feat: add Y combinator

Closes #130

src/Y.js

@@ -0,0 +1,30 @@
+/**
+ * Y-combinator
+ *
+ * The Y combinator is an interesting function which only works with functional languages,
+ * showing how recursion can still be done even without any variable or function declarations,
+ * only functions and parameters
+ *
+ * @func Y
+ * @memberOf RA
+ * @since {@link https://char0n.github.io/ramda-adjunct/2.3.0|v2.3.0}
+ * @category Function
+ * @sig (a, ... -> b -> b) -> (a, ... -> b)
+ * @param {Function} le Recursive function maker
+ * @return {Function}
+ * @see {@link http://kestas.kuliukas.com/YCombinatorExplained/|Y combinator explained}
+ * @example
+ *
+ * const makeFact = givenFact => (n) => {
+ *   if (n < 2) { return 1 }
+ *   return n * givenFact(n - 1);
+ * };
+ *
+ * const factorial = RA.Y(makeFact);
+ *
+ * factorial(5); //=> 120
+ */
+
+const Y = le => (f => f(f))(g => le(x => (g(g))(x)));
+
+export default Y;

src/index.d.ts

@@ -659,6 +659,15 @@ declare namespace RamdaAdjunct {
         defaultWhen<DefVal, Val>(predicate: Function, defaultVal: DefVal): (val: Val) => DefVal | Val;
         defaultWhen(predicate: Function): <DefVal, Val>(defaultVal: DefVal) => (val: Val) => DefVal | Val;
 
+        /**
+         * Y-combinator
+         *
+         * The Y combinator is an interesting function which only works with functional languages,
+         * showing how recursion can still be done even without any variable or function declarations,
+         * only functions and parameters
+         */
+        Y(le: Function): Function;
+
          /**
          * Identity type.
          */

src/index.js

@@ -80,6 +80,7 @@ export { default as curryRightN } from './curryRightN';
 export { default as curryRight } from './curryRight';
 export { default as resolveP } from './resolveP';
 export { default as rejectP } from './rejectP';
+export { default as Y } from './Y';
 // List
 export { default as pickIndexes } from './pickIndexes';
 export { default as list } from './list';

test/Y.js

@@ -0,0 +1,16 @@
+import * as RA from '../src/index';
+import eq from './shared/eq';
+
+
+describe('Y', function() {
+  it('tests for making a factorial function', function() {
+    const makeFact = givenFact => (n) => {
+      if (n < 2) { return 1 }
+      return n * givenFact(n - 1);
+    };
+
+    const factorial = RA.Y(makeFact);
+
+    eq(factorial(5), 120);
+  });
+});