Monads/Maybe monad: Difference between revisions
Added C++ implementation
m (Respect right-identity law) |
(Added C++ implementation) |
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{missing value, missing value, missing value, missing value, 0.5, 0.0, -0.346573590279, -0.499999999999, -0.549306144333, -0.69314718056, -0.804718956217}</pre>
=={{header|C++}}==
<lang cpp>#include <iostream>
#include <cmath>
#include <optional>
#include <vector>
using namespace std;
// std::optional can be a maybe monad. Use the >> operator as the bind function
template <typename T>
auto operator>>(const optional<T>& monad, auto f)
{
if(!monad.has_value())
{
// Return an empty maybe monad of the same type as if there
// was a value
return optional<remove_reference_t<decltype(*f(*monad))>>();
}
return f(*monad);
}
// The Pure function returns a maybe monad containing the value t
auto Pure(auto t)
{
return optional{t};
}
// A safe function to invert a value
auto SafeInverse(double v)
{
if (v == 0)
{
return optional<decltype(v)>();
}
else
{
return optional(1/v);
}
}
// A safe function to calculate the arc cosine
auto SafeAcos(double v)
{
if(v < -1 || v > 1)
{
// The input is out of range, return an empty monad
return optional<decltype(acos(v))>();
}
else
{
return optional(acos(v));
}
}
// Print the monad
template<typename T>
ostream& operator<<(ostream& s, optional<T> v)
{
s << (v ? to_string(*v) : "nothing");
return s;
}
int main()
{
// Use bind to compose SafeInverse and SafeAcos
vector<double> tests {-2.5, -1, -0.5, 0, 0.5, 1, 2.5};
cout << "x -> acos(1/x) , 1/(acos(x)\n";
for(auto v : tests)
{
auto maybeMonad = Pure(v);
auto inverseAcos = maybeMonad >> SafeInverse >> SafeAcos;
auto acosInverse = maybeMonad >> SafeAcos >> SafeInverse;
cout << v << " -> " << inverseAcos << ", " << acosInverse << "\n";
}
}
</lang>
{{out}}
<pre>
x -> acos(1/x) , 1/(acos(x)
-2.5 -> 1.982313, nothing
-1 -> 3.141593, 0.318310
-0.5 -> nothing, 0.477465
0 -> nothing, 0.636620
0.5 -> nothing, 0.954930
1 -> 0.000000, nothing
2.5 -> 1.159279, nothing
</pre>
=={{header|Clojure}}==
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