apl: ⍸ where — monadic indices-of-truthy + dyadic interval-index

- apl-where (monadic): ravel scan, filter non-zero, +⎕IO offsets
- apl-interval-index (dyadic): count of breaks ≤ y; broadcasts over
  scalar or vector Y
- Wired into apl-monadic-fn / apl-dyadic-fn cond chains
- +10 pipeline tests covering both arities, ⎕IO offsetting, edge cases
  (empty mask, all-truthy, y below/above all breaks)
- pipeline 109/109

lib/apl/runtime.sx

@@ -827,6 +827,31 @@
               ((new-ravel (reduce (fn (acc r) (append acc (map (fn (j) (nth ravel (+ (* r cols) j))) (range 0 cols)))) (list) kept-rows)))
               (make-array (cons (len kept-rows) (rest shape)) new-ravel))))))))
 
+(define
+  apl-where
+  (fn
+    (arr)
+    (let
+      ((ravel (get arr :ravel)) (io (disclose (apl-quad-io))))
+      (let
+        ((indices (filter (fn (i) (not (= (nth ravel i) 0))) (range 0 (len ravel)))))
+        (apl-vector (map (fn (i) (+ i io)) indices))))))
+
+(define
+  apl-interval-index
+  (fn
+    (breaks vals)
+    (let
+      ((b-ravel (get breaks :ravel))
+        (v-ravel
+          (if (scalar? vals) (list (disclose vals)) (get vals :ravel))))
+      (let
+        ((result (map (fn (y) (len (filter (fn (b) (<= b y)) b-ravel))) v-ravel)))
+        (if
+          (scalar? vals)
+          (apl-scalar (first result))
+          (make-array (get vals :shape) result))))))
+
 (define
   apl-primes
   (fn

lib/apl/tests/pipeline.sx

@@ -455,3 +455,45 @@
   (list 1 2 3))
 
 (apl-test "⍕ 42 → \"42\" (alias for ⎕FMT)" (apl-run "⍕ 42") "42")
+
+(begin
+  (apl-test
+    "⍸ where: indices of truthy cells"
+    (mkrv (apl-run "⍸ 0 1 0 1 1"))
+    (list 2 4 5))
+  (apl-test
+    "⍸ where: leading truthy"
+    (mkrv (apl-run "⍸ 1 0 0 1 1"))
+    (list 1 4 5))
+  (apl-test
+    "⍸ where: all-zero → empty"
+    (mkrv (apl-run "⍸ 0 0 0"))
+    (list))
+  (apl-test
+    "⍸ where: all-truthy"
+    (mkrv (apl-run "⍸ 1 1 1"))
+    (list 1 2 3))
+  (apl-test
+    "⍸ where: ⎕IO=1 (1-based)"
+    (mkrv (apl-run "⍸ (⍳5)=3"))
+    (list 3))
+  (apl-test
+    "⍸ interval-index: 2 4 6 ⍸ 5 → 2"
+    (mkrv (apl-run "2 4 6 ⍸ 5"))
+    (list 2))
+  (apl-test
+    "⍸ interval-index: 2 4 6 ⍸ 1 3 5 6 7 → 0 1 2 3 3"
+    (mkrv (apl-run "2 4 6 ⍸ 1 3 5 6 7"))
+    (list 0 1 2 3 3))
+  (apl-test
+    "⍸ interval-index: ⍳5 ⍸ 3 → 3"
+    (mkrv (apl-run "(⍳5) ⍸ 3"))
+    (list 3))
+  (apl-test
+    "⍸ interval-index: y below all → 0"
+    (mkrv (apl-run "10 20 30 ⍸ 5"))
+    (list 0))
+  (apl-test
+    "⍸ interval-index: y above all → len breaks"
+    (mkrv (apl-run "10 20 30 ⍸ 100"))
+    (list 3)))

lib/apl/transpile.sx

@@ -46,6 +46,7 @@
       ((= g "⍕") apl-quad-fmt)
       ((= g "⎕FMT") apl-quad-fmt)
       ((= g "⎕←") apl-quad-print)
+      ((= g "⍸") apl-where)
       (else (error "no monadic fn for glyph")))))
 
 (define
@@ -90,6 +91,7 @@
       ((= g "⍉") apl-transpose-dyadic)
       ((= g "⊢") (fn (a b) b))
       ((= g "⊣") (fn (a b) a))
+      ((= g "⍸") apl-interval-index)
       (else (error "no dyadic fn for glyph")))))
 
 (define