matux · July 21, 2022 15:28 · Jul 21, 2022
diff --git a/endocompo.dart b/endocompo.dart
@@ -0,0 +1,34 @@
+// Here be dragons //
+extension _EndomorphicComposition<T> on T Function(T) {
+  /// Mathematical composition of endomorphisms.
+  ///
+  /// In mathematics, function composition is an operation ∘ that takes two
+  /// functions ƒ and g, and produces a function h = g ∘ ƒ such that
+  /// h(x) = g(ƒ(x)).
+  ///
+  /// An endomorphism is any morphism ƒ whose domain is equal to its codomain
+  /// such that ƒ: a -> a or T ƒ(T)
+  T Function(T) operator <<(T Function(T) f) => (x) => f(this(x));
+
+  /// Forward composition of endomorphisms.
+  T Function(T) operator >>(T Function(T) f) => (x) => this(f(x));
+}
+
+int plusOne(int x) => x + 1;
+int mulTwo(int x) => x * 2;
+
+void thing() {
+  final xs = [1, 2, 3, 4];
+  final ys = xs.map(plusOne >> mulTwo); // [4, 6, 8, 10]
+  final zs = xs.map(plusOne << mulTwo); // [3, 5, 7, 9]
+  print('$ys\n$zs');
+
+  // imperatively
+  var yys = <int>[];
+  for (final x in xs) {
+    // yys.add(mulTwo(plusOne(x)));
+    final y = plusOne(x);
+    final z = mulTwo(y);
+    yys.add(z);
+  }
+}