Add fromLeft and fromRight

index.js

@@ -2362,6 +2362,52 @@
     impl: either
   };
 
+  //# fromLeft :: a -> Either a b -> a
+  //.
+  //. Takes a default value and an Either, and returns the Left value
+  //. if the Either is a Left; the default value otherwise.
+  //.
+  //. See also [`fromRight`](#fromRight).
+  //.
+  //. ```javascript
+  //. > S.fromLeft (0) (S.Left (42))
+  //. 42
+  //.
+  //. > S.fromLeft (0) (S.Right (42))
+  //. 0
+  //. ```
+  function fromLeft(x) {
+    return either (I) (K (x));
+  }
+  _.fromLeft = {
+    consts: {},
+    types: [a, $.Either (a) (b), a],
+    impl: fromLeft
+  };
+
+  //# fromRight :: b -> Either a b -> b
+  //.
+  //. Takes a default value and an Either, and returns the Right value
+  //. if the Either is a Right; the default value otherwise.
+  //.
+  //. See also [`fromLeft`](#fromLeft).
+  //.
+  //. ```javascript
+  //. > S.fromRight (0) (S.Right (42))
+  //. 42
+  //.
+  //. > S.fromRight (0) (S.Left (42))
+  //. 0
+  //. ```
+  function fromRight(x) {
+    return either (K (x)) (I);
+  }
+  _.fromRight = {
+    consts: {},
+    types: [b, $.Either (a) (b), b],
+    impl: fromRight
+  };
+
   //# fromEither :: b -> Either a b -> b
   //.
   //. Takes a default value and an Either, and returns the Right value

test/fromLeft.js

@@ -0,0 +1,15 @@
+'use strict';
+
+const S = require ('..');
+
+const eq = require ('./internal/eq');
+
+
+test ('fromLeft', () => {
+
+  eq (S.show (S.fromLeft)) ('fromLeft :: a -> Either a b -> a');
+
+  eq (S.fromLeft (0) (S.Left (42))) (42);
+  eq (S.fromLeft (0) (S.Right (42))) (0);
+
+});

test/fromRight.js

@@ -0,0 +1,15 @@
+'use strict';
+
+const S = require ('..');
+
+const eq = require ('./internal/eq');
+
+
+test ('fromRight', () => {
+
+  eq (S.show (S.fromRight)) ('fromRight :: b -> Either a b -> b');
+
+  eq (S.fromRight (0) (S.Left (42))) (0);
+  eq (S.fromRight (0) (S.Right (42))) (42);
+
+});