FunctionalPlus

helps you write concise and readable C++ code.

Introduction

Great code should mostly be self-documenting, but while using C++ in reality you can find yourself dealing with low-level stuff like iterators or hand-written loops that distract from the actual essence of the your code.

FunctionalPlus is a small header-only library supporting you in reducing code noise and in dealing with only one single level of abstraction at a time. By increasing brevity and maintainability of your code it can improve productivity (and fun!) in the long run. It tries to achieve these goals by providing easy-to-use functions that free you from implementing commonly used flows of control.

Say you have a list of numbers and are interested in the odd ones only.

bool is_odd(int x) { return x % 2 == 1; }
int main()
{
    typedef list<int> Ints;
    Ints numbers = { 24, 11, 65, 44, 80, 18, 73, 90, 69, 18 };
    // todo: get odd values from numbers ...
}

There are different possiblities to obtain your goal. Some of them are:

1. write a (range based) for loop (ommitting the curly braces)

    Ints odds;
    for (int x : numbers)
        if (is_odd(x))
            odds.push_back(x);

2. use std::copy_if from the STL

    Ints odds;
    std::copy_if(std::begin(numbers), std::end(numbers),
            std::back_inserter(odds), is_odd);

3. use keep_if from FunctionalPlus

    auto odds = fplus::keep_if(is_odd, numbers);

If you think version 3 could be the one most pleasant to work with, you might like FunctionalPlus. And if you still think the hand-written for loop is easier to understand, also consider what would happen if the loop body (i.e. a corresponding lambda function in the call to fplus::keep_if) would be much longer. When reading keep_if you would still immediately know that odds can only contain elements that came from numbers and were selected by some, possibly complicated, predicate. In the for loop case you have no idea what is happening until you read the whole loop body. The loop version probably would need a comment at the top stating what the use of keep_if would tell at first glance.

More examples

Below you find some short examples showing nice things you can do with functions and containers using FunctionalPlus.

The same old song

You can test the content of a container for various properties, e.g.

#include "fplus.h"
#include <iostream>
int main()
{
    std::list<std::string> things = {"same old", "same old"};
    if (fplus::all_the_same(things))
        std::cout << "All things being equal." << std::endl;
}

The I in our team

There also are some convenience functions for strings.

#include "fplus.h"
#include <iostream>
int main()
{
    std::string team = "Our team is great. I love everybody.";
    if (fplus::contains("I", fplus::split_words(team)))
        std::cout << "There actually is an I in team." << std::endl;
}

Silent Night

Predicates can be combined logically in a functional way.

#include "fplus.h"
#include <iostream>
struct Entity
{
    Entity() : calm_(true), bright_(true) {}
    bool calm_;
    bool bright_;
};

int main()
{
    auto isCalm = [](const Entity& e) { return e.calm_; };
    auto isBright = [](const Entity& e) { return e.bright_; };
    std::vector<Entity> entities(4);
    if (fplus::all_by(fplus::logical_and(isCalm, isBright), entities))
        std::cout << "Silent night." << std::endl;
}

all_by is a function that takes a unary predicate and checks if all elements in the container fulfill it. logical_and simply combines two unary predicates to one.

Transformations, function composition and binding

Let's say you have the following function given.

std::list<std::uint64_t> collatz_seq(std::uint64_t x);

And you want to create an std::map<std::uint64_t, std::string> containing string representations of the Collatz sequences for all numbers below 30. You can implement this nicely in a functional way too.

#include "fplus.h"
#include <iostream>

// std::list<std::uint64_t> collatz_seq(std::uint64_t x) { ... }

int main()
{
    typedef std::list<uint64_t> Ints;

    // [1, 2, 3 ... 29]
    auto numbers = fplus::generate_integral_range<Ints>(1, 30);

    // A function that does [1, 2, 3, 4, 5] -> "[1 => 2 => 3 => 4 => 5]"
    auto show_ints = fplus::bind_1_of_2(fplus::show_cont_with<Ints>, " => ");

    // A composed function that calculates a Collatz sequence and shows it.
    auto show_collats_seq = fplus::compose(collatz_seq, show_ints);

    // Apply it to all our numbers.
    auto seq_strs = fplus::transform(show_collats_seq, numbers);

    // Combine the numbers and their sequence representations into a map.
    auto collatz_dict = fplus::create_map(numbers, seq_strs);

    // Print some of the sequences.
    std::cout << collatz_dict[13] << std::endl;
    std::cout << collatz_dict[17] << std::endl;
}

Output:

[13 => 40 => 20 => 10 => 5 => 16 => 8 => 4 => 2 => 1]
[17 => 52 => 26 => 13 => 40 => 20 => 10 => 5 => 16 => 8 => 4 => 2 => 1]

The functions shown not only work with default STL containers like std::vector, std::list, std::deque, std::string etc., but also with custom containers providing a similar interface.

Type deduction and useful errors messages

FunctionalPlus deduces types for you where possible. Let's take one line of code from the Collatz example:

    auto show_collats_seq = fplus::compose(collatz_seq, show_ints);

collatz_seq is a Function taking an uint64_t and returning a list<uint64_t>. show_ints takes a list<uint64_t> and returns a string. Thanks to making use of function_traits written by kennyim it is possible to automatically deduce the expression fplus::compose(collatz_seq, show_ints) being a function taking an uint64_t and returning a string, so you do not have to manually provide type hints to the compiler.

In case you would accidentally pass two functions whose "connecting type" does not match, you will get a nice error message telling you exactly that, because FunctionalPlus uses compile time assertions where feasible to guard you from the sometimes confusingly long error messages compilers like to generate when faced with type errors in function templates.

By changing the way you think about programming from "writing your own loops and nested ifs" to "using and composing small functions" you will first perhaps get more errors at compile time, but this will pay out in having fewer errors at runtime and in spending less time debugging.

Finding the right functions

The library is splitted into several header files, but all functions live in namespace fplus. If you for example need a function splitting a sequence at elements with a specific property, use your IDE to auto-complete fplus::split... or just look into split.h and you will soon find fplus::split_by doing exactly this: split_by(is_even, true, [1,3,2,2,5,5,3,6,7,9]) == [[1,3],[],[5,5,3],[7,9]]

Performance

The basic functions are fast, thanks to C++'s concept of abstraction without overhead. Here are some measurements from the first example, taken on a standard desktop PC, compiled with GCC and the O3 flag.

10000 random numbers, keep odd ones, 1000 consecutive runs accumulated
----------------------------------------------------------------------

| Hand-written for loop | std::copy_if | fplus::keep_if |
|-----------------------|--------------|----------------|
|               0.468 s |      0.475 s |        0.463 s |

So the compiler seems to do a very good job in optimizing and inlining everthing to basically equal machine code performance-wise.

The more complex functions though can probably be written in a more optimized/optimizable way. Sometimes they are not even in the best possible time complexity class. If you use FunctionalPlus in a performance-critical scenario and profiling shows you need a faster version of a function please let me know or even help improving FunctionalPlus.

Additionally keep in mind that FunctionalPlus always produces copies and never operates in place. For example in the code for "The I in our team" there is this line:

if (fplus::contains("I", fplus::split_words(team)))

It first splits the whole string team into words and only then checks if it contains an "I". A hand-written version could be faster, because it could stop the splitting as soon as it encounters the word it is looking for. But since in my experience even in performance-critical software the vast majority of parts is not relevant for the overall performance, I think it is a good idea to strive for developer productivity and readability of code as the default. In many cases one can easily switch to something else later if it turns out to be needed.

Installation/Requirements

Just download FunctionalPlus and tell your compile to use the include directory.

A C++14-compatible compiler is needed. The tests run successfully on GCC 4.9, Clang 3.6 and Visual C++ 2015.

Disclaimer

The functionality in this library initially grew due to my personal need for it while using C++ on a regular basis. I try my best to make it error free and as comfortable to use as I can. It is still in an experimental state, and the API is likely to change in the future. If you have any suggestions, find errors, miss some functions or want to give general feedback/criticism, I'd love to hear from you. Of course, contributions are also very welcome.

License

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)