Implement dropRepeatsBy (ramda#3041) (ramda#3239)

* Implement dropRepeatsBy (ramda#3041)

* Improve tests for dropRepeatsBy

* Remove arrow function from dropRepeatsBy for compatibility purposes

* Improve test isolation for dropRepeatsBy and dropRepeatsWith

source/dropRepeatsBy.js

@@ -0,0 +1,30 @@
+import _curry2 from './internal/_curry2.js';
+import _dispatchable from './internal/_dispatchable.js';
+import _xdropRepeatsWith from './internal/_xdropRepeatsWith.js';
+import dropRepeatsWith from './dropRepeatsWith.js';
+import eqBy from './eqBy.js';
+
+
+/**
+ * Returns a new list without any consecutively repeating elements,
+ * based upon the value returned by applying the supplied function to
+ * each list element. [`R.equals`](#equals) is used to determine equality.
+ *
+ * Acts as a transducer if a transformer is given in list position.
+ *
+ * @func
+ * @memberOf R
+ * @category List
+ * @sig (a -> b) -> [a] -> [a]
+ * @param {Function} fn A function used to produce a value to use during comparisons.
+ * @param {Array} list The array to consider.
+ * @return {Array} `list` without repeating elements.
+ * @see R.transduce
+ * @example
+ *
+ *     R.dropRepeatsBy(Math.abs, [1, -1, -1, 2, 3, -4, 4, 2, 2]); //=> [1, 2, 3, -4, 2]
+ */
+var dropRepeatsBy = _curry2(function(fn, list) {
+  return _dispatchable([], _xdropRepeatsWith(eqBy(fn)), dropRepeatsWith(eqBy(fn)))(list);
+});
+export default dropRepeatsBy;

source/index.js

@@ -53,6 +53,7 @@ export { default as drop } from './drop.js';
 export { default as dropLast } from './dropLast.js';
 export { default as dropLastWhile } from './dropLastWhile.js';
 export { default as dropRepeats } from './dropRepeats.js';
+export { default as dropRepeatsBy } from './dropRepeatsBy.js';
 export { default as dropRepeatsWith } from './dropRepeatsWith.js';
 export { default as dropWhile } from './dropWhile.js';
 export { default as either } from './either.js';

test/dropRepeatsBy.js

@@ -0,0 +1,38 @@
+var R = require('../source/index.js');
+var eq = require('./shared/eq.js');
+
+
+describe('dropRepeatsBy', function() {
+  var objs = [
+    {i: 1}, {i: 2}, {i: 3}, {i: -4}, {i: 5}, {i: 3}
+  ];
+  var objs2 = [
+    {i: 1}, {i: -1}, {i: 1}, {i: 2}, {i: 3},
+    {i: 3}, {i: -4}, {i: 4}, {i: 5}, {i: 3}
+  ];
+  var fn = ({ i }) => ({ i: Math.abs(i) });
+
+  it('removes repeated elements based on predicate', function() {
+    eq(R.dropRepeatsBy(fn, objs2), objs);
+    eq(R.dropRepeatsBy(fn, objs), objs);
+  });
+
+  it('keeps elements from the left', function() {
+    eq(
+      R.dropRepeatsBy(
+        ({ n, ...rest }) => ({ ...rest }),
+        [{i: 1, n: 1}, {i: 1, n: 2}, {i: 1, n: 3}, {i: 4, n: 1}, {i: 4, n: 2}]
+      ),
+      [{i: 1, n: 1}, {i: 4, n: 1}]
+    );
+  });
+
+  it('returns an empty array for an empty array', function() {
+    eq(R.dropRepeatsBy(fn, []), []);
+  });
+
+  it('can act as a transducer', function() {
+    eq(R.into([], R.dropRepeatsBy(fn), objs2), objs);
+  });
+
+});

test/dropRepeatsWith.js

@@ -10,7 +10,7 @@ describe('dropRepeatsWith', function() {
     {i: 1}, {i: 1}, {i: 1}, {i: 2}, {i: 3},
     {i: 3}, {i: 4}, {i: 4}, {i: 5}, {i: 3}
   ];
-  var eqI = R.eqProps('i');
+  var eqI = (a, b) => a.i === b.i;
 
   it('removes repeated elements based on predicate', function() {
     eq(R.dropRepeatsWith(eqI, objs2), objs);