boost.tmp (Tacit Meta Programming)

Warning! still work in progress

I decided to rewrite all of kvasir::mpl on a slightly different paradigm to extend compiler support further back and get some more speed out of low arity calls. Therefore most of this code has seen little or no testing beyond the skimpy unit test coverage so far. Bug reports welcome, do not use this in anything you don't want to see blow up spectacularly... yet.

What is boost.tmp?

A (not yet submitted to) boost general purpose Template Meta Programming (TMP) library which covers the domains of boost.MPL, boost.Fusion, boost.mp11 and boost.hana.

Why do we need another metaprogramming library in boost?

There are a few reasons:

• a tacit DSL as the public interface allows for arbitrarily complex (almost) zero cost compositions of higher-order functions unlike current solutions
• pattern matching, encapsulation of expertise and leverage of intrinsics often can yield even faster compile times than code handwritten by experts
• the interface is lazy/short-circuiting by default which removes the need for things like std::conjunction, std::disjunction etc. and makes SFINAE friendly code much easier to write, understand and teach
• (still experimental) capture and monadic propagation of SFINAE failure allows for potentially more expressive, more intuitive and sometimes faster code in SFINAE contexts
• (still experimental) heterogeneous l-, r-value reference support (as opposed to just values in hana)
• (still experimental) heterogeneous (fusion) support in c++11
• The tacit DSL nature of the interface enables much of the inner workings to be left undefined allowing us to swap out the back end for either legacy or future support. This should (grain of salt here) allow code written for a c++11 compiler to benefit from c++23 (I hope) work of SG7 (just implement the call_ alias such that it passes everything on to the newfangled constexpr monster) without the need to change user code.

core concepts

existing concepts

Allow me to do a short review of existing metaprogramming concepts. If you are familiar with these feel free to skip ahead to new concepts.

template specialization

This is starting at the very beginning, skip ahead to lazy metafunctions if you know what class templates are.

The workhorse of C++ metaprogramming is a language feature called class templates and template specialization.

template<typename T>
struct remove_const{              //generic template

};

Above we have defined a class template named remove_const which takes one type parameter named T. This T parameter can be thought of a wildcard which can represent any type which the user passed in. We can pass in a parameter by naming it in a pair of trailing angle brackets:

remove_const<int> a{};    //note that this does not actually remove const (yet)

The above code defines a variable named a of type remove_const<int>. It should be noted that the type of a template instance consists of not only the template but all of its parameters. Therefore remove_const<int> and remove_const<bool> are two distinct types (or even remove_const<const int> for that matter). In order to make our remove_const<T> actually do what we intend we need to be able to do different things depending on whether the input is const or not. The right tool for the job, or the tool of choice for almost any kind of decision making or introspection in TMP for that matter, is called partial specialization.

template<typename T>
struct remove_const{              	//definition

};
template<typename T>
struct remove_const<const T>{     //partial specialization

};

template<typename T>
struct remove_const{                  //definition

};
template<typename T>
struct remove_const<const T>{     //partial specialization 1

};
template<typename T>
struct remove_const<const volatile T>{     //partial specialization 2

};
remove_const<int> a; //does not match 1 or 2
remove_const<const int> b; //matches 1 and not 2
remove_const<volatile const int> c; //matches 1 and 2

Above we can see that a only matches the generic definition, b matches specialization 1 and c matches both specializations. In the case of c specialization resolution is however not ambiguous because the pattern const volatile T can only match a smaller subset of all types compared to const T and is therefore considered more specialized. More specialized templates are considered a better match than less specialized templates.

lazy metafunctions

Lazy metafunctions are implemented using class templates.

template<typename T>
struct remove_const{              //generic template
    using type = T;
};
template<typename T>
struct remove_const<const T>{     //specialization
    using type = T;
};

Above we are using class template specialization in order to implement a metafunction which removes const from its input parameter type. By adding a member alias named "type" (the name type is a convention, we could have named it anything) we can return a type from this metafunction. In this case, the "type" is an alias to the input type which, in the specialization case, has its const qualifier stripped. This metafunction is lazy in that the instantiation (think 'evaluation' or 'making the damn thing') only happens when we actually call the metafunction. We can call a lazy metafunction by referring to its nested "type" alias.

template<typename T>
typename remove_const<T>::type foo(T& a);  

or more to the point

typedef remove_const<T> thing;    //not instantiated yet
typename thing<int>::type i;                      //instantiated here

Sadly above we must use the typename disambiguator in order to tell the compiler that the name "type" on the right-hand side of the :: is, in fact, a type and not a value, member function, template etc. This is one of the drawbacks of lazy metafunctions.

template<typename T>
struct remove_const<int>{     //crazy specialization for int
    void type();  //now "type" is a function
};
template<typename T>
struct remove_const<bool>{     //crazy specialization for bool
    int type;  //now "type" is a value
};
template<typename T>
struct remove_const<char>{     //crazy specialization for char
    template<int I>  //yes template parameters can also be values
    struct type{
        static int value = 1;
    };  //now "type" is a class template
};
template<typename T>
struct remove_const<long>{     //crazy specialization for long
    template<int I>  //yes template parameters can also be values
    int type();  //now "type" is a function template
};

As we can see above the name "type" could be any number of things and its not always clear which one the user is expecting. The default assumes that the user is expecting a nested name (think the thing on the right-hand side of a :: operator) of a class template to be a value. If we are expecting a function the compiler is also able to figure it out. If we are expecting a type rather than a value or function we need to prefix the expression with the typename disambiguator. If we are expecting a template for that matter we need to use the template disambiguator.

int a = add_const<bool>::template type<1>::value; //template disambiguator tells the
                                                  //compoler that "type" is expected to be a template 

add_const<long> b{};                              //create instance of add_const<long>
int c = b.template type<2>();                     //call member function template "type"

eager metafunctions

C++11 gave us a few new tools, not only variadic templates but also alias templates.

template<typename T>
using add_const = const T;

or

template<typename T>
using remove_const_t = typename remove_const<T>::type;

One advantage to alias templates is that the compiler knows the result of an alias call is a type so we no longer need to use the typename disambiguator. It also saves us typing out ::type all over the place. One major drawback, however, is that it does not allow us to be lazy, this is actually quite subtle at first, in the SFINAE section we will dive deeper into why we cannot deprecate lazy metafunctions from our public interface.

Substitution Failure Is Not An Error (SFINAE)

SFINAE is a rule in C++ which applies to so-called SFINAE contexts, certain errors (see list here) in such contexts do not result in a compilation error (referred to as a hard error), rather, they result in the template specialization or function template in which the SFINAE invocation occurs to be removed from the set of specializations/overloads.

void f(...) {}   //bad match but will match anything
template<typename T>
typename T::type f(T){ return {}; } //better match but will be removed by SFINAE if T does not have a ::type

This relates to eager vs. lazy metafunctions in a subtle way. Template instantiations do not short-circuit in the same way normal C++ expressions do. For example (thing1::value || thing2::value) will instantiate thing2 regardless of whether thing1::value returned true. Metaprograms which actually short circuit in cases where our programming logic allows short-circuiting, not only have a potential efficiency gain they also save us from triggering SFINAE in an evaluation whos result is not needed because the logic should have short-circuited.

Looking at an admittedly somewhat exotic example, let's say we want to to make sure that either our second parameter is an allocator or that both parameters have a common type (think if(is_allocator(b) || has_common_type(a,b))).

template<typename T, typename U>
std::enable_if<(is_allocator<U>::value || std::common_type_t<T,U>::value)> f(T,U);   //failed attempt, 
					//both sides are evaluated so common_type_t will 
					//SFINAE even if std::is_allocator<U>::value == true
					//also common_type_t yields a type and not a ::value

template<typename T, typename U>
std::enable_if<
    std::conditional<
        is_allocator<U>::value, 
        std::true_type, 
        std::is_convertible< std::common_type<T,U> 
    >::type::type::type::value> f(T,U);  //common pattern but its infinitely ugly
					//there are surely other 'hacks' but nothing 
					//that's not ugly and confusing

As we can see writing short-circuiting capable metaprogramming logic is complex, confusing and more subtly relies entirely on lazy metafunctions. On a side note the above code snippet assumes that is_allocator exists, it is not part of the standard library. I will present a method of implementation for is_allocator as well as a new way to be short-circuiting later in new concepts.

higher order metafunctions

Attempts at arbitrarily composable higher order metafunction are as old as metaprogramming itself, boost.MPL is the oldest popular metaprogramming library to support the concept but it seems that every template metaprogramming library since has come with its own flavor. The basic concept introduced in boost.MPL is based entirely on lazy matafunctions

using result = transform<input_list,std::add_pointer<_1>>;

in the above example, the algorithm transform is passed an input sequence of types as its first parameter and a higher order metafunction as its second parameter. But wait you say, std::add_pointer<> is just a lazy metafunction, that's entirely correct, however, we are exploiting the fact that it is not immediately evaluated and a placeholder in order to create a kind of DSL. The fact is that this std::add_pointer<_1> is never instantiated as such but is 'transpiled' into the boost.MPL flavor of composed higher-order metafunctions which replaces the placeholder with the respective input type from the input list and instantiates that. Interestingly we can, almost, achieve arbitrarily complex compositions in this paradigm

using input_list = mpl::vector<int,bool,char>;
using result = transform<input_list,
    std::is_same<
        std::remove_const<std::remove_reference<_1>>,
        std::add_pointer<_1>>>;

is roughly equivalent to

using result = mpl::vector<
    typename std::is_same<
        typename std::remove_const<typename std::remove_reference<int>::type>::type,
        typename std::add_pointer<int>::type>::type,
    typename std::is_same<
        typename std::remove_const<typename std::remove_reference<bool>::type>::type,
        typename std::add_pointer<bool>::type>::type,
    typename std::is_same<
        typename std::remove_const<typename std::remove_reference<char>::type>::type,
        typename std::add_pointer<char>::type>::type>;

the 'transpiler' behind the scenes is actually quite powerfull and capable of flowing the branches of a tree-like composition. There are however some caveats: only lazy metafunctions are supported, 'wrappers' (thik putting things in a list) are supported through a SFINAE test which can cause subtle surprising behavior and things get really complicated as soon as one tries to use a higher order function in another higher order function.

Many other libraries have emerged since boost.MPL which have experimented with different kinds of higher metafunctions and composition, including my (in my opinion) failed attempt in the brigand library. Before kvasir none of them could support arbitrary complexity without a considerable performance penalty and many of them require duplication of the public interface in one form or another (like the lazy namespace in brigand or quoted and non-quoted interface in boost.mp11).

new concepts

metaclosures

Above I have argued that lazy metafunctions are often clunky, require disambiguators, can be a performance hit and are hard to use in short-circuiting contexts. At the same time, eager metafunctions do not compose as ergonomically as higher-order metafunctions and cannot be lazy or short-circuiting. For these reasons I came up with a new metaprogramming concept: the metaclosure. Metaclosures can be thought of as a function object with a set of 'fixed' or 'bound' arguments (think lambda capture) as well as a 'metafunction call interface' (think lambda function call operator). So far I have just been throwing made up words around so let's look at a concrete example:

using result = call_<    //we use the call_ alias to call a metaclosure
    is_<int>,        //is_ is a metaclosure which takes one fixed arguement
                     //(int in this case)
                     //and compares its to any arguement passed to it returning 
                     //true_ if the types are the same and false_ otherwise.
    int>;            //here we are passing an int to the metafunction call interface of 
                     //is_<int> resulting in (in other words call_ resolves to an alias to) true_.

using result2 = call_<is_<int>, bool>; //result2 is an alias to false_

Collaboration

I have heard from many developers who must roll their own metaprogramming library at work because their lawyers are afraid to use something open source because the ownership structure is unclear (push to a repository with a certain license does not automatically mean that you are agreeing that your contribution can be used under that license, there is also the question of whether it was, in fact, the original author who pushed the code etc. etc.). Whether or not this concern is merited is another question (and one I cannot answer as I am not a lawyer), but I have decided to retain sole authorship of this library until I can get a Contributor License Agreement (CLA) worked out which is up to snuff. If you have knowledge in this field help would be much appreciated. I would be very happy for bug reports. I will offer this library under the boost license indefinitely (and may offer it under other licenses if needed), it may seem counter-intuitive but my motivation to restrict contributions (for now) is actually motivated by the intent to allow more people to use it.