diff --git a/src/Init/Control/Basic.lean b/src/Init/Control/Basic.lean
index 219130d1e1..636fd7245d 100644
--- a/src/Init/Control/Basic.lean
+++ b/src/Init/Control/Basic.lean
@@ -11,8 +11,13 @@ universe u v w
 /--
 A `ForIn'` instance, which handles `for h : x in c do`,
 can also handle `for x in x do` by ignoring `h`, and so provides a `ForIn` instance.
+
+Note that this instance will cause a potentially non-defeq duplication if both `ForIn` and `ForIn'`
+instances are provided for the same type.
 -/
-instance (priority := low) instForInOfForIn' [ForIn' m ρ α d] : ForIn m ρ α where
+-- We set the priority to 500 so it is below the default,
+-- but still above the low priority instance from `Stream`.
+instance (priority := 500) instForInOfForIn' [ForIn' m ρ α d] : ForIn m ρ α where
   forIn x b f := forIn' x b fun a _ => f a
 
 @[simp] theorem forIn'_eq_forIn [d : Membership α ρ] [ForIn' m ρ α d] {β} [Monad m] (x : ρ) (b : β)
diff --git a/src/Init/Data/Array/Basic.lean b/src/Init/Data/Array/Basic.lean
index 39e7d761ab..3e520e51b4 100644
--- a/src/Init/Data/Array/Basic.lean
+++ b/src/Init/Data/Array/Basic.lean
@@ -302,37 +302,6 @@ def modifyOp (self : Array α) (idx : Nat) (f : α → α) : Array α :=
   We claim this unsafe implementation is correct because an array cannot have more than `usizeSz` elements in our runtime.
 
   This kind of low level trick can be removed with a little bit of compiler support. For example, if the compiler simplifies `as.size < usizeSz` to true. -/
-@[inline] unsafe def forInUnsafe {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (as : Array α) (b : β) (f : α → β → m (ForInStep β)) : m β :=
-  let sz := as.usize
-  let rec @[specialize] loop (i : USize) (b : β) : m β := do
-    if i < sz then
-      let a := as.uget i lcProof
-      match (← f a b) with
-      | ForInStep.done  b => pure b
-      | ForInStep.yield b => loop (i+1) b
-    else
-      pure b
-  loop 0 b
-
-/-- Reference implementation for `forIn` -/
-@[implemented_by Array.forInUnsafe]
-protected def forIn {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (as : Array α) (b : β) (f : α → β → m (ForInStep β)) : m β :=
-  let rec loop (i : Nat) (h : i ≤ as.size) (b : β) : m β := do
-    match i, h with
-    | 0,   _ => pure b
-    | i+1, h =>
-      have h' : i < as.size            := Nat.lt_of_lt_of_le (Nat.lt_succ_self i) h
-      have : as.size - 1 < as.size     := Nat.sub_lt (Nat.zero_lt_of_lt h') (by decide)
-      have : as.size - 1 - i < as.size := Nat.lt_of_le_of_lt (Nat.sub_le (as.size - 1) i) this
-      match (← f as[as.size - 1 - i] b) with
-      | ForInStep.done b  => pure b
-      | ForInStep.yield b => loop i (Nat.le_of_lt h') b
-  loop as.size (Nat.le_refl _) b
-
-instance : ForIn m (Array α) α where
-  forIn := Array.forIn
-
-/-- See comment at `forInUnsafe` -/
 @[inline] unsafe def forIn'Unsafe {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (as : Array α) (b : β) (f : (a : α) → a ∈ as → β → m (ForInStep β)) : m β :=
   let sz := as.usize
   let rec @[specialize] loop (i : USize) (b : β) : m β := do
@@ -363,7 +332,9 @@ protected def forIn' {α : Type u} {β : Type v} {m : Type v → Type w} [Monad
 instance : ForIn' m (Array α) α inferInstance where
   forIn' := Array.forIn'
 
-/-- See comment at `forInUnsafe` -/
+-- No separate `ForIn` instance is required because it can be derived from `ForIn'`.
+
+/-- See comment at `forIn'Unsafe` -/
 @[inline]
 unsafe def foldlMUnsafe {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (f : β → α → m β) (init : β) (as : Array α) (start := 0) (stop := as.size) : m β :=
   let rec @[specialize] fold (i : USize) (stop : USize) (b : β) : m β := do
@@ -398,7 +369,7 @@ def foldlM {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (f : β
   else
     fold as.size (Nat.le_refl _)
 
-/-- See comment at `forInUnsafe` -/
+/-- See comment at `forIn'Unsafe` -/
 @[inline]
 unsafe def foldrMUnsafe {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (f : α → β → m β) (init : β) (as : Array α) (start := as.size) (stop := 0) : m β :=
   let rec @[specialize] fold (i : USize) (stop : USize) (b : β) : m β := do
@@ -437,7 +408,7 @@ def foldrM {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (f : α
   else
     pure init
 
-/-- See comment at `forInUnsafe` -/
+/-- See comment at `forIn'Unsafe` -/
 @[inline]
 unsafe def mapMUnsafe {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (f : α → m β) (as : Array α) : m (Array β) :=
   let sz := as.usize
diff --git a/src/Init/Data/Array/Lemmas.lean b/src/Init/Data/Array/Lemmas.lean
index 474491cb3b..c173fd0afb 100644
--- a/src/Init/Data/Array/Lemmas.lean
+++ b/src/Init/Data/Array/Lemmas.lean
@@ -104,25 +104,6 @@ We prefer to pull `List.toArray` outwards.
 @[simp] theorem back_toArray [Inhabited α] (l : List α) : l.toArray.back = l.getLast! := by
   simp only [back, size_toArray, Array.get!_eq_getElem!, getElem!_toArray, getLast!_eq_getElem!]
 
-@[simp] theorem forIn_loop_toArray [Monad m] (l : List α) (f : α → β → m (ForInStep β)) (i : Nat)
-    (h : i ≤ l.length) (b : β) :
-    Array.forIn.loop l.toArray f i h b = (l.drop (l.length - i)).forIn b f := by
-  induction i generalizing l b with
-  | zero => simp [Array.forIn.loop]
-  | succ i ih =>
-    simp only [Array.forIn.loop, size_toArray, getElem_toArray, ih, forIn_eq_forIn]
-    rw [Nat.sub_add_eq, List.drop_sub_one (by omega), List.getElem?_eq_getElem (by omega)]
-    simp only [Option.toList_some, singleton_append, forIn_cons]
-    have t : l.length - 1 - i = l.length - i - 1 := by omega
-    simp only [t]
-    congr
-
-@[simp] theorem forIn_toArray [Monad m] (l : List α) (b : β) (f : α → β → m (ForInStep β)) :
-    forIn l.toArray b f = forIn l b f := by
-  change l.toArray.forIn b f = l.forIn b f
-  rw [Array.forIn, forIn_loop_toArray]
-  simp
-
 @[simp] theorem forIn'_loop_toArray [Monad m] (l : List α) (f : (a : α) → a ∈ l.toArray → β → m (ForInStep β)) (i : Nat)
     (h : i ≤ l.length) (b : β) :
     Array.forIn'.loop l.toArray f i h b =
@@ -131,7 +112,7 @@ We prefer to pull `List.toArray` outwards.
   | zero =>
     simp [Array.forIn'.loop]
   | succ i ih =>
-    simp only [Array.forIn'.loop, size_toArray, getElem_toArray, ih, forIn_eq_forIn]
+    simp only [Array.forIn'.loop, size_toArray, getElem_toArray, ih]
     have t : drop (l.length - (i + 1)) l = l[l.length - i - 1] :: drop (l.length - i) l := by
       simp only [Nat.sub_add_eq]
       rw [List.drop_sub_one (by omega), List.getElem?_eq_getElem (by omega)]
@@ -145,7 +126,11 @@ We prefer to pull `List.toArray` outwards.
     forIn' l.toArray b f = forIn' l b (fun a m b => f a (mem_toArray.mpr m) b) := by
   change Array.forIn' _ _ _ = List.forIn' _ _ _
   rw [Array.forIn', forIn'_loop_toArray]
-  simp [List.forIn_eq_forIn]
+  simp
+
+@[simp] theorem forIn_toArray [Monad m] (l : List α) (b : β) (f : α → β → m (ForInStep β)) :
+    forIn l.toArray b f = forIn l b f := by
+  simpa using forIn'_toArray l b fun a m b => f a b
 
 theorem foldrM_toArray [Monad m] (f : α → β → m β) (init : β) (l : List α) :
     l.toArray.foldrM f init = l.foldrM f init := by
diff --git a/src/Init/Data/List/Control.lean b/src/Init/Data/List/Control.lean
index 2f945829ac..dd17b84167 100644
--- a/src/Init/Data/List/Control.lean
+++ b/src/Init/Data/List/Control.lean
@@ -215,27 +215,6 @@ def findSomeM? {m : Type u → Type v} [Monad m] {α : Type w} {β : Type u} (f
     | some b => pure (some b)
     | none   => findSomeM? f as
 
-@[inline] protected def forIn {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (as : List α) (init : β) (f : α → β → m (ForInStep β)) : m β :=
-  let rec @[specialize] loop
-    | [], b    => pure b
-    | a::as, b => do
-      match (← f a b) with
-      | ForInStep.done b  => pure b
-      | ForInStep.yield b => loop as b
-  loop as init
-
-instance : ForIn m (List α) α where
-  forIn := List.forIn
-
-@[simp] theorem forIn_eq_forIn [Monad m] : @List.forIn α β m _ = forIn := rfl
-
-@[simp] theorem forIn_nil [Monad m] (f : α → β → m (ForInStep β)) (b : β) : forIn [] b f = pure b :=
-  rfl
-
-@[simp] theorem forIn_cons [Monad m] (f : α → β → m (ForInStep β)) (a : α) (as : List α) (b : β)
-    : forIn (a::as) b f = f a b >>= fun | ForInStep.done b => pure b | ForInStep.yield b => forIn as b f :=
-  rfl
-
 @[inline] protected def forIn' {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (as : List α) (init : β) (f : (a : α) → a ∈ as → β → m (ForInStep β)) : m β :=
   let rec @[specialize] loop : (as' : List α) → (b : β) → Exists (fun bs => bs ++ as' = as) → m β
     | [], b, _    => pure b
@@ -254,16 +233,15 @@ instance : ForIn m (List α) α where
 instance : ForIn' m (List α) α inferInstance where
   forIn' := List.forIn'
 
+-- No separate `ForIn` instance is required because it can be derived from `ForIn'`.
+
 @[simp] theorem forIn'_eq_forIn' [Monad m] : @List.forIn' α β m _ = forIn' := rfl
 
-@[simp] theorem forIn'_eq_forIn {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (as : List α) (init : β) (f : α → β → m (ForInStep β)) : forIn' as init (fun a _ b => f a b) = forIn as init f := by
-  simp [forIn', forIn, List.forIn, List.forIn']
-  have : ∀ cs h, List.forIn'.loop cs (fun a _ b => f a b) as init h = List.forIn.loop f as init := by
-    intro cs h
-    induction as generalizing cs init with
-    | nil => intros; rfl
-    | cons a as ih => intros; simp [List.forIn.loop, List.forIn'.loop, ih]
-  apply this
+@[simp] theorem forIn'_nil [Monad m] (f : (a : α) → a ∈ [] → β → m (ForInStep β)) (b : β) : forIn' [] b f = pure b :=
+  rfl
+
+@[simp] theorem forIn_nil [Monad m] (f : α → β → m (ForInStep β)) (b : β) : forIn [] b f = pure b :=
+  rfl
 
 instance : ForM m (List α) α where
   forM := List.forM
diff --git a/src/Init/Data/List/Monadic.lean b/src/Init/Data/List/Monadic.lean
index f8ded7d6f2..5836d3d8ff 100644
--- a/src/Init/Data/List/Monadic.lean
+++ b/src/Init/Data/List/Monadic.lean
@@ -89,9 +89,6 @@ theorem mapM_eq_reverse_foldlM_cons [Monad m] [LawfulMonad m] (f : α → m β)
 
 /-! ### forIn' -/
 
-@[simp] theorem forIn'_nil [Monad m] (f : (a : α) → a ∈ [] → β → m (ForInStep β)) (b : β) : forIn' [] b f = pure b :=
-  rfl
-
 theorem forIn'_loop_congr [Monad m] {as bs : List α}
     {f : (a' : α) → a' ∈ as → β → m (ForInStep β)}
     {g : (a' : α) → a' ∈ bs → β → m (ForInStep β)}
@@ -122,6 +119,11 @@ theorem forIn'_loop_congr [Monad m] {as bs : List α}
     intros
     rfl
 
+@[simp] theorem forIn_cons [Monad m] (f : α → β → m (ForInStep β)) (a : α) (as : List α) (b : β) :
+    forIn (a::as) b f = f a b >>= fun | ForInStep.done b => pure b | ForInStep.yield b => forIn as b f := by
+  have := forIn'_cons (a := a) (as := as) (fun a' _ b => f a' b) b
+  simpa only [forIn'_eq_forIn]
+
 @[congr] theorem forIn'_congr [Monad m] {as bs : List α} (w : as = bs)
     {b b' : β} (hb : b = b')
     {f : (a' : α) → a' ∈ as → β → m (ForInStep β)}
diff --git a/src/Init/Data/Option/Instances.lean b/src/Init/Data/Option/Instances.lean
index cfce9a4038..580a7fb3ad 100644
--- a/src/Init/Data/Option/Instances.lean
+++ b/src/Init/Data/Option/Instances.lean
@@ -86,4 +86,6 @@ instance : ForIn' m (Option α) α inferInstance where
       match ← f a rfl init with
       | .done r | .yield r => return r
 
+-- No separate `ForIn` instance is required because it can be derived from `ForIn'`.
+
 end Option
diff --git a/src/Init/Data/Range.lean b/src/Init/Data/Range.lean
index 0de8c92228..55451cf1de 100644
--- a/src/Init/Data/Range.lean
+++ b/src/Init/Data/Range.lean
@@ -20,21 +20,6 @@ instance : Membership Nat Range where
 namespace Range
 universe u v
 
-@[inline] protected def forIn {β : Type u} {m : Type u → Type v} [Monad m] (range : Range) (init : β) (f : Nat → β → m (ForInStep β)) : m β :=
-  -- pass `stop` and `step` separately so the `range` object can be eliminated through inlining
-  let rec @[specialize] loop (fuel i stop step : Nat) (b : β) : m β := do
-    if i ≥ stop then
-      return b
-    else match fuel with
-     | 0   => pure b
-     | fuel+1 => match (← f i b) with
-        | ForInStep.done b  => pure b
-        | ForInStep.yield b => loop fuel (i + step) stop step b
-  loop range.stop range.start range.stop range.step init
-
-instance : ForIn m Range Nat where
-  forIn := Range.forIn
-
 @[inline] protected def forIn' {β : Type u} {m : Type u → Type v} [Monad m] (range : Range) (init : β) (f : (i : Nat) → i ∈ range → β → m (ForInStep β)) : m β :=
   let rec @[specialize] loop (start stop step : Nat) (f : (i : Nat) → start ≤ i ∧ i < stop → β → m (ForInStep β)) (fuel i : Nat) (hl : start ≤ i) (b : β) : m β := do
     if hu : i < stop then
@@ -50,6 +35,8 @@ instance : ForIn m Range Nat where
 instance : ForIn' m Range Nat inferInstance where
   forIn' := Range.forIn'
 
+-- No separate `ForIn` instance is required because it can be derived from `ForIn'`.
+
 @[inline] protected def forM {m : Type u → Type v} [Monad m] (range : Range) (f : Nat → m PUnit) : m PUnit :=
   let rec @[specialize] loop (fuel i stop step : Nat) : m PUnit := do
     if i ≥ stop then
diff --git a/src/Lean/Data/KVMap.lean b/src/Lean/Data/KVMap.lean
index 60db90bbb7..f56a30e5fd 100644
--- a/src/Lean/Data/KVMap.lean
+++ b/src/Lean/Data/KVMap.lean
@@ -177,7 +177,7 @@ def updateSyntax (m : KVMap) (k : Name) (f : Syntax → Syntax) : KVMap :=
 
 @[inline] protected def forIn.{w, w'} {δ : Type w} {m : Type w → Type w'} [Monad m]
   (kv : KVMap) (init : δ) (f : Name × DataValue → δ → m (ForInStep δ)) : m δ :=
-  kv.entries.forIn init f
+  forIn kv.entries init f
 
 instance : ForIn m KVMap (Name × DataValue) where
   forIn := KVMap.forIn
diff --git a/src/Std/Data/DHashMap/Raw.lean b/src/Std/Data/DHashMap/Raw.lean
index 1c01c2a4cd..08bbc72035 100644
--- a/src/Std/Data/DHashMap/Raw.lean
+++ b/src/Std/Data/DHashMap/Raw.lean
@@ -334,7 +334,7 @@ map in some order.
 
 /-- Support for the `for` loop construct in `do` blocks. -/
 @[inline] def forIn (f : (a : α) → β a → δ → m (ForInStep δ)) (init : δ) (b : Raw α β) : m δ :=
-  b.buckets.forIn init (fun bucket acc => bucket.forInStep acc f)
+  ForIn.forIn b.buckets init (fun bucket acc => bucket.forInStep acc f)
 
 instance : ForM m (Raw α β) ((a : α) × β a) where
   forM m f := m.forM (fun a b => f ⟨a, b⟩)
diff --git a/src/lake/Lake/Util/OrdHashSet.lean b/src/lake/Lake/Util/OrdHashSet.lean
index fea3d04a59..003af356e1 100644
--- a/src/lake/Lake/Util/OrdHashSet.lean
+++ b/src/lake/Lake/Util/OrdHashSet.lean
@@ -69,6 +69,6 @@ def ofArray (arr : Array α) : OrdHashSet α :=
   self.toArray.forM f
 
 @[inline] protected def forIn [Monad m] (self : OrdHashSet α) (init : β) (f : α → β → m (ForInStep β)) : m β :=
-  self.toArray.forIn init f
+  ForIn.forIn self.toArray init f
 
 instance : ForIn m (OrdHashSet α) α := ⟨OrdHashSet.forIn⟩
diff --git a/src/lake/Lake/Util/RBArray.lean b/src/lake/Lake/Util/RBArray.lean
index a0a18640dd..3794801f6c 100644
--- a/src/lake/Lake/Util/RBArray.lean
+++ b/src/lake/Lake/Util/RBArray.lean
@@ -63,7 +63,7 @@ def insert (self : RBArray α β cmp) (a : α) (b : β) : RBArray α β cmp :=
   self.toArray.forM f
 
 @[inline] protected def forIn [Monad m] (self : RBArray α β cmp) (init : σ) (f : β → σ → m (ForInStep σ)) : m σ :=
-  self.toArray.forIn init f
+  ForIn.forIn self.toArray init f
 
 instance : ForIn m (RBArray α β cmp) β := ⟨RBArray.forIn⟩
 
diff --git a/tests/bench/liasolver.lean b/tests/bench/liasolver.lean
index 8a5633b8e2..adcfab670f 100644
--- a/tests/bench/liasolver.lean
+++ b/tests/bench/liasolver.lean
@@ -53,7 +53,7 @@ namespace Lean.HashMap
 
   @[inline] protected def forIn {δ : Type w} {m : Type w → Type w'} [Monad m]
     (as : HashMap α β) (init : δ) (f : (α × β) → δ → m (ForInStep δ)) : m δ := do
-    as.val.buckets.val.forIn init fun bucket acc => do
+    forIn as.val.buckets.val init fun bucket acc => do
       let (done, v) ← bucket.forIn (false, acc) fun v (_, acc) => do
         let r ← f v acc
         match r with
diff --git a/tests/lean/run/array_simp.lean b/tests/lean/run/array_simp.lean
index d0ed4dd48d..4e6ff093b1 100644
--- a/tests/lean/run/array_simp.lean
+++ b/tests/lean/run/array_simp.lean
@@ -10,7 +10,7 @@ attribute [local simp] Id.run in
 #check_simp
   (Id.run do
     let mut s := 0
-    for i in [1,2,3,4].toArray do
+    for i in #[1,2,3,4] do
       s := s + i
     pure s) ~> 10
 
@@ -18,6 +18,6 @@ attribute [local simp] Id.run in
 #check_simp
   (Id.run do
     let mut s := 0
-    for h : i in [1,2,3,4].toArray do
+    for h : i in #[1,2,3,4] do
       s := s + i
     pure s) ~> 10
diff --git a/tests/lean/run/list_simp.lean b/tests/lean/run/list_simp.lean
index 6fd985face..ed4f89c716 100644
--- a/tests/lean/run/list_simp.lean
+++ b/tests/lean/run/list_simp.lean
@@ -461,6 +461,21 @@ end Pairwise
 /-! ### max? -/
 
 /-! ## Monadic operations -/
+attribute [local simp] Id.run in
+#check_simp
+  (Id.run do
+    let mut s := 0
+    for i in [1,2,3,4] do
+      s := s + i
+    pure s) ~> 10
+
+attribute [local simp] Id.run in
+#check_simp
+  (Id.run do
+    let mut s := 0
+    for h : i in [1,2,3,4] do
+      s := s + i
+    pure s) ~> 10
 
 /-! ### mapM -/
 
diff --git a/tests/lean/run/treeNode.lean b/tests/lean/run/treeNode.lean
index 961386d87b..7d333da8f6 100644
--- a/tests/lean/run/treeNode.lean
+++ b/tests/lean/run/treeNode.lean
@@ -13,13 +13,13 @@ def treeToList (t : TreeNode) : List String :=
      r := r ++ treeToList child
    return r
 
-@[simp] theorem treeToList_eq (name : String) (children : List TreeNode) : treeToList (.mkNode name children) =  name :: List.flatten (children.map treeToList) := by
-  simp only [treeToList, Id.run, Id.pure_eq, Id.bind_eq, List.forIn'_eq_forIn, forIn, List.forIn]
-  have : ∀ acc, (Id.run do List.forIn.loop (fun a b => ForInStep.yield (b ++ treeToList a)) children acc) = acc ++ List.flatten (List.map treeToList children) := by
-    intro acc
-    induction children generalizing acc with simp [List.forIn.loop, List.map, List.flatten, Id.run]
-    | cons c cs ih => simp [Id.run] at ih; simp [ih, List.append_assoc]
-  apply this
+@[simp] theorem treeToList_eq (name : String) (children : List TreeNode) : treeToList (.mkNode name children) = name :: List.flatten (children.map treeToList) := by
+  simp [treeToList, Id.run]
+  conv => rhs; rw [← List.singleton_append]
+  generalize [name] = as
+  induction children generalizing as with
+  | nil => simp
+  | cons c cs ih => simp [ih, List.append_assoc]
 
 mutual
   def numNames : TreeNode → Nat