/*=============================================================================
Copyright (c) 2014 Paul Fultz II
reveal.h
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
==============================================================================*/
#ifndef BOOST_HOF_GUARD_FUNCTION_REVEAL_H
#define BOOST_HOF_GUARD_FUNCTION_REVEAL_H
/// reveal
/// ======
///
/// Description
/// -----------
///
/// The `reveal` function adaptor helps shows the error messages that get
/// masked on some compilers. Sometimes an error in a function that causes a
/// substitution failure, will remove the function from valid overloads. On
/// compilers without a backtrace for substitution failure, this will mask the
/// error inside the function. The `reveal` adaptor will expose these error
/// messages while still keeping the function SFINAE-friendly.
///
/// Sample
/// ------
///
/// If we take the `print` example from the quick start guide like this:
///
/// namespace adl {
///
/// using std::begin;
///
/// template<class R>
/// auto adl_begin(R&& r) BOOST_HOF_RETURNS(begin(r));
/// }
///
/// BOOST_HOF_STATIC_LAMBDA_FUNCTION(for_each_tuple) = [](const auto& sequence, auto f) BOOST_HOF_RETURNS
/// (
/// boost::hof::unpack(boost::hof::proj(f))(sequence)
/// );
///
/// auto print = boost::hof::fix(boost::hof::first_of(
/// [](auto, const auto& x) -> decltype(std::cout << x, void())
/// {
/// std::cout << x << std::endl;
/// },
/// [](auto self, const auto& range) -> decltype(self(*adl::adl_begin(range)), void())
/// {
/// for(const auto& x:range) self(x);
/// },
/// [](auto self, const auto& tuple) -> decltype(for_each_tuple(tuple, self), void())
/// {
/// return for_each_tuple(tuple, self);
/// }
/// ));
///
/// Which prints numbers and vectors:
///
/// print(5);
///
/// std::vector<int> v = { 1, 2, 3, 4 };
/// print(v);
///
/// However, if we pass a type that can't be printed, we get an error like
/// this:
///
/// print.cpp:49:5: error: no matching function for call to object of type 'boost::hof::fix_adaptor<boost::hof::first_of_adaptor<(lambda at print.cpp:29:9), (lambda at print.cpp:33:9), (lambda at print.cpp:37:9)> >'
/// print(foo{});
/// ^~~~~
/// fix.hpp:158:5: note: candidate template ignored: substitution failure [with Ts = <foo>]: no matching function for call to object of type 'const boost::hof::first_of_adaptor<(lambda at
/// print.cpp:29:9), (lambda at print.cpp:33:9), (lambda at print.cpp:37:9)>'
/// operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
///
/// Which is short and gives very little information why it can't be called.
/// It doesn't even show the overloads that were try. However, using the
/// `reveal` adaptor we can get more info about the error like this:
///
/// print.cpp:49:5: error: no matching function for call to object of type 'boost::hof::reveal_adaptor<boost::hof::fix_adaptor<boost::hof::first_of_adaptor<(lambda at print.cpp:29:9), (lambda at print.cpp:33:9),
/// (lambda at print.cpp:37:9)> >, boost::hof::fix_adaptor<boost::hof::first_of_adaptor<(lambda at print.cpp:29:9), (lambda at print.cpp:33:9), (lambda at print.cpp:37:9)> > >'
/// boost::hof::reveal(print)(foo{});
/// ^~~~~~~~~~~~~~~~~~
/// reveal.hpp:149:20: note: candidate template ignored: substitution failure [with Ts = <foo>, $1 = void]: no matching function for call to object of type '(lambda at print.cpp:29:9)'
/// constexpr auto operator()(Ts&&... xs) const
/// ^
/// reveal.hpp:149:20: note: candidate template ignored: substitution failure [with Ts = <foo>, $1 = void]: no matching function for call to object of type '(lambda at print.cpp:33:9)'
/// constexpr auto operator()(Ts&&... xs) const
/// ^
/// reveal.hpp:149:20: note: candidate template ignored: substitution failure [with Ts = <foo>, $1 = void]: no matching function for call to object of type '(lambda at print.cpp:37:9)'
/// constexpr auto operator()(Ts&&... xs) const
/// ^
/// fix.hpp:158:5: note: candidate template ignored: substitution failure [with Ts = <foo>]: no matching function for call to object of type 'const boost::hof::first_of_adaptor<(lambda at
/// print.cpp:29:9), (lambda at print.cpp:33:9), (lambda at print.cpp:37:9)>'
/// operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
///
/// So now the error has a note for each of the lambda overloads it tried. Of
/// course this can be improved even further by providing custom reporting of
/// failures.
///
/// Synopsis
/// --------
///
/// template<class F>
/// reveal_adaptor<F> reveal(F f);
///
/// Requirements
/// ------------
///
/// F must be:
///
/// * [ConstInvocable](ConstInvocable)
/// * MoveConstructible
///
/// Reporting Failures
/// ------------------
///
/// By default, `reveal` reports the substitution failure by trying to call
/// the function. However, more detail expressions can be be reported from a
/// template alias by using `as_failure`. This is done by defining a nested
/// `failure` struct in the function object and then inheriting from
/// `as_failure`. Also multiple failures can be reported by using
/// `with_failures`.
///
/// Synopsis
/// --------
///
/// // Report failure by instantiating the Template
/// template<template<class...> class Template>
/// struct as_failure;
///
/// // Report multiple falures
/// template<class... Failures>
/// struct with_failures;
///
/// // Report the failure for each function
/// template<class... Fs>
/// struct failure_for;
///
/// // Get the failure of a function
/// template<class F>
/// struct get_failure;
///
/// Example
/// -------
///
/// #include <boost/hof.hpp>
/// #include <cassert>
///
/// struct sum_f
/// {
/// template<class T, class U>
/// using sum_failure = decltype(std::declval<T>()+std::declval<U>());
///
/// struct failure
/// : boost::hof::as_failure<sum_failure>
/// {};
///
/// template<class T, class U>
/// auto operator()(T x, U y) const BOOST_HOF_RETURNS(x+y);
/// };
///
/// int main() {
/// assert(sum_f()(1, 2) == 3);
/// }
///
#include <boost/hof/always.hpp>
#include <boost/hof/returns.hpp>
#include <boost/hof/is_invocable.hpp>
#include <boost/hof/identity.hpp>
#include <boost/hof/detail/move.hpp>
#include <boost/hof/detail/callable_base.hpp>
#include <boost/hof/detail/delegate.hpp>
#include <boost/hof/detail/holder.hpp>
#include <boost/hof/detail/join.hpp>
#include <boost/hof/detail/make.hpp>
#include <boost/hof/detail/static_const_var.hpp>
#include <boost/hof/detail/using.hpp>
#ifndef BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS
#ifdef __clang__
#define BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS 1
#else
#define BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS 0
#endif
#endif
namespace boost { namespace hof {
namespace detail {
template<class T, class=void>
struct has_failure
: std::false_type
{};
template<class T>
struct has_failure<T, typename holder<
typename T::failure
>::type>
: std::true_type
{};
struct identity_failure
{
template<class T>
T operator()(T&& x);
template<class T>
static T&& val();
#if BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS
template<template<class...> class Template, class... Ts>
BOOST_HOF_USING(defer, Template<Ts...>);
#else
template<template<class...> class Template, class... Ts>
static auto defer(Ts&&...) -> Template<Ts...>;
#endif
};
}
template<class F, class=void>
struct get_failure
{
template<class... Ts>
struct of
{
#if BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS
template<class Id>
using apply = decltype(Id()(std::declval<F>())(std::declval<Ts>()...));
#else
template<class Id>
static auto apply(Id id) -> decltype(id(std::declval<F>())(std::declval<Ts>()...));
#endif
};
};
template<class F>
struct get_failure<F, typename std::enable_if<detail::has_failure<F>::value>::type>
: F::failure
{};
template<template<class...> class Template>
struct as_failure
{
template<class... Ts>
struct of
{
#if BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS
template<class Id>
using apply = typename Id::template defer<Template, Ts...>;
#else
template<class Id>
static auto apply(Id) -> decltype(Id::template defer<Template, Ts...>());
#endif
};
};
namespace detail {
template<class Failure, class... Ts>
BOOST_HOF_USING_TYPENAME(apply_failure, Failure::template of<Ts...>);
template<class F, class Failure>
struct reveal_failure
{
// Add default constructor to make clang 3.4 happy
constexpr reveal_failure()
{}
// This is just a placeholder to produce a note in the compiler, it is
// never called
template<
class... Ts,
class=typename std::enable_if<(!is_invocable<F, Ts...>::value)>::type
>
constexpr auto operator()(Ts&&... xs) const
#if BOOST_HOF_REVEAL_USE_TEMPLATE_ALIAS
-> typename apply_failure<Failure, Ts...>::template apply<boost::hof::detail::identity_failure>;
#else
-> decltype(apply_failure<Failure, Ts...>::apply(boost::hof::detail::identity_failure()));
#endif
};
template<class F, class Failure=get_failure<F>, class=void>
struct traverse_failure
: reveal_failure<F, Failure>
{
constexpr traverse_failure()
{}
};
template<class F, class Failure>
struct traverse_failure<F, Failure, typename holder<
typename Failure::children
>::type>
: Failure::children::template overloads<F>
{
constexpr traverse_failure()
{}
};
template<class Failure, class Transform, class=void>
struct transform_failures
: Transform::template apply<Failure>
{};
template<class Failure, class Transform>
struct transform_failures<Failure, Transform, typename holder<
typename Failure::children
>::type>
: Failure::children::template transform<Transform>
{};
}
template<class Failure, class... Failures>
struct failures;
template<class... Fs>
struct with_failures
{
typedef BOOST_HOF_JOIN(failures, Fs...) children;
};
template<class Failure, class... Failures>
struct failures
{
template<class Transform>
BOOST_HOF_USING(transform, with_failures<detail::transform_failures<Failure, Transform>, detail::transform_failures<Failures, Transform>...>);
template<class F, class FailureBase=BOOST_HOF_JOIN(failures, Failures...)>
struct overloads
: detail::traverse_failure<F, Failure>, FailureBase::template overloads<F>
{
constexpr overloads()
{}
using detail::traverse_failure<F, Failure>::operator();
using FailureBase::template overloads<F>::operator();
};
};
template<class Failure>
struct failures<Failure>
{
template<class Transform>
BOOST_HOF_USING(transform, with_failures<detail::transform_failures<Failure, Transform>>);
template<class F>
BOOST_HOF_USING(overloads, detail::traverse_failure<F, Failure>);
};
template<class Transform, class... Fs>
struct failure_map
: with_failures<detail::transform_failures<get_failure<Fs>, Transform>...>
{};
template<class... Fs>
struct failure_for
: with_failures<get_failure<Fs>...>
{};
template<class F, class Base=detail::callable_base<F>>
struct reveal_adaptor
: detail::traverse_failure<Base>, Base
{
typedef reveal_adaptor fit_rewritable1_tag;
using detail::traverse_failure<Base>::operator();
using Base::operator();
BOOST_HOF_INHERIT_CONSTRUCTOR(reveal_adaptor, Base);
};
// Avoid double reveals, it causes problem on gcc 4.6
template<class F>
struct reveal_adaptor<reveal_adaptor<F>>
: reveal_adaptor<F>
{
typedef reveal_adaptor fit_rewritable1_tag;
BOOST_HOF_INHERIT_CONSTRUCTOR(reveal_adaptor, reveal_adaptor<F>);
};
BOOST_HOF_DECLARE_STATIC_VAR(reveal, detail::make<reveal_adaptor>);
}} // namespace boost::hof
#endif