feat(lens): viewOr implemented

src/index.d.ts

@@ -425,6 +425,14 @@ declare namespace RamdaAdjunct {
         lensNotSatisfies(predicate: Function, lens: Function): (data: any) => Boolean
         lensNotSatisfies(predicate: Function): (lens: Function) => (data: any) => Boolean
 
+        /**
+         * Applies lens to given data structure and returns view value if present.
+         * Otherwise returns `defaultValue`.
+         */
+        viewOr(defaultValue: any, lens: Function, data: any): any
+        viewOr(defaultValue: any, lens: Function): (data: any) => any
+        viewOr(defaultValue: any): (lens: Function) => (data: any) => any
+
         /**
          * Identity type.
          */

src/index.js

@@ -67,6 +67,8 @@ import lensEq from './lensEq';
 import lensNotEq from './lensNotEq';
 import lensSatisfies from './lensSatisfies';
 import lensNotSatisfies from './lensNotSatisfies';
+// Lens
+import viewOr from './viewOr';
 
 // Type
 export { default as isNotUndefined } from './isNotUndefined';
@@ -145,6 +147,8 @@ export { default as lensEq } from './lensEq';
 export { default as lensNotEq } from './lensNotEq';
 export { default as lensSatisfies } from './lensSatisfies';
 export { default as lensNotSatisfies } from './lensNotSatisfies';
+// Lens
+export { default as viewOr } from './viewOr';
 
 /**
  * @namespace RA
@@ -228,6 +232,8 @@ const RA = {
   lensNotEq,
   lensSatisfies,
   lensNotSatisfies,
+  // Lens
+  viewOr,
 };
 
 export default RA;

src/viewOr.js

@@ -0,0 +1,28 @@
+import { view, defaultTo, curry } from 'ramda';
+
+/**
+ * If the data structure "viewed" through `lens` contains value, the value is returned.
+ * Otherwise returns the provided `defaultValue`.
+ * 
+ * @func viewOr
+ * @memberOf RA
+ * @since {@link https://char0n.github.io/ramda-adjunct/1.13.0|1.13.0}
+ * @category Lens
+ * @sig * -> lens -> data -> *
+ * @param {*} defaultValue Defauly value
+ * @param {Function} lens Van Laarhoven lens
+ * @param {*} data The data structure to apply the lens to
+ * 
+ * @example
+ * 
+ * RA.viewOr('N/A', R.lensProp('x'), {}); // => 'N/A'
+ * RA.viewOr('N/A', R.lensProp('x'), { x: 1 }); // => 1
+ * RA.viewOr('some', R.lensProp('y'), { y: null }); // => 'some'
+ * RA.viewOr('some', R.lensProp('y'), { y: false }); // => false
+ */
+
+const viewOr = curry(
+  (defaultValue, lens, data) => defaultTo(defaultValue, view(lens, data))
+);
+
+export default viewOr;

test/viewOr.js

@@ -0,0 +1,17 @@
+import { lensIndex, lensProp } from 'ramda';
+import RA from '../src/index';
+import eq from './shared/eq';
+
+describe('viewOr', function () {
+  it('tests "view" through lens with fallback to default value', function () {
+    eq(RA.viewOr('foo', lensProp('bar'), {}), 'foo');
+    eq(RA.viewOr('foo', lensProp('bar'), { bar: 'foobar' }), 'foobar');
+
+    eq(RA.viewOr('foo', lensProp('bar'), { bar: undefined }), 'foo');
+    eq(RA.viewOr('foo', lensProp('bar'), { bar: null }), 'foo');
+    eq(RA.viewOr('foo', lensProp('bar'), { bar: false }), false);
+
+    eq(RA.viewOr('foo', lensIndex(11), []), 'foo');
+    eq(RA.viewOr('foo', lensIndex(11), {}), 'foo');
+  });
+});