13 files changed, 825 insertions, 0 deletions
diff --git a/extra/guile/scm/detail/convert.hpp b/extra/guile/scm/detail/convert.hpp
new file mode 100644
index 000000000000..4c87ff185669
--- /dev/null
+++ b/extra/guile/scm/detail/convert.hpp
@@ -0,0 +1,58 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <scm/detail/util.hpp>
+
+#include <cstdint>
+#include <type_traits>
+#include <utility>
+
+#include <libguile.h>
+
+namespace scm {
+namespace detail {
+
+template <typename T, typename Enable=void>
+struct convert;
+
+template <typename T>
+auto to_scm(T&& v)
+    -> SCM_DECLTYPE_RETURN(
+        convert<std::decay_t<T>>::to_scm(std::forward<T>(v)));
+
+template <typename T>
+auto to_cpp(SCM v)
+    -> SCM_DECLTYPE_RETURN(
+        convert<std::decay_t<T>>::to_cpp(v));
+
+} // namespace detail
+} // namespace scm
+
+#define SCM_DECLARE_NUMERIC_TYPE(cpp_name__, scm_name__)                \
+    namespace scm {                                                     \
+    namespace detail {                                                  \
+    template <>                                                         \
+    struct convert<cpp_name__> {                                        \
+        static cpp_name__ to_cpp(SCM v) { return scm_to_ ## scm_name__(v); } \
+        static SCM to_scm(cpp_name__ v) { return scm_from_ ## scm_name__(v); } \
+    };                                                                  \
+    }} /* namespace scm::detail */                                      \
+    /**/
+
+SCM_DECLARE_NUMERIC_TYPE(float,         double);
+SCM_DECLARE_NUMERIC_TYPE(double,        double);
+SCM_DECLARE_NUMERIC_TYPE(std::int8_t,   int8);
+SCM_DECLARE_NUMERIC_TYPE(std::int16_t,  int16);
+SCM_DECLARE_NUMERIC_TYPE(std::int32_t,  int32);
+SCM_DECLARE_NUMERIC_TYPE(std::int64_t,  int64);
+SCM_DECLARE_NUMERIC_TYPE(std::uint8_t,  uint8);
+SCM_DECLARE_NUMERIC_TYPE(std::uint16_t, uint16);
+SCM_DECLARE_NUMERIC_TYPE(std::uint32_t, uint32);
+SCM_DECLARE_NUMERIC_TYPE(std::uint64_t, uint64);
diff --git a/extra/guile/scm/detail/define.hpp b/extra/guile/scm/detail/define.hpp
new file mode 100644
index 000000000000..08b6e763381f
--- /dev/null
+++ b/extra/guile/scm/detail/define.hpp
@@ -0,0 +1,36 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#ifndef SCM_AUTO_EXPORT
+#define SCM_AUTO_EXPORT 1
+#endif
+
+#include <scm/detail/subr_wrapper.hpp>
+#include <scm/list.hpp>
+
+namespace scm {
+namespace detail {
+
+template <typename Tag, typename Fn>
+static void define_impl(const std::string& name, Fn fn)
+{
+    using args_t = function_args_t<Fn>;
+    constexpr auto args_size = pack_size_v<args_t>;
+    constexpr auto has_rest  = std::is_same<pack_last_t<args_t>, scm::args>{};
+    constexpr auto arg_count = args_size - has_rest;
+    auto subr = (scm_t_subr) +subr_wrapper_aux<Tag>(fn, args_t{});
+    scm_c_define_gsubr(name.c_str(), arg_count, 0, has_rest, subr);
+#if SCM_AUTO_EXPORT
+    scm_c_export(name.c_str());
+#endif
+}
+
+} // namespace detail
+} // namespace scm
diff --git a/extra/guile/scm/detail/finalizer_wrapper.hpp b/extra/guile/scm/detail/finalizer_wrapper.hpp
new file mode 100644
index 000000000000..258249eb2c73
--- /dev/null
+++ b/extra/guile/scm/detail/finalizer_wrapper.hpp
@@ -0,0 +1,62 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <scm/detail/invoke.hpp>
+#include <scm/detail/function_args.hpp>
+#include <scm/detail/convert.hpp>
+
+namespace scm {
+namespace detail {
+// this anonymous namespace should help avoiding registration clashes
+// among translation units.
+namespace {
+
+template <typename Tag, typename Fn>
+auto finalizer_wrapper_impl(Fn fn, pack<>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] { invoke(fn_); };
+}
+template <typename Tag, typename Fn, typename T1>
+auto finalizer_wrapper_impl(Fn fn, pack<T1>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] (SCM a1) { invoke(fn_, to_cpp<T1>(a1)); };
+}
+template <typename Tag, typename Fn, typename T1, typename T2>
+auto finalizer_wrapper_impl(Fn fn, pack<T1, T2>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] (SCM a1, SCM a2) {
+        invoke(fn_, to_cpp<T1>(a1), to_cpp<T2>(a2));
+    };
+}
+template <typename Tag, typename Fn, typename T1, typename T2, typename T3>
+auto finalizer_wrapper_impl(Fn fn, pack<T1, T2, T3>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] (SCM a1, SCM a2, SCM a3) {
+        invoke(fn_, to_cpp<T1>(a1), to_cpp<T2>(a2), to_cpp<T3>(a3));
+    };
+}
+
+template <typename Tag, typename Fn>
+auto finalizer_wrapper(Fn fn)
+{
+    return finalizer_wrapper_impl<Tag>(fn, function_args_t<Fn>{});
+}
+
+} // anonymous namespace
+} // namespace detail
+} // namespace scm
diff --git a/extra/guile/scm/detail/function_args.hpp b/extra/guile/scm/detail/function_args.hpp
new file mode 100644
index 000000000000..809e3eb1979b
--- /dev/null
+++ b/extra/guile/scm/detail/function_args.hpp
@@ -0,0 +1,21 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <scm/detail/pack.hpp>
+#include <boost/callable_traits/args.hpp>
+
+namespace scm {
+namespace detail {
+
+template <typename Fn>
+using function_args_t = boost::callable_traits::args_t<Fn, pack>;
+
+} // namespace detail
+} // namespace scm
diff --git a/extra/guile/scm/detail/invoke.hpp b/extra/guile/scm/detail/invoke.hpp
new file mode 100644
index 000000000000..d9f2b37ccece
--- /dev/null
+++ b/extra/guile/scm/detail/invoke.hpp
@@ -0,0 +1,39 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+// Adapted from the official std::invoke proposal:
+// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4169.html
+
+#include <type_traits>
+#include <functional>
+
+namespace scm {
+namespace detail {
+
+template <typename Functor, typename... Args>
+std::enable_if_t<
+    std::is_member_pointer<std::decay_t<Functor>>::value,
+    std::result_of_t<Functor&&(Args&&...)>>
+invoke(Functor&& f, Args&&... args)
+{
+    return std::mem_fn(f)(std::forward<Args>(args)...);
+}
+
+template <typename Functor, typename... Args>
+std::enable_if_t<
+    !std::is_member_pointer<std::decay_t<Functor>>::value,
+    std::result_of_t<Functor&&(Args&&...)>>
+invoke(Functor&& f, Args&&... args)
+{
+    return std::forward<Functor>(f)(std::forward<Args>(args)...);
+}
+
+} // namespace detail
+} // namespace scm
diff --git a/extra/guile/scm/detail/pack.hpp b/extra/guile/scm/detail/pack.hpp
new file mode 100644
index 000000000000..9a1813570bcd
--- /dev/null
+++ b/extra/guile/scm/detail/pack.hpp
@@ -0,0 +1,52 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+namespace scm {
+namespace detail {
+
+struct none_t;
+
+template <typename... Ts>
+struct pack {};
+
+template <typename Pack>
+struct pack_size;
+
+template <typename... Ts>
+struct pack_size<pack<Ts...>>
+{
+    static constexpr auto value = sizeof...(Ts);
+};
+
+template <typename Pack>
+constexpr auto pack_size_v = pack_size<Pack>::value;
+
+template <typename Pack>
+struct pack_last
+{
+    using type = none_t;
+};
+
+template <typename T, typename ...Ts>
+struct pack_last<pack<T, Ts...>>
+    : pack_last<pack<Ts...>>
+{};
+
+template <typename T>
+struct pack_last<pack<T>>
+{
+    using type = T;
+};
+
+template <typename Pack>
+using pack_last_t = typename pack_last<Pack>::type;
+
+} // namespace detail
+} // namespace scm
diff --git a/extra/guile/scm/detail/subr_wrapper.hpp b/extra/guile/scm/detail/subr_wrapper.hpp
new file mode 100644
index 000000000000..fc11ff1c51ec
--- /dev/null
+++ b/extra/guile/scm/detail/subr_wrapper.hpp
@@ -0,0 +1,111 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <scm/detail/invoke.hpp>
+#include <scm/detail/function_args.hpp>
+#include <scm/detail/convert.hpp>
+
+namespace scm {
+namespace detail {
+
+// this anonymous namespace should help avoiding registration clashes
+// among translation units.
+namespace {
+
+template <typename Tag, typename R, typename Fn>
+auto subr_wrapper_impl(Fn fn, pack<R>, pack<>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] () -> SCM { return to_scm(invoke(fn_)); };
+}
+template <typename Tag, typename Fn, typename R, typename T1>
+auto subr_wrapper_impl(Fn fn, pack<R>, pack<T1>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] (SCM a1) -> SCM {
+        return to_scm(invoke(fn_, to_cpp<T1>(a1)));
+    };
+}
+template <typename Tag, typename Fn, typename R, typename T1, typename T2>
+auto subr_wrapper_impl(Fn fn, pack<R>, pack<T1, T2>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] (SCM a1, SCM a2) -> SCM {
+        return to_scm(invoke(fn_, to_cpp<T1>(a1), to_cpp<T2>(a2)));
+    };
+}
+template <typename Tag, typename Fn, typename R, typename T1, typename T2,
+          typename T3>
+auto subr_wrapper_impl(Fn fn, pack<R>, pack<T1, T2, T3>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] (SCM a1, SCM a2, SCM a3) -> SCM {
+        return to_scm(invoke(fn_, to_cpp<T1>(a1), to_cpp<T2>(a2),
+                             to_cpp<T3>(a3)));
+    };
+}
+
+template <typename Tag, typename Fn>
+auto subr_wrapper_impl(Fn fn, pack<void>, pack<>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] () -> SCM { invoke(fn_); return SCM_UNSPECIFIED; };
+}
+template <typename Tag, typename Fn, typename T1>
+auto subr_wrapper_impl(Fn fn, pack<void>, pack<T1>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] (SCM a1) -> SCM {
+        invoke(fn_, to_cpp<T1>(a1)); return SCM_UNSPECIFIED;
+    };
+}
+template <typename Tag, typename Fn, typename T1, typename T2>
+auto subr_wrapper_impl(Fn fn, pack<void>, pack<T1, T2>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] (SCM a1, SCM a2) -> SCM {
+        invoke(fn_, to_cpp<T1>(a1), to_cpp<T2>(a2));
+        return SCM_UNSPECIFIED;
+    };
+}
+template <typename Tag, typename Fn, typename T1, typename T2, typename T3>
+auto subr_wrapper_impl(Fn fn, pack<void>, pack<T1, T2, T3>)
+{
+    check_call_once<Tag, Fn>();
+    static const Fn fn_ = fn;
+    return [] (SCM a1, SCM a2, SCM a3) -> SCM {
+        invoke(fn_, to_cpp<T1>(a1), to_cpp<T2>(a2), to_cpp<T3>(a3));
+        return SCM_UNSPECIFIED;
+    };
+}
+
+template <typename Tag, typename Fn, typename... Args>
+auto subr_wrapper_aux(Fn fn, pack<Args...>)
+{
+    return subr_wrapper_impl<Tag>(
+        fn, pack<std::result_of_t<Fn(Args...)>>{}, pack<Args...>{});
+}
+
+template <typename Tag, typename Fn>
+auto subr_wrapper(Fn fn)
+{
+    return subr_wrapper_aux<Tag>(fn, function_args_t<Fn>{});
+}
+
+} // anonymous namespace
+} // namespace detail
+} // namespace scm
diff --git a/extra/guile/scm/detail/util.hpp b/extra/guile/scm/detail/util.hpp
new file mode 100644
index 000000000000..fdc323722e99
--- /dev/null
+++ b/extra/guile/scm/detail/util.hpp
@@ -0,0 +1,49 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <libguile.h>
+
+namespace scm {
+namespace detail {
+
+#define SCM_DECLTYPE_RETURN(...)                \
+    decltype(__VA_ARGS__)                       \
+    { return __VA_ARGS__; }                     \
+    /**/
+
+template <typename... Ts>
+constexpr bool is_valid_v = true;
+
+template <typename... Ts>
+using is_valid_t = void;
+
+template <typename... Ts>
+void check_call_once()
+{
+    static bool called = false;
+    if (called) scm_misc_error (nullptr, "Double defined binding. \
+This may be caused because there are multiple C++ binding groups in the same \
+translation unit.  You may solve this by using different type tags for each \
+binding group.", SCM_EOL);
+    called = true;
+}
+
+struct move_sequence
+{
+    move_sequence() = default;
+    move_sequence(const move_sequence&) = delete;
+    move_sequence(move_sequence&& other)
+    { other.moved_from_ = true; };
+
+    bool moved_from_ = false;
+};
+
+} // namespace detail
+} // namespace scm
diff --git a/extra/guile/scm/group.hpp b/extra/guile/scm/group.hpp
new file mode 100644
index 000000000000..69cd385820cb
--- /dev/null
+++ b/extra/guile/scm/group.hpp
@@ -0,0 +1,88 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <scm/detail/define.hpp>
+#include <string>
+
+namespace scm {
+namespace detail {
+
+template <typename Tag, int Seq=0>
+struct definer
+{
+    using this_t = definer;
+    using next_t = definer<Tag, Seq + 1>;
+
+    std::string group_name_ = {};
+
+    definer() = default;
+    definer(definer&&) = default;
+
+    template <int Seq2,
+              typename Enable=std::enable_if_t<Seq2 + 1 == Seq>>
+    definer(definer<Tag, Seq2>)
+    {}
+
+    template <typename Fn>
+    next_t define(std::string name, Fn fn) &&
+    {
+        define_impl<this_t>(name, fn);
+        return { std::move(*this) };
+    }
+
+    template <typename Fn>
+    next_t maker(Fn fn) &&
+    {
+        define_impl<this_t>("make", fn);
+        return { std::move(*this) };
+    }
+};
+
+template <typename Tag, int Seq=0>
+struct group_definer
+{
+    using this_t = group_definer;
+    using next_t = group_definer<Tag, Seq + 1>;
+
+    std::string group_name_ = {};
+
+    group_definer(std::string name)
+        : group_name_{std::move(name)} {}
+
+    group_definer(group_definer&&) = default;
+
+    template <int Seq2,
+              typename Enable=std::enable_if_t<Seq2 + 1 == Seq>>
+    group_definer(group_definer<Tag, Seq2>)
+    {}
+
+    template <typename Fn>
+    next_t define(std::string name, Fn fn) &&
+    {
+        define_impl<this_t>(group_name_ + "-" + name, fn);
+        return { std::move(*this) };
+    }
+};
+
+} // namespace detail
+
+template <typename Tag=void>
+detail::definer<Tag> group()
+{
+    return {};
+}
+
+template <typename Tag=void>
+detail::group_definer<Tag> group(std::string name)
+{
+    return { std::move(name) };
+}
+
+} // namespace scm
diff --git a/extra/guile/scm/list.hpp b/extra/guile/scm/list.hpp
new file mode 100644
index 000000000000..dc162c200244
--- /dev/null
+++ b/extra/guile/scm/list.hpp
@@ -0,0 +1,54 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <scm/val.hpp>
+#include <iostream>
+
+namespace scm {
+
+struct list : detail::wrapper
+{
+    using base_t = detail::wrapper;
+    using base_t::base_t;
+
+    using iterator = list;
+    using value_type = val;
+
+    list() : base_t{SCM_EOL} {};
+    list end() const { return {}; }
+    list begin() const { return *this; }
+
+    explicit operator bool() { return handle_ != SCM_EOL; }
+
+    val operator* () const { return val{scm_car(handle_)}; }
+
+    list& operator++ ()
+    {
+        handle_ = scm_cdr(handle_);
+        return *this;
+    }
+
+    list operator++ (int)
+    {
+        auto result = *this;
+        result.handle_ = scm_cdr(handle_);
+        return result;
+    }
+};
+
+struct args : list
+{
+    using list::list;
+};
+
+} // namespace scm
+
+SCM_DECLARE_WRAPPER_TYPE(scm::list);
+SCM_DECLARE_WRAPPER_TYPE(scm::args);
diff --git a/extra/guile/scm/scm.hpp b/extra/guile/scm/scm.hpp
new file mode 100644
index 000000000000..f4e4989a44a9
--- /dev/null
+++ b/extra/guile/scm/scm.hpp
@@ -0,0 +1,14 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <scm/val.hpp>
+#include <scm/list.hpp>
+#include <scm/group.hpp>
+#include <scm/type.hpp>
diff --git a/extra/guile/scm/type.hpp b/extra/guile/scm/type.hpp
new file mode 100644
index 000000000000..da53ed46ef8b
--- /dev/null
+++ b/extra/guile/scm/type.hpp
@@ -0,0 +1,153 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <scm/detail/finalizer_wrapper.hpp>
+#include <scm/detail/define.hpp>
+#include <string>
+
+namespace scm {
+namespace detail {
+
+template <typename T>
+struct foreign_type_storage
+{
+    static SCM data;
+};
+
+template <typename T>
+SCM foreign_type_storage<T>::data = SCM_UNSPECIFIED;
+
+template <typename T>
+struct convert_foreign_type
+{
+    using storage_t = foreign_type_storage<T>;
+    static T& to_cpp(SCM v)
+    {
+        assert(storage_t::data != SCM_UNSPECIFIED &&
+               "can not convert to undefined type");
+        scm_assert_foreign_object_type(storage_t::data, v);
+        return *(T*)scm_foreign_object_ref(v, 0);
+    }
+
+    template <typename U>
+    static SCM to_scm(U&& v)
+    {
+        assert(storage_t::data != SCM_UNSPECIFIED &&
+               "can not convert from undefined type");
+        return scm_make_foreign_object_1(
+            storage_t::data,
+            new (scm_gc_malloc(sizeof(T), "scmpp")) T(
+                std::forward<U>(v)));
+    }
+};
+
+// Assume that every other type is foreign
+template <typename T>
+struct convert<T,
+               std::enable_if_t<!std::is_fundamental<T>::value &&
+                                // only value types are supported at
+                                // the moment but the story might
+                                // change later...
+                                !std::is_pointer<T>::value>>
+    : convert_foreign_type<T>
+{
+};
+
+template <typename Tag, typename T, int Seq=0>
+struct type_definer : move_sequence
+{
+    using this_t = type_definer;
+    using next_t = type_definer<Tag, T, Seq + 1>;
+
+    std::string type_name_ = {};
+    scm_t_struct_finalize finalizer_ = nullptr;
+
+    type_definer(type_definer&&) = default;
+
+    type_definer(std::string type_name)
+        : type_name_(std::move(type_name))
+    {}
+
+    ~type_definer()
+    {
+        if (!moved_from_) {
+            using storage_t = detail::foreign_type_storage<T>;
+            assert(storage_t::data == SCM_UNSPECIFIED);
+            storage_t::data = scm_make_foreign_object_type(
+                scm_from_utf8_symbol(("<" + type_name_ + ">").c_str()),
+                scm_list_1(scm_from_utf8_symbol("data")),
+                finalizer_);
+        }
+    }
+
+    template <int Seq2, typename Enable=std::enable_if_t<Seq2 + 1 == Seq>>
+    type_definer(type_definer<Tag, T, Seq2> r)
+        : move_sequence{std::move(r)}
+        , type_name_{std::move(r.type_name_)}
+        , finalizer_{std::move(r.finalizer_)}
+    {}
+
+    next_t constructor() &&
+    {
+        define_impl<this_t>(type_name_, [] { return T{}; });
+        return { std::move(*this) };
+    }
+
+    template <typename Fn>
+    next_t constructor(Fn fn) &&
+    {
+        define_impl<this_t>(type_name_, fn);
+        return { std::move(*this) };
+    }
+
+    next_t finalizer() &&
+    {
+        finalizer_ = (scm_t_struct_finalize) +finalizer_wrapper<Tag>(
+            [] (T& x) { x.~T(); });
+        return { std::move(*this) };
+    }
+
+    template <typename Fn>
+    next_t finalizer(Fn fn) &&
+    {
+        finalizer_ = (scm_t_struct_finalize) +finalizer_wrapper<Tag>(fn);
+        return { std::move(*this) };
+    }
+
+    next_t maker() &&
+    {
+        define_impl<this_t>("make-" + type_name_, [] { return T{}; });
+        return { std::move(*this) };
+    }
+
+    template <typename Fn>
+    next_t maker(Fn fn) &&
+    {
+        define_impl<this_t>("make-" + type_name_, fn);
+        return { std::move(*this) };
+    }
+
+    template <typename Fn>
+    next_t define(std::string name, Fn fn) &&
+    {
+        define_impl<this_t>(type_name_ + "-" + name, fn);
+        return { std::move(*this) };
+    }
+};
+
+} // namespace detail
+
+template <typename Tag, typename T=Tag>
+detail::type_definer<Tag, T> type(std::string type_name)
+{
+    return { type_name };
+}
+
+} // namespace scm
diff --git a/extra/guile/scm/val.hpp b/extra/guile/scm/val.hpp
new file mode 100644
index 000000000000..63d7189262da
--- /dev/null
+++ b/extra/guile/scm/val.hpp
@@ -0,0 +1,88 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <scm/detail/convert.hpp>
+
+namespace scm {
+namespace detail {
+
+template <typename T>
+struct convert_wrapper_type
+{
+    static T to_cpp(SCM v) { return T{v}; }
+    static SCM to_scm(T v) { return v.get(); }
+};
+
+struct wrapper
+{
+    wrapper() = default;
+    wrapper(SCM hdl) : handle_{hdl} {}
+    SCM get() const { return handle_; }
+    operator SCM () const { return handle_; }
+
+    bool operator==(wrapper other) { return handle_ == other.handle_; }
+    bool operator!=(wrapper other) { return handle_ != other.handle_; }
+
+protected:
+    SCM handle_ = SCM_UNSPECIFIED;
+};
+
+} // namespace detail
+
+struct val : detail::wrapper
+{
+    using base_t = detail::wrapper;
+    using base_t::base_t;
+
+    template <typename T,
+              typename = std::enable_if_t<
+                  (!std::is_same<std::decay_t<T>, val>{} &&
+                   !std::is_same<std::decay_t<T>, SCM>{})>>
+    val(T&& x)
+        : base_t(detail::to_scm(std::forward<T>(x)))
+    {}
+
+    template <typename T,
+              typename = std::enable_if_t<
+                  std::is_same<T, decltype(detail::to_cpp<T>(SCM{}))>{}>>
+    operator T() const { return detail::to_cpp<T>(handle_); }
+
+    template <typename T,
+              typename = std::enable_if_t<
+                  std::is_same<T&, decltype(detail::to_cpp<T>(SCM{}))>{}>>
+    operator T& () const { return detail::to_cpp<T>(handle_); }
+
+    template <typename T,
+              typename = std::enable_if_t<
+                  std::is_same<const T&, decltype(detail::to_cpp<T>(SCM{}))>{}>>
+    operator const T& () const { return detail::to_cpp<T>(handle_); }
+
+    val operator() () const
+    { return val{scm_call_0(get())}; }
+    val operator() (val a0) const
+    { return val{scm_call_1(get(), a0)}; }
+    val operator() (val a0, val a1) const
+    { return val{scm_call_2(get(), a0, a1)}; }
+    val operator() (val a0, val a1, val a3) const
+    { return val{scm_call_3(get(), a0, a1, a3)}; }
+};
+
+} // namespace scm
+
+#define SCM_DECLARE_WRAPPER_TYPE(cpp_name__)                            \
+    namespace scm {                                                     \
+    namespace detail {                                                  \
+    template <>                                                         \
+    struct convert<cpp_name__>                                          \
+        : convert_wrapper_type<cpp_name__> {};                          \
+    }} /* namespace scm::detail */                                      \
+    /**/
+
+SCM_DECLARE_WRAPPER_TYPE(val);