(new validation) Adds a function to aggregate lists of validations (#71)

This closes #71.

src/data/validation/collect.js

@@ -0,0 +1,24 @@
+//----------------------------------------------------------------------
+//
+// This source file is part of the Folktale project.
+//
+// Licensed under MIT. See LICENCE for full licence information.
+// See CONTRIBUTORS for the list of contributors to the project.
+//
+//----------------------------------------------------------------------
+
+
+const { Success } = require('./validation');
+
+
+/*~
+ * stability: experimental
+ * type: |
+ *   forall a, b: (Array (Validation a b)) => Validation a b
+ *   where a is Semigroup
+ */
+const collect = (validations) =>
+  validations.reduce((a, b) => a.concat(b), Success());
+
+
+module.exports = collect;

src/data/validation/index.js

@@ -10,6 +10,7 @@
 /*~ stability: experimental */
 module.exports = {
   ...require('./validation'),
+  collect: require('./collect'),
 
   /*~
    * type: |

test/specs-src/data.validation.js

@@ -11,7 +11,32 @@ const { property, forall} = require('jsverify');
 const _ = require('folktale/data/validation');
 const laws = require('../helpers/fantasy-land-laws');
 
+
 describe('Data.Validation', () => {
+  describe('collect(validations)', () => {
+    it('failures are concatenated', () => {
+      const a = _.Failure('a');
+      const b = _.Failure('b');
+
+      $ASSERT(_.collect([a, b]) == _.Failure('ab'));
+    });
+
+    it('successes are ignored in the presence of failures', () => {
+      const a = _.Failure('a');
+      const b = _.Failure('b');
+      const c = _.Success('c');
+
+      $ASSERT(_.collect([a, b, c]) == _.Failure('ab'));
+    });
+
+    it('keeps the last success', () => {
+      const a = _.Success(1);
+      const b = _.Success(2);
+
+      $ASSERT(_.collect([a, b]) == _.Success(2));
+    });
+  });
+
   describe('Functor', () => {
     property('map', 'json', 'json -> json', (a, f) => {
       return _.of(f(a)).equals(_.of(a).map(f))