Functional Programming in C++

Functional Programming in C++

Can someone guide me how do functional programming in C++? Is there some good online material that I can refer?

Please note that I know about the library FC++. I want to know how to do that with C++ standard library alone.

Thanks.

Answer by Eli Bendersky for Functional Programming in C++

Update August 2014: This answer was posted in 2009. C++11 improved matters considerably for functional programming in C++, so this answer is no longer accurate. I'm leaving it below for a historical record.

Since this answer stuck as the accepted one - I'm turning it into a community Wiki. Feel free to collaboratively improve it to add real tips on function programming with modern C++.

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You can not do true functional programming with C++. All you can do is approximate it with a large amount of pain and complexity (although in C++11 it's a bit easier). Therefore, this approach isn't recommended. C++ supports other programming paradigms relatively well, and IMHO should not be bent to paradigms it supports less well - in the end it will make unreadable code only the author understands.

Answer by Javier for Functional Programming in C++

I don't think that you can't to true, real, functional programming in C++; but it's certainly not the easiest or natural way to use it. Also, you might just use a couple of functional-like idioms and not the whole mindset (i.e. 'fluent style')

My advise would be to learn a functional language, maybe start with Scheme, then move to Haskell. Then use what you've learned when programming in C++. maybe you won't use an obvious functional style; but you might get the biggest advantages (i.e. using immutable structures).

Answer by Derrick Turk for Functional Programming in C++

You can accomplish a surprising amount of "functional programming" style with modern C++. In fact, the language has been trending in that direction since its' standardization.

The standard library contains algorithms analogous to map, reduce, etc (for_each, transform, adjacent_sum...). The next revision, C++0x, contains many features designed to let programmers work with these in a more functional style (lambda expressions, etc.).

Look into the various Boost libraries for more fun. Just to illustrate that standard C++ contains plenty of functional goodness, here's a factorial function in continuation-passing style in standard C++.

#include     // abstract base class for a continuation functor  struct continuation {      virtual void operator() (unsigned) const = 0;  };    // accumulating continuation functor  struct accum_cont: public continuation {      private:          unsigned accumulator_;          const continuation &enclosing_;      public:          accum_cont(unsigned accumulator, const continuation &enclosing)              : accumulator_(accumulator), enclosing_(enclosing) {};           virtual void operator() (unsigned n) const {              enclosing_(accumulator_ * n);          };  };    void fact_cps (unsigned n, const continuation &c)  {      if (n == 0)          c(1);      else          fact_cps(n - 1, accum_cont(n, c));  }    int main ()  {      // continuation which displays its' argument when called      struct disp_cont: public continuation {          virtual void operator() (unsigned n) const {              std::cout << n << std::endl;          };      } dc;        // continuation which multiplies its' argument by 2      // and displays it when called      struct mult_cont: public continuation {          virtual void operator() (unsigned n) const {              std::cout << n * 2 << std::endl;          };      } mc;        fact_cps(4, dc); // prints 24      fact_cps(5, mc); // prints 240        return 0;  }  

Ok, I lied a little bit. It's a factorial functor. After all, closures are a poor man's objects... and vice versa. Most of the functional techniques used in C++ rely on the use of functors (i.e. function objects)---you'll see this extensively in the STL.

Answer by rvirding for Functional Programming in C++

There is a book called Functional C by Pieter Hartel and Henk Muller which might help. If it still available. A link to some info on it is here. IIRC it wasn't too bad.

Answer by daven11 for Functional Programming in C++

Probably a bit late but for anyone else looking - I use lua as a functional programming extension to C++ and it's great. lua

Answer by GAS for Functional Programming in C++

Consider my 3 research projects:

• Functional and Declarative Design in C++. (GitHub) (Slides (Rus)) (Talk (Rus))

This project is a working prototype of 'Amber' game. The code demonstrates many of major functional concepts: immutability, lambdas, monads, combinators, pure functions, declarative code design. It uses Qt C++ and C++11 features.

For a quick example, see how tasks can be chained into one big task that will modify the Amber's world when it's applied:

const AmberTask tickOneAmberHour = [](const amber::Amber& amber)  {      auto action1Res = magic::anyway(inflateShadowStorms, magic::wrap(amber));      auto action2Res = magic::anyway(affectShadowStorms, action1Res);      auto action3Res = magic::onFail(shadowStabilization, action2Res);      auto action4Res = magic::anyway(tickWorldTime, action3Res);      return action4Res.amber;  };  

• Lenses in C++. (GitHub) (Slides (Eng)) (Talk (Rus))

This is a showcase of generic functional lenses in C++. The implementatoin is built with using of Variadic Templates, some intresting (and valid) C++ hacks to make lenses composable and neat-looking. The library is just a demo for the talk and thus it provides only a few of most important combinators, namely: set(), view(), traverse(), bind(), infix literal combinator to, over() and other.

(Note that there exist the 'C++ Lenses' project: but it's not about real 'lenses', it's about class properties with getters and setters in sense of C# or Java properties.)

Quick example

Car car1 = {"x555xx", "Ford Focus", 0, {}};  Car car2 = {"y555yy", "Toyota Corolla", 10000, {}};    std::vector cars = {car1, car2};    auto zoomer = traversed() to modelL();    std::function variator = [](std::string) { return std::string("BMW x6"); };  std::vector result = over(zoomer, cars, variator);    QVERIFY(result.size() == 2);  QVERIFY(result[0].model == "BMW x6");  QVERIFY(result[1].model == "BMW x6");  

• Functional 'Life': parallel celullar automata and comonads. (GitHub) (Slides (Eng)) (Talk (Rus))

You have probably heard about monads. Monads are everywhere in talks about functional programming now. It's a buzzword. But what about comonads? I presented 1D and 2D celullar automata with the concept of comonads under the hood. The aim was to show how easy it is to move from the single-flow code to the parallel one using std::future as a Par monad. The project also benchmarks and compares two these approaches.

Quick example

template   UUB fmap(      const func& f,      const UUUUA& uuu)  {      const func f2 = [=](const UUUA& uuu2)      {          UB newUt;          newUt.position = uuu2.position;          newUt.field = fp::map(f, uuu2.field);          return newUt;      };        return { fp::map(f2, uuu.field), uuu.position };  }  

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