haskell: classic program fib.hs + source-order top-level binding (+2 tests, 388/388)

lib/haskell/eval.sx

@@ -677,16 +677,22 @@ plus a b = a + b
   hk-bind-decls!
   (fn
     (env decls)
-    (let ((groups (dict)) (pat-binds (list)))
-      ;; Pass 1: collect fun-clause groups by name; collect pat-binds
-      ;; in source order. Pre-seed env so any name can already be
-      ;; looked up by closures built in pass 2.
+    (let
+      ((groups (dict))
+        (group-order (list))
+        (pat-binds (list)))
+      ;; Pass 1: collect fun-clause groups by name; track first-seen
+      ;; order so pass 3 can evaluate 0-arity bodies in source order
+      ;; (forward references to other 0-arity definitions still need
+      ;; the earlier name to be bound first).
       (for-each
         (fn (d)
           (cond
             ((= (first d) "fun-clause")
               (let
                 ((name (nth d 1)))
+                (when (not (has-key? groups name))
+                  (append! group-order name))
                 (dict-set!
                   groups
                   name
@@ -703,8 +709,9 @@ plus a b = a + b
               (append! pat-binds d))
             (:else nil)))
         decls)
-      ;; Pass 2: install multifuns for arity > 0; mark 0-arity for
-      ;; pass 3. The mutable env means recursive references work.
+      ;; Pass 2: install multifuns (arity > 0) — order doesn't matter
+      ;; because they're closures; collect 0-arity names in source
+      ;; order for pass 3.
       (let ((zero-arity (list)))
         (for-each
           (fn (name)
@@ -717,8 +724,11 @@ plus a b = a + b
                       name
                       (hk-mk-multifun arity clauses env)))
                   (:else (append! zero-arity name))))))
-          (keys groups))
-        ;; Pass 3: evaluate 0-arity bodies and pat-binds.
+          group-order)
+        ;; Pass 3: evaluate 0-arity bodies and pat-binds in source
+        ;; order — forward references to a later 0-arity name will
+        ;; still see its placeholder (nil) and fail noisily, but the
+        ;; common case of a top-down program works.
         (for-each
           (fn (name)
             (let ((clauses (get groups name)))

lib/haskell/tests/program-fib.sx

@@ -0,0 +1,45 @@
+;; fib.hs — infinite Fibonacci stream classic program.
+;;
+;; The canonical artefact lives at lib/haskell/tests/programs/fib.hs.
+;; The source is mirrored here as an SX string because the evaluator
+;; doesn't have read-file in the default env. If you change one, keep
+;; the other in sync — there's a runner-level cross-check against the
+;; expected first-15 list.
+
+(define
+  hk-prog-val
+  (fn
+    (src name)
+    (hk-deep-force (get (hk-eval-program (hk-core src)) name))))
+
+(define hk-as-list
+  (fn (xs)
+    (cond
+      ((and (list? xs) (= (first xs) "[]")) (list))
+      ((and (list? xs) (= (first xs) ":"))
+        (cons (nth xs 1) (hk-as-list (nth xs 2))))
+      (:else xs))))
+
+(define
+  hk-fib-source
+  "zipPlus (x:xs) (y:ys) = x + y : zipPlus xs ys
+zipPlus _ _ = []
+myFibs = 0 : 1 : zipPlus myFibs (tail myFibs)
+result = take 15 myFibs
+")
+
+(hk-test
+  "fib.hs — first 15 Fibonacci numbers"
+  (hk-as-list (hk-prog-val hk-fib-source "result"))
+  (list 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377))
+
+;; Spot-check that the user-defined zipPlus is also reachable
+(hk-test
+  "fib.hs — zipPlus is a multi-clause user fn"
+  (hk-as-list
+    (hk-prog-val
+      (str hk-fib-source "extra = zipPlus [1, 2, 3] [10, 20, 30]\n")
+      "extra"))
+  (list 11 22 33))
+
+{:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}

lib/haskell/tests/programs/fib.hs

@@ -0,0 +1,15 @@
+-- fib.hs — infinite Fibonacci stream.
+--
+-- The classic two-line definition: `fibs` is a self-referential
+-- lazy list built by zipping itself with its own tail, summing the
+-- pair at each step. Without lazy `:` (cons cell with thunked head
+-- and tail) this would diverge before producing any output; with
+-- it, `take 15 fibs` evaluates exactly as much of the spine as
+-- demanded.
+
+zipPlus (x:xs) (y:ys) = x + y : zipPlus xs ys
+zipPlus _ _ = []
+
+myFibs = 0 : 1 : zipPlus myFibs (tail myFibs)
+
+result = take 15 myFibs

plans/haskell-on-sx.md

@@ -82,7 +82,7 @@ Key mappings:
 - [x] `seq`, `deepseq` from Prelude
 - [x] Do-notation for a stub `IO` monad (just threading, no real side effects yet)
 - [ ] Classic programs in `lib/haskell/tests/programs/`:
-  - [ ] `fib.hs` — infinite Fibonacci stream
+  - [x] `fib.hs` — infinite Fibonacci stream
   - [ ] `sieve.hs` — lazy sieve of Eratosthenes
   - [ ] `quicksort.hs` — naive QS
   - [ ] `nqueens.hs`
@@ -114,6 +114,23 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-25** — First classic program: `fib.hs`. Canonical Haskell
+  source lives at `lib/haskell/tests/programs/fib.hs` (the
+  two-cons-cell self-referential fibs definition plus a hand-rolled
+  `zipPlus`). The runner at `lib/haskell/tests/program-fib.sx`
+  mirrors the source as an SX string (the OCaml server's
+  `read-file` lives in the page-helpers env, not the default load
+  env, so direct file reads from inside `eval` aren't available).
+  Tests: `take 15 myFibs == [0,1,1,2,3,5,8,13,21,34,55,89,144,233,377]`,
+  plus a spot-check that the user-defined `zipPlus` is also
+  reachable. Found and fixed an ordering bug in `hk-bind-decls!`:
+  pass 3 (0-arity body evaluation) iterated `(keys groups)` whose
+  order is implementation-defined, so a top-down program where
+  `result = take 15 myFibs` came after `myFibs = …` could see
+  `myFibs` still bound to its `nil` placeholder. Now group names
+  are tracked in source order via a parallel list and pass 3 walks
+  that. 388/388 green.
+
 - **2026-04-25** — Phase 3 do-notation + stub IO monad. Added a
   `hk-desugar-do` pass that follows Haskell 98 §3.14 verbatim:
   `do { e } = e`, `do { e ; ss } = e >> do { ss }`,