feat: add liftFN

Ref #56

package.json

@@ -95,6 +95,7 @@
     "mocha": "=3.2.0",
     "mocha-junit-reporter": "=1.13.0",
     "mocha-multi-reporters": "=1.1.4",
+    "monet": "=0.8.10",
     "nsp": "=2.6.3",
     "nyc": "=10.2.0",
     "phantom": "=4.0.2",

src/index.js

@@ -38,6 +38,7 @@ import isNotInteger from './isNotInteger';
 // Function
 import stubUndefined from './stubUndefined';
 import noop from './noop';
+import liftFN from './liftFN';
 // List
 import pickIndexes from './pickIndexes';
 import list from './list';
@@ -86,6 +87,7 @@ export { default as isNotInteger } from './isNotInteger';
 // Function
 export { default as stubUndefined } from './stubUndefined';
 export { default as noop } from './noop';
+export { default as liftFN } from './liftFN';
 // List
 export { default as pickIndexes } from './pickIndexes';
 export { default as list } from './list';
@@ -138,6 +140,7 @@ const RA = {
   // Function
   stubUndefined,
   noop,
+  liftFN,
   // List
   pickIndexes,
   list,

src/liftFN.js

@@ -0,0 +1,41 @@
+import { curry, last, slice, reverse, reduce, pipe, ap, curryN, map, flip } from 'ramda';
+
+/**
+ * "lifts" a function to be the specified arity, so that it may "map over" objects that satisfy
+ * the Apply spec of algebraic structures. This function is not compatible
+ * with {@link https://github.com/fantasyland/fantasy-land#apply|FantasyLand Apply spec}.
+ *
+ * Lifting is specific for {@link https://github.com/scalaz/scalaz|scalaz} and {@link http://www.functionaljava.org/|functional java} implementations.
+ * One of the mainstream libraries that uses this Apply spec is {@link https://cwmyers.github.io/monet.js/|monet.js}.
+ * This function acts as interop for ramda and monet.js.
+ *
+ * More info {@link https://github.com/fantasyland/fantasy-land/issues/50|here}.
+ *
+ * @func liftFN
+ * @memberOf RA
+ * @since {@link https://char0n.github.io/ramda-adjunct/1.2.0|v1.2.0}
+ * @category Function
+ * @sig Apply a => Number -> (a... -> a) -> (a... -> a)
+ * @param {Function} fn The function to lift into higher context
+ * @return {Function} The lifted function
+ * @see {@link http://ramdajs.com/docs/#lift|lift}, {@link http://ramdajs.com/docs/#ap|ap}
+ * @example
+ *
+ * const { Maybe } = require('monet');
+ *
+ * const add3 = (a, b, c) => a + b + c;
+ * const madd3 = RA.liftFN(3, add3);
+ *
+ * madd3(Maybe.Some(10), Maybe.Some(15), Maybe.Some(17)); //=> Maybe.Some(42)
+ * madd3(Maybe.Some(10), Maybe.Nothing(), Maybe.Some(17)); //=> Maybe.Nothing()
+ */
+const liftFN = curry((arity, fn) => {
+  const lifted = curryN(arity, fn);
+  return curryN(arity, (...args) => {
+    const accumulator = map(lifted, last(args));
+    const apps = pipe(slice(0, arity - 1), reverse)(args);
+    return reduce(flip(ap), accumulator, apps);
+  });
+});
+
+export default liftFN;

test/liftN.js

@@ -0,0 +1,38 @@
+import { Maybe } from 'monet';
+import { add, reduce, curry } from 'ramda';
+
+import RA from '../src/index';
+import eq from './shared/eq';
+
+
+const addN = (...args) => reduce(add, 0, args);
+const add3 = curry((a, b, c) => a + b + c);
+
+describe('liftFN', function() {
+  const addN3 = RA.liftFN(3, addN);
+  const addN4 = RA.liftFN(4, addN);
+  const addN5 = RA.liftFN(5, addN);
+
+  it('returns a function', function() {
+    eq(typeof RA.liftFN(3, add3), 'function');
+  });
+
+  it('limits a variadic function to the specified arity', function() {
+    eq(addN3(Maybe.Some(1), Maybe.Some(1), Maybe.Some(1), Maybe.Some(1)), Maybe.Some(3));
+  });
+
+  it('can lift functions of any arity', function() {
+    eq(addN3(Maybe.Some(1), Maybe.Some(1), Maybe.Some(1)), Maybe.Some(3));
+    eq(addN4(Maybe.Some(1), Maybe.Some(1), Maybe.Some(1), Maybe.Some(1)), Maybe.Some(4));
+    eq(
+      addN5(Maybe.Some(1), Maybe.Some(1), Maybe.Some(1), Maybe.Some(1), Maybe.Some(1)),
+      Maybe.Some(5)
+    );
+  });
+
+  it('is curried', function() {
+    const f4 = RA.liftFN(4);
+    eq(typeof f4, 'function');
+    eq(f4(addN)(Maybe.Some(1), Maybe.Some(1), Maybe.Some(1), Maybe.Some(1)), Maybe.Some(4));
+  });
+});