How do I get the value OUT of my maybe<> monad? - c++

Question

For educational reasons, I'm trying to implement a maybe monad in C++14. My (perhaps overly simplistic) understanding of monads is that they let you define a computation as a series of composable function calls. The wikipedia article on monads calls them "programmable semicolons" because they let you define what happens between what would otherwise be a set of discreet function calls. The maybe monad is a monad that interrupts computation if a failure occurs.
template<class T>
struct maybe
{
maybe( const T& t ) : argument( t ), valid( true ) {}
maybe() : argument(), valid( false ) {}
T argument;
bool valid;
};
template<class T>
maybe<T> just( const T& t ) { return maybe<T>(t); }
template<class T>
maybe<T> nothing() { return maybe<T>(); }
auto terminal_maybe = [] ( auto term ) {
return [=] ( auto func ) {
return func( term );
};
};
auto fmap_maybe = [] ( auto f ) {
return [=] ( auto t ) {
if( t.valid ) {
try {
t.argument = f( t.argument );
printf("argument = %d\n",t.argument);
}
catch(...) {
t.valid = false;
}
}
return (t.valid) ? terminal_maybe( just( t.argument ) ) : terminal_maybe( nothing<decltype(t.argument)>() );
};
};
int main( int argc, char* argv[] )
{
auto plus_2 = [] ( auto arg ) { return arg + 2; };
auto minus_2 = [] ( auto arg ) { return arg - 2; };
maybe<int> forty = just(40);
terminal_maybe(forty)
(fmap_maybe( plus_2 ))
(fmap_maybe( plus_2 ));
printf("result = %d\n",forty.argument);
return 0;
}
As you can see I am super close! I can chain multiple calls together monadically (and I can tell from printf that my value does what I expect (increments from 40 to 42 and then from 42 to 44)). The problem is that I have no way to get the final value OUT! I tried making terminal_maybe accept a reference (auto&) and that forced me to modify fmap's return statement (to just return terminal_maybe( t ) rather than a new maybe). But it still didn't have the correct value for the final printf.

Answer 1

This works, but I don't know if it makes sense from a FP point of view.
auto unwrap = [](auto const &f) {
return f;
};
int main( int argc, char* argv[] )
{
auto plus_2 = [] ( auto arg ) { return arg + 2; };
auto minus_2 = [] ( auto arg ) { return arg - 2; };
maybe<int> forty = just(40);
auto const &outv = terminal_maybe(forty)
(fmap_maybe( plus_2 ))
(fmap_maybe( plus_2 ))
(unwrap);
std::printf("result = %d\n",outv.argument);
return 0;
}