enabling R.scan to be used as transducer. (ramda#2817)

* Add R.scan transducer tests

* R.scan transducer enabler: R.into should always call the @@transducer/init method

* enable R.scan as transducer

* pull upstream & resolve conflicts

* add docs about R.scan can act as a transducer

source/internal/_flatCat.js

@@ -4,30 +4,27 @@ import _xArrayReduce from './_xArrayReduce.js';
 import _xReduce from './_xReduce.js';
 import _xfBase from './_xfBase.js';
 
-var preservingReduced = function(xf) {
-  return {
-    '@@transducer/init': _xfBase.init,
-    '@@transducer/result': function(result) {
-      return xf['@@transducer/result'](result);
-    },
-    '@@transducer/step': function(result, input) {
-      var ret = xf['@@transducer/step'](result, input);
-      return ret['@@transducer/reduced'] ? _forceReduced(ret) : ret;
-    }
-  };
+var tInit = '@@transducer/init';
+var tStep = '@@transducer/step';
+var tResult = '@@transducer/result';
+function XPreservingReduced(xf) {
+  this.xf = xf;
+}
+XPreservingReduced.prototype[tInit] = _xfBase.init;
+XPreservingReduced.prototype[tResult] = _xfBase.result;
+XPreservingReduced.prototype[tStep] = function(result, input) {
+  var ret = this.xf[tStep](result, input);
+  return ret['@@transducer/reduced'] ? _forceReduced(ret) : ret;
 };
 
-var _flatCat = function _xcat(xf) {
-  var rxf = preservingReduced(xf);
-  return {
-    '@@transducer/init': _xfBase.init,
-    '@@transducer/result': function(result) {
-      return rxf['@@transducer/result'](result);
-    },
-    '@@transducer/step': function(result, input) {
-      return !_isArrayLike(input) ? _xArrayReduce(rxf, result, [input]) : _xReduce(rxf, result, input);
-    }
-  };
+function XFlatCat(xf) {
+  this.xf = new XPreservingReduced(xf);
+}
+XFlatCat.prototype[tInit] = _xfBase.init;
+XFlatCat.prototype[tResult] = _xfBase.result;
+XFlatCat.prototype[tStep] = function(result, input) {
+  return !_isArrayLike(input) ? _xArrayReduce(this.xf, result, [input]) : _xReduce(this.xf, result, input);
 };
+var _flatCat = function _xcat(xf) { return new XFlatCat(xf); };
 
 export default _flatCat;

source/internal/_xscan.js

@@ -0,0 +1,27 @@
+import _curry3 from './_curry3.js';
+import _xfBase from './_xfBase.js';
+
+var tInit = '@@transducer/init';
+var tStep = '@@transducer/step';
+
+function XScan(reducer, acc, xf) {
+  this.xf = xf;
+  this.f = reducer;
+  this.acc = acc;
+}
+XScan.prototype[tInit] = function() {
+  return this.xf[tStep](this.xf[tInit](), this.acc);
+};
+XScan.prototype['@@transducer/result'] = _xfBase.result;
+XScan.prototype[tStep] = function(result, input) {
+  if (result['@@transducer/reduced']) {
+    return result;
+  }
+  this.acc = this.f(this.acc, input);
+  return this.xf[tStep](result, this.acc);
+};
+
+var _xscan = _curry3(function _xscan(reducer, acc, xf) {
+  return new XScan(reducer, acc, xf);
+});
+export default _xscan;

source/into.js

@@ -1,4 +1,3 @@
-import _clone from './internal/_clone.js';
 import _curry3 from './internal/_curry3.js';
 import _isTransformer from './internal/_isTransformer.js';
 import _xReduce from './internal/_xReduce.js';
@@ -44,9 +43,8 @@ import _stepCat from './internal/_stepCat.js';
  *      const intoArray = R.into([]);
  *      intoArray(transducer, numbers); //=> [2, 3]
  */
-var into = _curry3(function into(acc, xf, list) {
-  return _isTransformer(acc)
-    ? _xReduce(xf(acc), acc['@@transducer/init'](), list)
-    : _xReduce(xf(_stepCat(acc)), _clone(acc, false), list);
+var into = _curry3(function into(acc, transducer, list) {
+  var xf = transducer(_isTransformer(acc) ? acc : _stepCat(acc));
+  return _xReduce(xf, xf['@@transducer/init'](), list);
 });
 export default into;

source/scan.js

@@ -1,9 +1,13 @@
 import _curry3 from './internal/_curry3.js';
+import _dispatchable from './internal/_dispatchable.js';
+import _xscan from './internal/_xscan.js';
 
 
 /**
  * Scan is similar to [`reduce`](#reduce), but returns a list of successively
- * reduced values from the left
+ * reduced values from the left.
+ *
+ * Acts as a transducer if a transformer is given in list position.
  *
  * @func
  * @memberOf R
@@ -22,7 +26,7 @@ import _curry3 from './internal/_curry3.js';
  *      const factorials = R.scan(R.multiply, 1, numbers); //=> [1, 1, 2, 6, 24]
  * @symb R.scan(f, a, [b, c]) = [a, f(a, b), f(f(a, b), c)]
  */
-var scan = _curry3(function scan(fn, acc, list) {
+var scan = _curry3(_dispatchable([], _xscan, function scan(fn, acc, list) {
   var idx = 0;
   var len = list.length;
   var result = [acc];
@@ -32,5 +36,5 @@ var scan = _curry3(function scan(fn, acc, list) {
     idx += 1;
   }
   return result;
-});
+}));
 export default scan;

test/scan.js

@@ -1,10 +1,12 @@
 var R = require('../source/index.js');
 var eq = require('./shared/eq.js');
+var sinon = require('sinon');
 
 
 describe('scan', function() {
   var add = function(a, b) {return a + b;};
   var mult = function(a, b) {return a * b;};
+  var double = function(x) {return 2 * x;};
 
   it('scans simple functions over arrays with the supplied accumulator', function() {
     eq(R.scan(add, 0, [1, 2, 3, 4]), [0, 1, 3, 6, 10]);
@@ -16,4 +18,25 @@ describe('scan', function() {
     eq(R.scan(mult, 1, []), [1]);
   });
 
+  it('works with transducers', function() {
+    eq(R.into([], R.scan(add, 0), [1, 2, 3, 4]), [0, 1, 3, 6, 10]);
+    eq(R.into([], R.scan(mult, 1), []), [1]);
+  });
+
+  it('composes with other transducers', function() {
+    eq(R.into([], R.compose(R.scan(mult, 1), R.take(2)), [1, 2, 3, 4]), [1, 1]);
+    eq(R.into([], R.compose(R.scan(mult, 1), R.map(double)), [1, 2, 3, 4]), [2, 2, 4, 12, 48]);
+  });
+
+  it('works lazily: taking 3 elements must call reducer twice', function() {
+    var reducer = sinon.spy();
+    R.into([], R.compose(R.scan(reducer, 0), R.take(3)), [1, 2, 3, 4]);
+    sinon.assert.calledTwice(reducer);
+  });
+
+  it('works lazily: taking 0 elements must call reducer 0 times', function() {
+    var reducer = sinon.spy();
+    R.into([], R.compose(R.scan(reducer, 0), R.take(0)), [1, 2, 3, 4]);
+    sinon.assert.notCalled(reducer);
+  });
 });