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reveal.hpp
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/*============================================================================= 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