Monads/List monad: Difference between revisions

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 {{Out}}
 <lang AppleScript>{{3, 4, 5}, {5, 12, 13}, {6, 8, 10}, {7, 24, 25}, {8, 15, 17}, {9, 12, 15}, {12, 16, 20}, {15, 20, 25}}</lang>
+=={{header|C++}}==
+<lang cpp>#include <iostream>
+#include <vector>
+using namespace std;
+// std::vector can be a list monad.  Use the >> operator as the bind function
+template <typename T>
+auto operator>>(const vector<T>& monad, auto f)
+{
+    // Declare a vector of the same type that the function f returns
+    vector<remove_reference_t<decltype(f(monad.front()).front())>> result;
+    for(auto& item : monad)
+    {
+        // Apply the function f to each item in the monad. f will return a
+        // new list monad containing 0 or more items.
+        const auto r = f(item);
+        // Concatenate the results of f with previous results
+        result.insert(result.end(), begin(r), end(r));
+    }
+    return result;
+}
+// The Pure function returns a vector containing one item, t
+auto Pure(auto t)
+{
+    return vector{t};
+}
+// A function to double items in the list monad
+auto Double(int i)
+{
+    return Pure(2 * i);
+}
+// A function to increment items
+auto Increment(int i)
+{
+    return Pure(i + 1);
+}
+// A function to convert items to a string
+auto NiceNumber(int i)
+{
+    return Pure(to_string(i) + " is a nice number\n");
+}
+// A function to map an item to a sequence ending at max value
+// for example: 497 -> {497, 498, 499, 500}
+auto UpperSequence = [](auto startingVal)
+{
+    const int MaxValue = 500;
+    vector<decltype(startingVal)> sequence;
+    while(startingVal <= MaxValue)
+        sequence.push_back(startingVal++);
+    return sequence;
+};
+// Print contents of a vector
+void PrintVector(const auto& vec)
+{
+    cout << " ";
+    for(auto value : vec)
+    {
+        cout << value << " ";
+    }
+    cout << "\n";
+}
+// Print the Pythagorean triples
+void PrintTriples(const auto& vec)
+{
+    cout << "Pythagorean triples:\n";
+    for(auto it = vec.begin(); it != vec.end();)
+    {
+        auto x = *it++;
+        auto y = *it++;
+        auto z = *it++;
+        cout << x << ", " << y << ", " << z << "\n";
+    }
+    cout << "\n";
+}
+int main()
+{
+    // Apply Increment, Double, and NiceNumber to {2, 3, 4} using the monadic bind
+    auto listMonad =
+        vector<int> {2, 3, 4} >>
+        Increment >>
+        Double >>
+        NiceNumber;
+    PrintVector(listMonad);
+    // Find Pythagorean triples using the list monad.  The 'x' monad list goes
+    // from 1 to the max; the 'y' goes from the current 'x' to the max; and 'z'
+    // goes from the current 'y' to the max.  The last bind returns the triplet
+    // if it is Pythagorean, otherwise it returns an empty list monad.
+    auto pythagoreanTriples = UpperSequence(1) >>
+        [](int x){return UpperSequence(x) >>
+        [x](int y){return UpperSequence(y) >>
+        [x, y](int z){return (x*x + y*y == z*z) ? vector{x, y, z} : vector<int>{};};};};
+    PrintTriples(pythagoreanTriples);
+}
+</lang>
+{{out}}
+<pre>
+is a nice number
+is a nice number
+is a nice number
+Pythagorean triples:
+, 4, 5
+, 12, 13
+, 8, 10
+, 24, 25
+, 15, 17
+, 12, 15
+, 40, 41
+, 24, 26
+, 60, 61
+ . . .
+ . . .
+, 336, 464
+, 360, 485
+, 357, 493
+</pre>
 =={{header|Clojure}}==