haskell: lazy : + ranges + Prelude (repeat/iterate/fibs/take, +25 tests, 359/359)

plans/haskell-on-sx.md

@@ -78,7 +78,7 @@ Key mappings:
 - [x] Transpile to thunk-wrapped SX: every application arg becomes `(make-thunk (lambda () <arg>))`
 - [x] `force` = SX eval-thunk-to-WHNF primitive
 - [x] Pattern match forces scrutinee before matching
-- [ ] Infinite structures: `repeat x`, `iterate f x`, `[1..]`, Fibonacci stream, sieve of Eratosthenes
+- [x] Infinite structures: `repeat x`, `iterate f x`, `[1..]`, Fibonacci stream (sieve deferred — needs lazy `++` and is exercised under `Classic programs`)
 - [ ] `seq`, `deepseq` from Prelude
 - [ ] Do-notation for a stub `IO` monad (just threading, no real side effects yet)
 - [ ] Classic programs in `lib/haskell/tests/programs/`:
@@ -114,6 +114,35 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-24** — Phase 3 infinite structures + Prelude. Two
+  evaluator changes turn the lazy primitives into a working
+  language:
+  1. Op-form `:` is now non-strict in both args — `hk-eval-op`
+     special-cases it before the eager force-and-binop path, so a
+     cons-cell holds two thunks. This is what makes `repeat x =
+     x : repeat x`, `iterate f x = x : iterate f (f x)`, and the
+     classic `fibs = 0 : 1 : zipWith plus fibs (tail fibs)`
+     terminate when only a finite prefix is consumed.
+  2. Operators are now first-class values via a small
+     `hk-make-binop-builtin` helper, so `(+)`, `(*)`, `(==)` etc.
+     can be passed to `zipWith` and `map`.
+  Added range support across parser + evaluator: `[from..to]` and
+  `[from,next..to]` evaluate eagerly via `hk-build-range` (handles
+  step direction); `[from..]` parses to a new `:range-from` node
+  that the evaluator desugars to `iterate (+ 1) from`. New
+  `hk-load-into!` runs the regular pipeline (parse → desugar →
+  register data → bind decls) on a source string, and `hk-init-env`
+  preloads `hk-prelude-src` with the Phase-3 Prelude:
+  `head`, `tail`, `fst`, `snd`, `take`, `drop`, `repeat`, `iterate`,
+  `length`, `map`, `filter`, `zipWith`, plus `fibs` and `plus`.
+  25 new tests in `lib/haskell/tests/infinite.sx`, including
+  `take 10 fibs == [0,1,1,2,3,5,8,13,21,34]`,
+  `head (drop 99 [1..])`, `iterate (\x -> x * 2) 1` powers of two,
+  user-defined `ones = 1 : ones`, `naturalsFrom`, range edge cases,
+  composed `map`/`filter`, and a custom `mySum`. 359/359 green.
+  Sieve of Eratosthenes is deferred — it needs lazy `++` plus a
+  `mod` primitive — and lives under `Classic programs` anyway.
+
 - **2026-04-24** — Phase 3 laziness foundation. Added a thunk type to
   `lib/haskell/eval.sx` (`hk-mk-thunk` / `hk-is-thunk?`) backed by a
   one-shot memoizing `hk-force` that evaluates the deferred AST, then