test:

src/Lean/Data/PersistentArray.lean

@@ -277,9 +277,25 @@ instance [Monad m] : ForIn m (PersistentArray α) α where
   | node cs => cs.forM (fun c => forMAux f c)
   | leaf vs => vs.forM f
 
-@[specialize] def forM (t : PersistentArray α) (f : α → m PUnit) : m PUnit :=
+@[specialize] def forMFrom0 (t : PersistentArray α) (f : α → m PUnit) : m PUnit :=
   forMAux f t.root *> t.tail.forM f
 
+@[specialize] private partial def forFromMAux (f : α → m PUnit) : PersistentArrayNode α → USize → USize → m PUnit
+  | node cs, i, shift => do
+    let j    := (div2Shift i shift).toNat
+    forFromMAux f cs[j]! (mod2Shift i shift) (shift - initShift)
+    cs.forM (start := j+1) (forMAux f)
+  | leaf vs, i, _ => vs.forM (start := i.toNat) f
+
+@[specialize] def forM (t : PersistentArray α) (f : α → m PUnit) (start : Nat := 0) : m PUnit := do
+  if start == 0 then
+    forMFrom0 t f
+  else if start >= t.tailOff then
+    t.tail.forM (start := start - t.tailOff) f
+  else do
+    forFromMAux f t.root (USize.ofNat start) t.shift
+    t.tail.forM f
+
 end
 
 @[inline] def foldl (t : PersistentArray α) (f : β → α → β) (init : β) (start : Nat := 0) : β :=

tests/lean/run/PArray_forM.lean

@@ -0,0 +1,29 @@
+import Lean.Data.PersistentArray
+/-!
+Test `PersistentArray.forM` with nonzero start position.
+-/
+
+def mk (n : Nat) : Lean.PersistentArray Nat :=
+  List.range n |>.toPArray'
+
+def sum1 (start : Nat) (s : List Nat) : Nat :=
+  let (_, s) := StateT.run (m := Id) (s.drop start |>.forM fun val => modify (· + val)) 0
+  s
+
+def sum2 (start : Nat) (s : Lean.PArray Nat) : Nat :=
+  let (_, s) := StateT.run (m := Id)  (s.forM (start := start) (fun val => modify (· + val))) 0
+  s
+
+def check (s₁ : List Nat) : IO Unit := do
+  let s₂ := s₁.toPArray'
+  let n  := s₂.size
+  for i in *...n do
+    unless sum1 i s₁ == sum2 i s₂ do
+      throw <| .userError "failed"
+  IO.println "ok"
+
+#eval check (List.range 10)
+#eval check (List.range 0)
+#eval check (List.range 2000)
+#eval check (List.replicate 1000 1)
+#eval check (List.replicate 10 2)