apl: commute f⍨ (+10 tests, 173/173)

lib/apl/runtime.sx

@@ -1058,3 +1058,7 @@
                               (rest pairs))))
                         (range 0 b-post-size)))
                     (range 0 a-pre-size)))))))))))
+
+(define apl-commute (fn (f x) (f x x)))
+
+(define apl-commute-dyadic (fn (f x y) (f y x)))

lib/apl/tests/operators.sx

@@ -455,4 +455,66 @@
       apl-mul
       (make-array (list 1) (list 6))
       (make-array (list 1) (list 7))))
-  (list 42))
+  (list 42))
+
+(apl-test
+  "commute +⍨ scalar doubles"
+  (rv (apl-commute apl-add (apl-scalar 5)))
+  (list 10))
+
+(apl-test
+  "commute ×⍨ vector squares"
+  (rv (apl-commute apl-mul (make-array (list 4) (list 1 2 3 4))))
+  (list 1 4 9 16))
+
+(apl-test
+  "commute +⍨ vector doubles"
+  (rv (apl-commute apl-add (make-array (list 3) (list 1 2 3))))
+  (list 2 4 6))
+
+(apl-test
+  "commute +⍨ shape preserved"
+  (sh (apl-commute apl-add (make-array (list 3) (list 1 2 3))))
+  (list 3))
+
+(apl-test
+  "commute ×⍨ matrix shape preserved"
+  (sh (apl-commute apl-mul (make-array (list 2 2) (list 1 2 3 4))))
+  (list 2 2))
+
+(apl-test
+  "commute-dyadic -⍨ swaps subtraction"
+  (rv (apl-commute-dyadic apl-sub (apl-scalar 5) (apl-scalar 3)))
+  (list -2))
+
+(apl-test
+  "commute-dyadic ÷⍨ swaps division"
+  (rv (apl-commute-dyadic apl-div (apl-scalar 4) (apl-scalar 12)))
+  (list 3))
+
+(apl-test
+  "commute-dyadic -⍨ on vectors"
+  (rv
+    (apl-commute-dyadic
+      apl-sub
+      (make-array (list 3) (list 10 20 30))
+      (make-array (list 3) (list 1 2 3))))
+  (list -9 -18 -27))
+
+(apl-test
+  "commute-dyadic +⍨ commutative same result"
+  (rv
+    (apl-commute-dyadic
+      apl-add
+      (make-array (list 3) (list 1 2 3))
+      (make-array (list 3) (list 10 20 30))))
+  (list 11 22 33))
+
+(apl-test
+  "commute-dyadic ×⍨ commutative same result"
+  (rv
+    (apl-commute-dyadic
+      apl-mul
+      (make-array (list 3) (list 2 3 4))
+      (make-array (list 3) (list 5 6 7))))
+  (list 10 18 28))

plans/apl-on-sx.md

@@ -78,7 +78,7 @@ Core mapping:
 - [x] Each `f¨` — applies `f` to each scalar/element
 - [x] Outer product `∘.f` — `1 2 3 ∘.× 1 2 3` ↦ multiplication table
 - [x] Inner product `f.g` — `+.×` is matrix multiply
-- [ ] Commute `f⍨` — `f⍨ x` ↔ `x f x`, `x f⍨ y` ↔ `y f x`
+- [x] Commute `f⍨` — `f⍨ x` ↔ `x f x`, `x f⍨ y` ↔ `y f x`
 - [ ] Compose `f∘g` — applies `g` first then `f`
 - [ ] Power `f⍣n` — apply f n times; `f⍣≡` until fixed point
 - [ ] Rank `f⍤k` — apply f at sub-rank k
@@ -118,6 +118,7 @@ data; format for string templating.
 
 _Newest first._
 
+- 2026-05-06: Phase 4 step 6 — commute f⍨ (apl-commute monadic dup, apl-commute-dyadic swap); 173/173 tests
 - 2026-05-06: Phase 4 step 5 — inner product f.g (apl-inner); +.× matrix multiply, ∧.= equal-vectors; 163/163 tests
 - 2026-05-06: Phase 4 step 4 — outer product ∘.f (apl-outer); rank-doubling result shape = a-shape++b-shape; 151/151 tests
 - 2026-05-06: Phase 4 step 3 — each f¨ (monadic apl-each + dyadic apl-each-dyadic); scalar broadcast both sides; 139/139 tests