diff --git a/theories/heap_lang/lib/array.v b/theories/heap_lang/lib/array.v
index e46424e4aff56fa17326a55ea4485c39a3d6c2d0..f5c894e92cbde6e242b5542c6cc14425c13a4815 100644
--- a/theories/heap_lang/lib/array.v
+++ b/theories/heap_lang/lib/array.v
@@ -175,6 +175,57 @@ Section proof.
       by rewrite app_assoc.
   Qed.
 
+  Local Lemma twp_array_init_loop {A : Type} (g : A → val) (Q : nat → A → iProp Σ)
+          (xs : list A) i n l (f : val) stk E :
+    0 < n →
+    length xs = i →
+    i ≤ n →
+    ([∗ list] k↦x∈xs, Q k x) -∗
+    ([∗ list] j ∈ seq i (n-i), WP f #(j : nat) @ stk; E [{ v, ∃ x : A, ⌜v = g x⌝ ∗ Q j x }]) -∗
+    l ↦∗ ((g <$> xs) ++ replicate (n - i) #()) -∗
+    WP array_init_loop #l #i #n f @ stk; E [{ _, ∃ ys,
+       l ↦∗ (g <$> (xs ++ ys)) ∗ ⌜length (xs++ys) = n⌝ ∗ ([∗ list] k↦x∈(xs++ys), Q k x) }].
+  Proof.
+    iIntros (Hn Hxs Hi) "Hxs Hf Hl". iRevert (Hxs Hi).
+    remember (n - i) as k. iRevert (Heqk).
+    iInduction k as [|k] "IH" forall (xs i); iIntros (Heqk Hxs Hi).
+    - wp_rec. wp_pures. case_bool_decide; simplify_eq/=; wp_pures.
+      + iExists []. iFrame.
+        rewrite !app_nil_r. eauto with iFrame.
+      + assert (length xs ≠ n) by congruence. lia.
+    - wp_rec. wp_pures. case_bool_decide; simplify_eq/=; wp_pures.
+      + exfalso. lia.
+      + wp_bind (f #(length xs)).
+        iSimpl in "Hf". iDestruct "Hf" as "[H Hf]".
+        iApply (twp_wand with "H").
+        iIntros (v). iDestruct 1 as (x) "[-> Hx]".
+        wp_apply (twp_store_offset with "Hl").
+        { apply lookup_lt_is_Some_2.
+          rewrite app_length /=.
+          assert (S n - length xs > 0) by lia.
+        rewrite fmap_length replicate_length. lia. }
+        iIntros "Hl". wp_pures.
+        assert ((Z.of_nat (length xs) + 1)%Z = Z.of_nat (length xs + 1)) as -> by lia.
+
+        iSpecialize ("IH" $! (xs++[x]) (length xs+1) with "[Hx Hxs] [Hf] [Hl] [%] [%] [%]").
+        { rewrite big_sepL_app /= Nat.add_0_r. by iFrame. }
+        { by rewrite Nat.add_1_r. }
+        { assert (length xs = length xs + 0) as Hlen1 by lia.
+          rewrite {1}Hlen1.
+        rewrite -{1}(fmap_length g xs).
+        rewrite insert_app_r fmap_app /=.
+        rewrite app_assoc_reverse /= //. }
+        { lia. }
+        { by rewrite app_length. }
+        { lia. }
+        iApply (twp_wand with "IH").
+        iIntros (_). iDestruct 1 as (ys) "(Hys & Hlen & HQs)".
+        iDestruct "Hlen" as %Hlen.
+        rewrite -app_assoc.
+        iExists ([x] ++ ys). iFrame. iPureIntro.
+        by rewrite app_assoc.
+  Qed.
+
   Theorem wp_array_init {A : Type} (g : A → val) (Q : nat → A → iProp Σ)
           n (f : val) stk E :
     (0 < n)%Z →
@@ -195,6 +246,26 @@ Section proof.
     wp_pures. iApply "HΦ".
     iFrame "Hl HQs". iPureIntro. lia.
   Qed.
+  Theorem twp_array_init {A : Type} (g : A → val) (Q : nat → A → iProp Σ)
+          n (f : val) stk E :
+    (0 < n)%Z →
+    [[{ [∗ list] i ∈ seq 0 (Z.to_nat n), WP f #(i : nat) @ stk; E [{ v, ∃ x : A, ⌜v = g x⌝ ∗ Q i x }] }]]
+      array_init #n f @ stk; E
+    [[{ l xs, RET #l; l ↦∗ (g <$> xs) ∗ ⌜Z.of_nat (length xs) = n⌝ ∗ ([∗ list] k↦x∈xs, Q k x) }]].
+  Proof.
+    intros Hn. iIntros (Φ) "Hf HΦ".
+    wp_rec. wp_pures. wp_alloc l as "Hl"; first done.
+    wp_pures.
+    iPoseProof (twp_array_init_loop g Q [] 0 (Z.to_nat n) with "[//] [Hf] [Hl]") as "H"; try by (simpl; lia).
+    { simpl. assert (Z.to_nat n - 0 = Z.to_nat n) as -> by lia. done. }
+    { simpl. assert (Z.to_nat n - 0 = Z.to_nat n) as -> by lia. done. }
+    assert (Z.of_nat 0%nat = 0%Z) as -> by lia.
+    assert (Z.of_nat (Z.to_nat n) = n) as -> by lia.
+    wp_apply (twp_wand with "H").
+    iIntros (?). iDestruct 1 as (vs) "(Hl & % & HQs)".
+    wp_pures. iApply "HΦ".
+    iFrame "Hl HQs". iPureIntro. lia.
+  Qed.
 
   (* Version of [wp_array_init] with the auxiliary type [A] set to
   [val], and with the persistent assumption on the function [f]. *)