haskell: Phase 10 — Fractional stub: recip + fromRational (+3 tests, 31/31)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/haskell/eval.sx

@@ -809,6 +809,11 @@
           env
           "truncate"
           (hk-mk-builtin "truncate" (fn (x) (truncate x)) 1))
+        (dict-set! env "recip" (hk-mk-builtin "recip" (fn (x) (/ 1 x)) 1))
+        (dict-set!
+          env
+          "fromRational"
+          (hk-mk-builtin "fromRational" (fn (x) x) 1))
         (let
           ((--sx-to-hk-- (fn (lst) (if (empty? lst) (list "[]") (list ":" (first lst) (--sx-to-hk-- (rest lst))))))
             (--words--

lib/haskell/tests/numerics.sx

@@ -156,4 +156,13 @@
   (hk-deep-force (hk-run "main = truncate (negate 3.7)"))
   -3)
 
+(hk-test "recip 4.0 = 0.25" (hk-deep-force (hk-run "main = recip 4.0")) 0.25)
+
+(hk-test "1.0 / 4.0 = 0.25" (hk-deep-force (hk-run "main = 1.0 / 4.0")) 0.25)
+
+(hk-test
+  "fromRational 0.5 = 0.5 (identity)"
+  (hk-deep-force (hk-run "main = fromRational 0.5"))
+  0.5)
+
 {:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}

plans/haskell-completeness.md

@@ -162,7 +162,9 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   tests like `show 42 = "42"` rely on this rendering. Documented in `numerics.sx`._
 - [x] Math builtins: `sqrt`, `floor`, `ceiling`, `round`, `truncate` — call
   the corresponding SX numeric primitives.
-- [ ] `Fractional` typeclass stub: `(/)`, `recip`, `fromRational`.
+- [x] `Fractional` typeclass stub: `(/)`, `recip`, `fromRational`. _(/)
+  already a binop; `recip x = 1 / x` and `fromRational x = x` registered as
+  builtins in the post-prelude block._
 - [ ] `Floating` typeclass stub: `pi`, `exp`, `log`, `sin`, `cos`, `(**)`
   (power operator, maps to SX exponentiation).
 - [ ] Tests in `lib/haskell/tests/numeric.sx` (≥ 15 tests: fromIntegral
@@ -301,6 +303,10 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 10 Fractional stub (recip, fromRational):
+- `(/)` already a binop. Added `recip` and `fromRational` as builtins
+  post-prelude. 3 new tests in `numerics.sx` (now 31/31).
+
 **2026-05-07** — Phase 10 math builtins (sqrt/floor/ceiling/round/truncate):
 - Inserted in the post-prelude `begin` block so they override the prelude's
   identity stubs. `ceiling` is the only one needing a definition (SX doesn't