e1f4eba
2 changed files with 25 additions and 5 deletions
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4
test/integration/lazy-infinite-test.janet
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@ -52,9 +52,7 @@
["take 3 partition-all 2 range" "(quote ((0 1) (2 3) (4 5)))" "(take 3 (partition-all 2 (range)))"]
["take 3 map-indexed vector range" "(quote ([0 0] [1 1] [2 2]))" "(take 3 (map-indexed vector (range)))"]
["take 3 distinct cycle" "(quote (1 2 3))" "(take 3 (distinct (cycle [1 2 1 3 1])))"]
# NOTE: mapcat on infinite inputs hangs because apply forces the lazy map
# result. This requires Step 4 (fix apply to lazily spread lazy-seqs).
# ["take 6 mapcat dup range" "(quote (0 0 1 1 2 2))" "(take 6 (mapcat (fn [x] [x x]) (range)))"]
["take 6 mapcat dup range" "(quote (0 0 1 1 2 2))" "(take 6 (mapcat (fn [x] [x x]) (range)))"]
["take 3 take-nth 2 range" "(quote (0 2 4))" "(take 3 (take-nth 2 (range)))"]
["take 3 interpose :x range" "(quote (0 :x 1))" "(take 3 (interpose :x (range)))"]
["take 3 map vector range iterate" "(quote ([0 100] [1 101] [2 102]))" "(take 3 (map vector (range) (iterate inc 100)))"]