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Match Clojure's lazy seq realization model

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jolt's seq layer realized one element ahead of Clojure, so a side-effecting
lazy seq ran its producer too eagerly. Four changes bring it in line:

- rest is Clojure's more(): it returns the tail without realizing it. An
  unforced tail (vector / string / lazy-seq cell) comes back as a deferred
  seq, so (rest (iterate f x)) does not call f. next still realizes one.
- iterate applies f lazily, inside the tail thunk, so (first (iterate f x))
  is x with no call to f (clojure.lang.Iterate parity).
- take realizes exactly n: the last element terminates without touching the
  rest, instead of forcing one more element of the source.
- an empty realized lazy seq is still a sequence value, printing "()" not
  "nil" (a JVM LazySeq is never nil).

Also: the map transducer's step fn now takes multiple inputs
([result input & inputs]) so a multi-collection transduce applies f across
all of them. Fixes medley's join/window/sequence-padded laziness and
multi-input transducer tests (now 293/293). The rest change also fixed a
latent overrun in distinct/dedupe over a map's empty tail.

iterate is a seed source, re-minted.
This commit is contained in:
Yogthos 2026-06-26 19:41:02 -04:00
parent 331a41ee26
commit 448611a5df
6 changed files with 57 additions and 12 deletions
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8
test/chez/corpus.edn
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@ -3261,4 +3261,12 @@
{:suite "printer / print-level" :label "scalars print regardless of level" :expected "true" :actual "(= \"[1 2]\" (binding [*print-level* 1] (pr-str [1 2])))"}
{:suite "reader / default-data-reader-fn" :label "consulted for an unregistered tag" :expected "true" :actual "(= [(quote foo) 42] (binding [*default-data-reader-fn* (fn [tag v] [tag v])] (read-string \"#foo 42\")))"}
{:suite "printer / print-vars" :label "print-length / print-level / default-data-reader-fn default to nil" :expected "[true true true]" :actual "[(nil? *print-length*) (nil? *print-level*) (nil? *default-data-reader-fn*)]"}
{:suite "seqs / laziness" :label "an empty lazy seq is () not nil" :expected "true" :actual "(= () (lazy-seq nil))"}
{:suite "seqs / laziness" :label "rest of a one-element coll is ()" :expected "true" :actual "(= () (rest [1]))"}
{:suite "seqs / laziness" :label "first of iterate does not call f" :expected "true" :actual "(let [a (atom 0)] (first (iterate (fn [x] (swap! a inc) (inc x)) 0)) (= 0 @a))"}
{:suite "seqs / laziness" :label "rest of iterate does not realize the next element" :expected "true" :actual "(let [a (atom 0)] (rest (iterate (fn [x] (swap! a inc) (inc x)) 0)) (= 0 @a))"}
{:suite "seqs / laziness" :label "take from an infinite iterate" :expected "[0 1 2 3 4]" :actual "(vec (take 5 (iterate inc 0)))"}
{:suite "seqs / laziness" :label "distinct over a rest-derived seq does not overrun" :expected "2" :actual "(count (distinct (map inc (rest [10 20 30]))))"}
{:suite "transducers / map multi-input" :label "the map transducer applies f across all inputs" :expected "[6]" :actual "(((map +) conj) [] 1 2 3)"}
{:suite "transducers / map multi-input" :label "single input is unchanged" :expected "[3]" :actual "(((map +) conj) [] 3)"}
]