diff --git a/src/include/Array.hip.hpp b/src/include/Array.hip.hpp
index 33cc547a97..5e00d32a09 100644
--- a/src/include/Array.hip.hpp
+++ b/src/include/Array.hip.hpp
@@ -34,14 +34,6 @@ struct Array
 
     __host__ __device__ TData& operator()(index_t i) { return mData[i]; }
 
-    template <index_t I>
-    __host__ __device__ constexpr TData Get(Number<I>) const
-    {
-        static_assert(I < NSize, "wrong!");
-
-        return mData[I];
-    }
-
     template <index_t I>
     __host__ __device__ constexpr void Set(Number<I>, TData x)
     {
@@ -50,16 +42,33 @@ struct Array
         mData[I] = x;
     }
 
+    __host__ __device__ constexpr void Set(index_t I, TData x) { mData[I] = x; }
+
+    struct lambda_PushBack // emulate constexpr lambda
+    {
+        const Array<TData, NSize>& old_array;
+        Array<TData, NSize + 1>& new_array;
+
+        __host__ __device__ constexpr lambda_PushBack(const Array<TData, NSize>& old_array_,
+                                                      Array<TData, NSize + 1>& new_array_)
+            : old_array(old_array_), new_array(new_array_)
+        {
+        }
+
+        template <index_t I>
+        __host__ __device__ constexpr void operator()(Number<I>) const
+        {
+            new_array.Set(Number<I>{}, old_array[I]);
+        }
+    };
+
     __host__ __device__ constexpr auto PushBack(TData x) const
     {
         Array<TData, NSize + 1> new_array;
 
-        static_for<0, NSize, 1>{}([&](auto I) {
-            constexpr index_t i = I.Get();
-            new_array(i)        = mData[i];
-        });
+        static_for<0, NSize, 1>{}(lambda_PushBack(*this, new_array));
 
-        new_array(NSize) = x;
+        new_array.Set(Number<NSize>{}, x);
 
         return new_array;
     }
@@ -81,18 +90,13 @@ __host__ __device__ constexpr auto make_zero_array()
 
 template <class TData, index_t NSize, index_t... IRs>
 __host__ __device__ constexpr auto reorder_array_given_new2old(const Array<TData, NSize>& old_array,
-                                                               Sequence<IRs...> new2old)
+                                                               Sequence<IRs...> /*new2old*/)
 {
-    Array<TData, NSize> new_array;
-
     static_assert(NSize == sizeof...(IRs), "NSize not consistent");
 
-    static_for<0, NSize, 1>{}([&](auto IDim) {
-        constexpr index_t idim = IDim.Get();
-        new_array[idim]        = old_array[new2old.Get(IDim)];
-    });
+    static_assert(is_valid_sequence_map<Sequence<IRs...>>::value, "wrong! invalid reorder map");
 
-    return new_array;
+    return Array<TData, NSize>{old_array.mSize[IRs]...};
 }
 
 template <class TData, index_t NSize, class MapOld2New>
@@ -120,12 +124,14 @@ struct lambda_reorder_array_given_old2new
 
 template <class TData, index_t NSize, index_t... IRs>
 __host__ __device__ constexpr auto reorder_array_given_old2new(const Array<TData, NSize>& old_array,
-                                                               Sequence<IRs...> old2new)
+                                                               Sequence<IRs...> /*old2new*/)
 {
     Array<TData, NSize> new_array;
 
     static_assert(NSize == sizeof...(IRs), "NSize not consistent");
 
+    static_assert(is_valid_sequence_map<Sequence<IRs...>>::value, "wrong! invalid reorder map");
+
     static_for<0, NSize, 1>{}(
         lambda_reorder_array_given_old2new<TData, NSize, Sequence<IRs...>>(old_array, new_array));
 
@@ -141,25 +147,44 @@ __host__ __device__ constexpr auto extract_array(const Array<TData, NSize>& old_
 
     static_assert(new_size <= NSize, "wrong! too many extract");
 
-    static_for<0, new_size, 1>{}([&](auto I) {
-        constexpr index_t i = I.Get();
-        new_array(i)        = old_array[ExtractSeq::Get(I)];
-    });
+    static_for<0, new_size, 1>{}([&](auto I) { new_array(I) = old_array[ExtractSeq::Get(I)]; });
 
     return new_array;
 }
 
+template <class F, class X, class Y, class Z> // emulate constepxr lambda for array math
+struct lambda_array_math
+{
+    const F& f;
+    const X& x;
+    const Y& y;
+    Z& z;
+
+    __host__ __device__ constexpr lambda_array_math(const F& f_, const X& x_, const Y& y_, Z& z_)
+        : f(f_), x(x_), y(y_), z(z_)
+    {
+    }
+
+    template <index_t IDim_>
+    __host__ __device__ constexpr void operator()(Number<IDim_>) const
+    {
+        constexpr auto IDim = Number<IDim_>{};
+
+        z.Set(IDim, f(x[IDim], y[IDim]));
+    }
+};
+
 // Array = Array + Array
 template <class TData, index_t NSize>
 __host__ __device__ constexpr auto operator+(Array<TData, NSize> a, Array<TData, NSize> b)
 {
     Array<TData, NSize> result;
 
-    static_for<0, NSize, 1>{}([&](auto I) {
-        constexpr index_t i = I.Get();
+    auto f = mod_conv::plus<index_t>{};
 
-        result(i) = a[i] + b[i];
-    });
+    static_for<0, NSize, 1>{}(
+        lambda_array_math<decltype(f), decltype(a), decltype(b), decltype(result)>(
+            f, a, b, result));
 
     return result;
 }
@@ -170,11 +195,11 @@ __host__ __device__ constexpr auto operator-(Array<TData, NSize> a, Array<TData,
 {
     Array<TData, NSize> result;
 
-    static_for<0, NSize, 1>{}([&](auto I) {
-        constexpr index_t i = I.Get();
+    auto f = mod_conv::minus<index_t>{};
 
-        result(i) = a[i] - b[i];
-    });
+    static_for<0, NSize, 1>{}(
+        lambda_array_math<decltype(f), decltype(a), decltype(b), decltype(result)>(
+            f, a, b, result));
 
     return result;
 }
@@ -187,11 +212,11 @@ __host__ __device__ constexpr auto operator+(Array<TData, NSize> a, Sequence<Is.
 
     Array<TData, NSize> result;
 
-    static_for<0, NSize, 1>{}([&](auto I) {
-        constexpr index_t i = I.Get();
+    auto f = mod_conv::plus<index_t>{};
 
-        result(i) = a[i] + b.Get(I);
-    });
+    static_for<0, NSize, 1>{}(
+        lambda_array_math<decltype(f), decltype(a), decltype(b), decltype(result)>(
+            f, a, b, result));
 
     return result;
 }
@@ -204,11 +229,11 @@ __host__ __device__ constexpr auto operator-(Array<TData, NSize> a, Sequence<Is.
 
     Array<TData, NSize> result;
 
-    static_for<0, NSize, 1>{}([&](auto I) {
-        constexpr index_t i = I.Get();
+    auto f = mod_conv::minus<index_t>{};
 
-        result(i) = a[i] - b.Get(I);
-    });
+    static_for<0, NSize, 1>{}(
+        lambda_array_math<decltype(f), decltype(a), decltype(b), decltype(result)>(
+            f, a, b, result));
 
     return result;
 }
@@ -221,11 +246,11 @@ __host__ __device__ constexpr auto operator*(Array<TData, NSize> a, Sequence<Is.
 
     Array<TData, NSize> result;
 
-    static_for<0, NSize, 1>{}([&](auto I) {
-        constexpr index_t i = I.Get();
+    auto f = mod_conv::multiplies<index_t>{};
 
-        result(i) = a[i] * b.Get(I);
-    });
+    static_for<0, NSize, 1>{}(
+        lambda_array_math<decltype(f), decltype(a), decltype(b), decltype(result)>(
+            f, a, b, result));
 
     return result;
 }
@@ -238,11 +263,11 @@ __host__ __device__ constexpr auto operator-(Sequence<Is...> a, Array<TData, NSi
 
     Array<TData, NSize> result;
 
-    static_for<0, NSize, 1>{}([&](auto I) {
-        constexpr index_t i = I.Get();
+    auto f = mod_conv::minus<index_t>{};
 
-        result(i) = a.Get(I) - b[i];
-    });
+    static_for<0, NSize, 1>{}(
+        lambda_array_math<decltype(f), decltype(a), decltype(b), decltype(result)>(
+            f, a, b, result));
 
     return result;
 }
@@ -255,10 +280,7 @@ accumulate_on_array(const Array<TData, NSize>& a, Reduce f, TData init)
 
     static_assert(NSize > 0, "wrong");
 
-    static_for<0, NSize, 1>{}([&](auto I) {
-        constexpr index_t i = I.Get();
-        result              = f(result, a[i]);
-    });
+    static_for<0, NSize, 1>{}([&](auto I) { result = f(result, a[I]); });
 
     return result;
 }
diff --git a/src/include/ConstantTensorDescriptor.hip.hpp b/src/include/ConstantTensorDescriptor.hip.hpp
index 845e6e0190..f28cb32733 100644
--- a/src/include/ConstantTensorDescriptor.hip.hpp
+++ b/src/include/ConstantTensorDescriptor.hip.hpp
@@ -48,13 +48,13 @@ struct ConstantTensorDescriptor
     template <index_t I>
     __host__ __device__ static constexpr index_t GetLength(Number<I>)
     {
-        return Lengths{}.Get(Number<I>{});
+        return Lengths::Get(Number<I>{});
     }
 
     template <index_t I>
     __host__ __device__ static constexpr index_t GetStride(Number<I>)
     {
-        return Strides{}.Get(Number<I>{});
+        return Strides::Get(Number<I>{});
     }
 
     struct lambda_AreDimensionsContinuous
@@ -131,7 +131,7 @@ struct ConstantTensorDescriptor
         template <class X>
         __host__ __device__ constexpr void operator()(X IDim) const
         {
-            offset += multi_id.Get(IDim) * Type::GetStride(IDim);
+            offset += multi_id[IDim] * Type::GetStride(IDim);
         }
     };
 
diff --git a/src/include/Sequence.hip.hpp b/src/include/Sequence.hip.hpp
index fdb3634363..a29506f215 100644
--- a/src/include/Sequence.hip.hpp
+++ b/src/include/Sequence.hip.hpp
@@ -2,6 +2,9 @@
 #include "integral_constant.hip.hpp"
 #include "functional.hip.hpp"
 
+template <class Seq>
+struct is_valid_sequence_map;
+
 template <index_t... Is>
 struct Sequence
 {
@@ -40,27 +43,24 @@ struct Sequence
     template <index_t... IRs>
     __host__ __device__ static constexpr auto ReorderGivenNew2Old(Sequence<IRs...> /*new2old*/)
     {
-#if 0 // require sequence_sort, which is not implemented yet
-        static_assert(is_same<sequence_sort<Sequence<IRs...>>::SortedSeqType,
-                              arithmetic_sequence_gen<0, mSize, 1>::SeqType>::value,
-                      "wrong! invalid new2old map");
-#endif
-
         static_assert(sizeof...(Is) == sizeof...(IRs),
-                      "wrong! new2old map should have the same size as Sequence to be rerodered");
+                      "wrong! reorder map should have the same size as Sequence to be rerodered");
 
-        return Sequence<Type{}.Get(Number<IRs>{})...>{};
+        static_assert(is_valid_sequence_map<Sequence<IRs...>>::value, "wrong! invalid reorder map");
+
+        return Sequence<Type::Get(Number<IRs>{})...>{};
     }
 
 #if 0 // require sequence_sort, which is not implemented yet
     template <class MapOld2New>
     __host__ __device__ static constexpr auto ReorderGivenOld2New(MapOld2New /*old2new*/)
     {
-#if 0
-        static_assert(is_same<sequence_sort<MapOld2New>::SortedSeqType,
-                              arithmetic_sequence_gen<0, mSize, 1>::SeqType>::value,
-                      "wrong! invalid old2new map");
-#endif
+        static_assert(sizeof...(Is) == MapOld2New::GetSize(),
+                      "wrong! reorder map should have the same size as Sequence to be rerodered");
+
+        static_assert(is_valid_sequence_map<MapOld2New>::value, 
+                      "wrong! invalid reorder map");
+
         constexpr auto map_new2old = typename sequence_map_inverse<MapOld2New>::SeqMapType{};
 
         return ReorderGivenNew2Old(map_new2old);
@@ -106,13 +106,13 @@ struct Sequence
     template <index_t... Ns>
     __host__ __device__ static constexpr auto Extract(Number<Ns>...)
     {
-        return Sequence<Type{}.Get(Number<Ns>{})...>{};
+        return Sequence<Type::Get(Number<Ns>{})...>{};
     }
 
     template <index_t... Ns>
     __host__ __device__ static constexpr auto Extract(Sequence<Ns...>)
     {
-        return Sequence<Type{}.Get(Number<Ns>{})...>{};
+        return Sequence<Type::Get(Number<Ns>{})...>{};
     }
 
     template <index_t I, index_t X>
@@ -316,6 +316,7 @@ struct sequence_map_inverse<Sequence<Is...>>
 };
 
 #endif
+
 template <class Seq>
 struct is_valid_sequence_map
 {
diff --git a/src/include/base.hip.hpp b/src/include/base.hip.hpp
new file mode 100644
index 0000000000..dd6fc19b1f
--- /dev/null
+++ b/src/include/base.hip.hpp
@@ -0,0 +1,113 @@
+#pragma once
+
+__device__ index_t get_thread_local_1d_id() { return threadIdx.x; }
+
+__device__ index_t get_block_1d_id() { return blockIdx.x; }
+
+template <class T1, class T2>
+struct is_same
+{
+    static constexpr bool value = false;
+};
+
+template <class T>
+struct is_same<T, T>
+{
+    static constexpr bool value = true;
+};
+
+template <class X, class Y>
+__host__ __device__ constexpr bool is_same_type(X, Y)
+{
+    return is_same<X, Y>::value;
+}
+
+namespace mod_conv { // namespace mod_conv
+template <class T, T s>
+struct scales
+{
+    __host__ __device__ constexpr T operator()(T a) const { return s * a; }
+};
+
+template <class T>
+struct plus
+{
+    __host__ __device__ constexpr T operator()(T a, T b) const { return a + b; }
+};
+
+template <class T>
+struct minus
+{
+    __host__ __device__ constexpr T operator()(T a, T b) const { return a - b; }
+};
+
+template <class T>
+struct multiplies
+{
+    __host__ __device__ constexpr T operator()(T a, T b) const { return a * b; }
+};
+
+template <class T>
+struct integer_divide_ceiler
+{
+    __host__ __device__ constexpr T operator()(T a, T b) const
+    {
+        static_assert(is_same<T, index_t>::value || is_same<T, int>::value, "wrong type");
+
+        return (a + b - 1) / b;
+    }
+};
+
+template <class T>
+__host__ __device__ constexpr T integer_divide_ceil(T a, T b)
+{
+    static_assert(is_same<T, index_t>::value || is_same<T, int>::value, "wrong type");
+
+    return (a + b - 1) / b;
+}
+
+template <class T>
+__host__ __device__ constexpr T max(T x, T y)
+{
+    return x > y ? x : y;
+}
+
+template <class T, class... Ts>
+__host__ __device__ constexpr T max(T x, Ts... xs)
+{
+    static_assert(sizeof...(xs) > 0, "not enough argument");
+
+    auto y = max(xs...);
+
+    static_assert(is_same<decltype(y), T>::value, "not the same type");
+
+    return x > y ? x : y;
+}
+
+template <class T>
+__host__ __device__ constexpr T min(T x, T y)
+{
+    return x < y ? x : y;
+}
+
+template <class T, class... Ts>
+__host__ __device__ constexpr T min(T x, Ts... xs)
+{
+    static_assert(sizeof...(xs) > 0, "not enough argument");
+
+    auto y = min(xs...);
+
+    static_assert(is_same<decltype(y), T>::value, "not the same type");
+
+    return x < y ? x : y;
+}
+
+// this is wrong
+// TODO: implement correct least common multiple, instead of calling max()
+template <class T, class... Ts>
+__host__ __device__ constexpr T lcm(T x, Ts... xs)
+{
+    return max(x, xs...);
+}
+
+} // namespace mod_conv
diff --git a/src/include/blockwise_generic_tensor_slice_op.hip.hpp b/src/include/blockwise_generic_tensor_slice_op.hip.hpp
index 5665fb2726..ef49f7a33e 100644
--- a/src/include/blockwise_generic_tensor_slice_op.hip.hpp
+++ b/src/include/blockwise_generic_tensor_slice_op.hip.hpp
@@ -203,20 +203,18 @@ struct BlockwiseGenericTensorSliceCopy_v1
             make_ConstantTensorDescriptor_packed(thread_sub_tensor_lengths * repeat_lengths);
 
         static_ford<decltype(repeat_lengths)>{}([&](auto repeat_multi_id_) {
-#if 0
+#if 1
             constexpr auto repeat_multi_id = sequence2array(decltype(repeat_multi_id_){});
 
-            const auto src_thread_data_multi_id_begin =
-                repeat_multi_id * data_per_cluster_per_dims; // cannot not constexpr, why?
+            const auto src_thread_data_multi_id_begin = repeat_multi_id * data_per_cluster_per_dims;
 
-            const auto clipboard_data_multi_id_begin =
-                repeat_multi_id * thread_sub_tensor_lengths; // cannot not constexpr, why?
+            const auto clipboard_data_multi_id_begin = repeat_multi_id * thread_sub_tensor_lengths;
 
-            const index_t src_offset = SrcDesc{}.GetOffsetFromMultiIndex(
-                src_thread_data_multi_id_begin); // cannot not constexpr, why?
+            const index_t src_offset =
+                SrcDesc{}.GetOffsetFromMultiIndex(src_thread_data_multi_id_begin);
 
-            const index_t clipboard_offset = thread_tensor_desc.GetOffsetFromMultiIndex(
-                clipboard_data_multi_id_begin); // cannot not constexpr, why?
+            const index_t clipboard_offset =
+                thread_tensor_desc.GetOffsetFromMultiIndex(clipboard_data_multi_id_begin);
 #else
             constexpr auto repeat_multi_id = decltype(repeat_multi_id_){};
 
@@ -258,20 +256,17 @@ struct BlockwiseGenericTensorSliceCopy_v1
             make_ConstantTensorDescriptor_packed(thread_sub_tensor_lengths * repeat_lengths);
 
         static_ford<decltype(repeat_lengths)>{}([&](auto repeat_multi_id_) {
-#if 0
+#if 1
             constexpr auto repeat_multi_id = sequence2array(decltype(repeat_multi_id_){});
 
-            const auto clipboard_data_multi_id_begin =
-                repeat_multi_id * thread_sub_tensor_lengths; // cannot not constexpr, why?
+            const auto clipboard_data_multi_id_begin = repeat_multi_id * thread_sub_tensor_lengths;
 
-            const auto dst_data_multi_id_begin =
-                repeat_multi_id * data_per_cluster_per_dims; // cannot not constexpr, why?
+            const auto dst_data_multi_id_begin = repeat_multi_id * data_per_cluster_per_dims;
 
-            const index_t clipboard_offset = thread_tensor_desc.GetOffsetFromMultiIndex(
-                clipboard_data_multi_id_begin); // cannot not constexpr, why?
+            const index_t clipboard_offset =
+                thread_tensor_desc.GetOffsetFromMultiIndex(clipboard_data_multi_id_begin);
 
-            const index_t dst_offset = DstDesc{}.GetOffsetFromMultiIndex(
-                dst_data_multi_id_begin); // cannot not constexpr, why?
+            const index_t dst_offset = DstDesc{}.GetOffsetFromMultiIndex(dst_data_multi_id_begin);
 #else
             constexpr auto repeat_multi_id = decltype(repeat_multi_id_){};
 
diff --git a/src/include/common.hip.hpp b/src/include/common.hip.hpp
index 28ff7003bf..f59b74d0fd 100644
--- a/src/include/common.hip.hpp
+++ b/src/include/common.hip.hpp
@@ -1,4 +1,5 @@
 #pragma once
+#include "base.hip.hpp"
 #include "vector_type.hip.hpp"
 #include "integral_constant.hip.hpp"
 #include "Sequence.hip.hpp"
@@ -10,109 +11,3 @@
 #if USE_AMD_INLINE_ASM
 #include "amd_inline_asm.hip.hpp"
 #endif
-
-__device__ index_t get_thread_local_1d_id() { return threadIdx.x; }
-
-__device__ index_t get_block_1d_id() { return blockIdx.x; }
-
-template <class T1, class T2>
-struct is_same
-{
-    static constexpr bool value = false;
-};
-
-template <class T>
-struct is_same<T, T>
-{
-    static constexpr bool value = true;
-};
-
-template <class X, class Y>
-__host__ __device__ constexpr bool is_same_type(X, Y)
-{
-    return is_same<X, Y>::value;
-}
-
-namespace mod_conv { // namespace mod_conv
-template <class T, T s>
-struct scales
-{
-    __host__ __device__ constexpr T operator()(T a) const { return s * a; }
-};
-
-template <class T>
-struct plus
-{
-    __host__ __device__ constexpr T operator()(T a, T b) const { return a + b; }
-};
-
-template <class T>
-struct multiplies
-{
-    __host__ __device__ constexpr T operator()(T a, T b) const { return a * b; }
-};
-
-template <class T>
-struct integer_divide_ceiler
-{
-    __host__ __device__ constexpr T operator()(T a, T b) const
-    {
-        static_assert(is_same<T, index_t>::value || is_same<T, int>::value, "wrong type");
-
-        return (a + b - 1) / b;
-    }
-};
-
-template <class T>
-__host__ __device__ constexpr T integer_divide_ceil(T a, T b)
-{
-    static_assert(is_same<T, index_t>::value || is_same<T, int>::value, "wrong type");
-
-    return (a + b - 1) / b;
-}
-
-template <class T>
-__host__ __device__ constexpr T max(T x, T y)
-{
-    return x > y ? x : y;
-}
-
-template <class T, class... Ts>
-__host__ __device__ constexpr T max(T x, Ts... xs)
-{
-    static_assert(sizeof...(xs) > 0, "not enough argument");
-
-    auto y = max(xs...);
-
-    static_assert(is_same<decltype(y), T>::value, "not the same type");
-
-    return x > y ? x : y;
-}
-
-template <class T>
-__host__ __device__ constexpr T min(T x, T y)
-{
-    return x < y ? x : y;
-}
-
-template <class T, class... Ts>
-__host__ __device__ constexpr T min(T x, Ts... xs)
-{
-    static_assert(sizeof...(xs) > 0, "not enough argument");
-
-    auto y = min(xs...);
-
-    static_assert(is_same<decltype(y), T>::value, "not the same type");
-
-    return x < y ? x : y;
-}
-
-// this is wrong
-// TODO: implement correct least common multiple, instead of calling max()
-template <class T, class... Ts>
-__host__ __device__ constexpr T lcm(T x, Ts... xs)
-{
-    return max(x, xs...);
-}
-
-} // namespace mod_conv
diff --git a/src/include/functional3.hip.hpp b/src/include/functional3.hip.hpp
index 78b95200c5..4019725c4c 100644
--- a/src/include/functional3.hip.hpp
+++ b/src/include/functional3.hip.hpp
@@ -11,7 +11,7 @@ struct static_ford_impl
     // F signature: F(Sequence<...> multi_id)
     // CurrentMultiIndex: Sequence<...>
     template <class F, class CurrentMultiIndex>
-    __host__ __device__ void operator()(F f, CurrentMultiIndex) const
+    __host__ __device__ constexpr void operator()(F f, CurrentMultiIndex) const
     {
         static_assert(RemainLengths::GetSize() > 0, "wrong! should not get here");
 
@@ -28,7 +28,7 @@ struct static_ford_impl<Sequence<>>
     // F signature: F(Sequence<...> multi_id)
     // CurrentMultiIndex: Sequence<...>
     template <class F, class CurrentMultiIndex>
-    __host__ __device__ void operator()(F f, CurrentMultiIndex) const
+    __host__ __device__ constexpr void operator()(F f, CurrentMultiIndex) const
     {
         f(CurrentMultiIndex{});
     }
@@ -40,7 +40,7 @@ struct static_ford
 {
     // F signature: F(Sequence<...> multi_id)
     template <class F>
-    __host__ __device__ void operator()(F f) const
+    __host__ __device__ constexpr void operator()(F f) const
     {
         static_assert(Lengths::GetSize() > 0, "wrong! Lengths is empty");
 
@@ -55,7 +55,7 @@ struct ford_impl
     // CurrentMultiIndex: Array<...>
     // RemainLengths: Sequence<...>
     template <class F, class CurrentMultiIndex, class RemainLengths>
-    __host__ __device__ void
+    __host__ __device__ constexpr void
     operator()(F f, CurrentMultiIndex current_multi_id, RemainLengths) const
     {
         static_assert(RemainLengths::GetSize() == RemainDim, "wrong!");
@@ -77,7 +77,7 @@ struct ford_impl<1>
     // CurrentMultiIndex: Array<...>
     // RemainLengths: Sequence<...>
     template <class F, class CurrentMultiIndex, class RemainLengths>
-    __host__ __device__ void
+    __host__ __device__ constexpr void
     operator()(F f, CurrentMultiIndex current_multi_id, RemainLengths) const
     {
         static_assert(RemainLengths::GetSize() == 1, "wrong!");
@@ -97,7 +97,7 @@ struct ford
 {
     // F signature: F(Array<...> multi_id)
     template <class F>
-    __host__ __device__ void operator()(F f) const
+    __host__ __device__ constexpr void operator()(F f) const
     {
         constexpr index_t first_length = Lengths{}.Front();