laocheng / fit

C++ function library

Overview

Fit is a header-only C++11 library that provides utilities for functions and function objects.

Fit is:

• Modern: Fit takes advantages of modern C++11/C++14 features. It support both constexpr initialization and constexpr evaluation of functions. It takes advantage of type deduction, varidiac templates, and perfect forwarding to provide a simple and modern interface.
• Relevant: Fit provides utilities for functions and does not try to implement a functional language in C++. As such, Fit solves many problems relevant to C++ programmers, including initialization of function objects and lambdas, overloading with ordering, improved return type deduction, and much more.
• Lightweight: Fit builds simple lightweight abstraction on top of function objects. It does not require subscribing to an entire framework. Just use the parts you need.

Fit is divided into three components:

• Function Adaptors: These take functions and return a new function that provides an additional capability to the previous function.
• Functions: These return functions that achieve a specific purpose.
• Utilities: These are general utilities that are useful when defining or using functions

Quick Start

Function Objects

In C++, a function object is just a class that overrides the call operator like this:

// A sum function object
struct sum_f
{
    template<class T, class U>
    auto operator()(T x, U y) const
    {
        return x + y;
    }
};

There are few things to note about this. First, the call operator member function is always declared const, which is generally required to be used with Fit.(Note: The mutable_ adaptor can be used to make a mutable function object have a const call operator, but this should generally be avoided). Secondly, the sum_f class must be constructed first before it can be called:

auto three = sum_f()(1, 2);

We can make it behave like a regular function if we construct the class as a global variable. However, we want to statically initialize it(ie initialize it at compile-time) to avoid the static initialization order fiasco. We can do that using constexpr like this:

const constexpr sum_f sum = {};

Adaptors

Now we have defined the function as a function object we can add new "enhancements" to the function. We could make the function pipable using the pipable adaptor:

const constexpr pipable_adaptor<sum_f> sum = {};

So it could be called like this:

auto three = 1 | sum(2);

Or we could make it an infix named operator as well using the infix adaptor:

const constexpr infix_adaptor<sum_f> sum = {};

And it could be called like this:

auto three = 1 <sum> 2;

Additionally each of the adaptors have a corresponding function version without the _adaptor suffix. So we could pass sum as a variable to the adaptors to make new functions. So we can do things like partial application and function composition if we wanted to:

auto add_1 = partial(sum)(1);
auto add_2 = compose(add_1, add_1);
auto three = add_2(1);

Lambdas

Instead of writing function objects which can be a little verbose, we can write the functions as lambdas instead. However, by default lambdas cannot be statically initialized at compile time. So we can use the FIT_STATIC_LAMBDA to initialize them at compile time:

const constexpr auto sum = FIT_STATIC_LAMBDA(auto x, auto y)
{
    return x + y;
};

And we can apply the same adaptors as well:

// Pipable sum
const constexpr auto sum = pipable(FIT_STATIC_LAMBDA(auto x, auto y)
{
    return x + y;
});

We can also statically initialize the lambda after the adaptors have been applied by using FIT_STATIC_FUNCTION:

// Pipable sum
FIT_STATIC_FUNCTION(sum) = pipable([](auto x, auto y)
{
    return x + y;
});

As we will see, this can help make it cleaner when we are defining several lambdas, such as for overloading.

Overloading

Now, Fit provides two ways of doing overloading. The match adaptor will call a function based on C++ overload resolution, which tries to find the best match, like this:

FIT_STATIC_FUNCTION(print) = match(
    [](int x)
    {
        std::cout << "Integer: " << x << std::endl;
    },
    [](const std::string& x)
    {
        std::cout << "String: " << x << std::endl;
    }
);

However, when trying to do overloading involving something more generic, it can lead to ambiguities. So the conditional adaptor will pick the first function that is callable. This allows ordering the functions based on which one is more important. So if we wanted to write a function to print all the values in a type including primitive types as well as ranges, we could write something like this:

#define REQUIRES(...) typename std::enable_if<(__VA_ARGS__), int>::type = 0

FIT_STATIC_FUNCTION(print) = conditional(
    [](auto x, REQUIRES(std::is_fundamental<decltype(x)>()))
    {
        std::cout << x << std::endl;
    },
    [](const std::string& x)
    {
        std::cout << x << std::endl;
    },
    [](const auto& range)
    {
        for(const auto& x:range) 
            std::cout << x << std::endl;
    }
);

We constraint the first lambda to just fundamental types using REQUIRES. The last lambda we assume it is a range(we should check if it is a range but that is beyond the scope of this guide), and it is only called if the first two lambdas cannot be called(ie the type is not a fundamental nor a string).

Recursive

Additionally, we could go a step further and make the print function recursive. We can use the fix adaptor, which implements a fix point combinator. So the first parameter of the function will be the function itself:

FIT_STATIC_FUNCTION(print) = fix(conditional(
    [](auto, auto x, REQUIRES(std::is_fundamental<decltype(x)>()))
    {
        std::cout << x << std::endl;
    },
    [](auto, const std::string& x)
    {
        std::cout << x << std::endl;
    },
    [](auto self, const auto& range)
    {
        for(const auto& x:range) self(x);
    }
));

Requirements

This requires a C++11 compiler. There a no third-party dependencies. This has been tested on clang 3.4 and gcc 4.6-4.9.