Monads/Writer monad: Difference between revisions

=={{header|C++}}==

<lang cpp>#include <cmath>

#include <iostream>

#include <string>

using namespace std;

// Use a struct as the monad

struct LoggingMonad

{

double Value;

string Log;

};

// Use the >> operator as the bind function

auto operator>>(const LoggingMonad& monad, auto f)

{

auto result = f(monad.Value);

return LoggingMonad{result.Value, monad.Log + "\n" + result.Log};

}

// Define the three simple functions

auto Root = [](double x){ return sqrt(x); };

auto AddOne = [](double x){ return x + 1; };

auto Half = [](double x){ return x / 2.0; };

// Define a function to create writer monads from the simple functions

auto Writer = [](auto f, string message)

{

return [=](double x){return LoggingMonad(f(x), message);};

};

// Derive writer versions of the simple functions

auto writerRoot = Writer(Root, "Taking square root");

auto writerAddOne = Writer(AddOne, "Adding 1");

auto writerHalf = Writer(Half, "Dividing by 2");

int main()

{

// Compose the writers to compute the golden ratio

auto result = LoggingMonad{5, "Starting with 5"} >> writerRoot >> writerAddOne >> writerHalf;

cout << result.Log << "\nResult: " << result.Value;

}

</lang>

{{out}}

<pre>

Starting with 5

Taking square root

Adding 1

Dividing by 2

Result: 1.61803

</pre>