[Ramda] Added memoizeWith definition (#24895)

* Ramda: added memoizeWith definition

* Ramda: added memoizeWith definition

* Ramda: added memoizeWith definition - narrowed generic

types/ramda/index.d.ts

@@ -17,6 +17,7 @@
 //                 Ethan Resnick <https://github.com/ethanresnick>
 //                 Jack Leigh <https://github.com/leighman>
 //                 Keagan McClelland <https://github.com/CaptJakk>
+//                 Tomas Szabo <https://github.com/deftomat>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.6
 
@@ -1027,6 +1028,13 @@ declare namespace R {
          */
         memoize<T = any>(fn: (...a: any[]) => T): (...a: any[]) => T;
 
+        /**
+         * A customisable version of R.memoize. memoizeWith takes an additional function that will be applied to a given
+         * argument set and used to create the cache key under which the results of the function to be memoized will be stored.
+         * Care must be taken when implementing key generation to avoid clashes that may overwrite previous entries erroneously.
+         */
+        memoizeWith<T extends (...args: any[]) => any>(keyFn: (...v: any[]) => string, fn: T): T;
+
         /**
          * Create a new object with the own properties of a
          * merged with the own properties of object b.

types/ramda/ramda-tests.ts

@@ -309,6 +309,36 @@ R.times(i, 5);
     const isLong = memoStringLength('short') > 10; // false
 })();
 
+(() => {
+    interface Vector {
+      x: number;
+      y: number;
+    }
+
+    let numberOfCalls = 0;
+
+    function vectorSum(a: Vector, b: Vector): Vector {
+        numberOfCalls += 1;
+        return {
+            x: a.x + b.x,
+            y: a.y + b.y
+        };
+    }
+
+    const memoVectorSum = R.memoizeWith(JSON.stringify, vectorSum);
+
+    memoVectorSum({ x: 1, y: 1 }, { x: 2, y: 2 }); // => { x: 3, y: 3 }
+    numberOfCalls; // => 1
+    memoVectorSum({ x: 1, y: 1 }, { x: 2, y: 2 }); // => { x: 3, y: 3 }
+    numberOfCalls; // => 1
+    memoVectorSum({ x: 1, y: 2 }, { x: 2, y: 3 }); // => { x: 3, y: 5 }
+    numberOfCalls; // => 2
+
+    // Note that argument order matters
+    memoVectorSum({ x: 2, y: 3 }, { x: 1, y: 2 }); // => { x: 3, y: 5 }
+    numberOfCalls; // => 3
+})();
+
 (() => {
     const addOneOnce = R.once((x: number) => x + 1);
     addOneOnce(10); // => 11