kleisli.cpp

#include <cmath>
#include <functional>
#include <iostream>

using std::function;
using std::cout;
using std::endl;

template<class A> class optional {
    bool _isValid;
    A _value;
public:
    optional() : _isValid(false) {}
    optional(A v) : _isValid(true), _value(v) {}
    bool isValid() const { return _isValid; }
    A value() const { return _value; }
    void print() {
        if (_isValid) {
            cout << _value << endl;
        } else {
            cout << "invalid!" << endl;
        }
    }
};

optional<double> safe_root(double x) {
    if (x >= 0) return optional<double>{sqrt(x)};
    else return optional<double>{};
}

optional<double> safe_reciprocal(double x) {
    if (x != 0) return optional<double>{1/x};
    else return optional<double>{};
}

template<class A, class B, class C>
function<optional<C>(A)> compose(function<optional<B>(A)> m1,
                                 function<optional<C>(B)> m2)
{
    return [m1, m2](A a) {
        auto p1 = m1(a);
        if (!p1.isValid()) {
            return optional<C>{};
        }

        auto p2 = m2(p1.value());
        if (!p2.isValid()) {
            return optional<C>{};
        }

        return optional<C>(p2.value());
    };
}

template<class A>
optional<A> identity(A a) {
    return optional<A>(a);
}

int main() {
    auto c = compose<double, double, double>(identity<double>, safe_root);
    auto d = compose<double, double, double>(c, safe_reciprocal);
    auto r1 = d(4);
    auto r2 = d(-1);

    r1.print();
    r2.print();
}