Rename fromEither to fromRight, add fromLeft and new fromEither

index.js

@@ -2320,24 +2320,70 @@
     impl: isRight
   };
 
-  //# fromEither :: b -> Either a b -> b
+  //# 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.fromEither (0) (S.Right (42))
+  //. > S.fromRight (0) (S.Right (42))
   //. 42
   //.
-  //. > S.fromEither (0) (S.Left (42))
+  //. > S.fromRight (0) (S.Left (42))
   //. 0
   //. ```
-  function fromEither(x) {
+  function fromRight(x) {
     return either (K (x)) (I);
   }
-  _.fromEither = {
+  _.fromRight = {
     consts: {},
     types: [b, $.Either (a) (b), b],
+    impl: fromRight
+  };
+
+  //# fromEither :: Either a a -> a
+  //.
+  //. Takes an Either with the same type on the Left and on the Right
+  //. and return whichever value exists.
+  //.
+  //. ```javascript
+  //. > S.fromEither (S.Right (42))
+  //. 42
+  //.
+  //. > S.fromEither (S.Left (42))
+  //. 42
+  //. ```
+  function fromEither(x) {
+    return either (I) (I) (x);
+  }
+  _.fromEither = {
+    consts: {},
+    types: [$.Either (a) (a), a],
     impl: fromEither
   };
 

test/fromEither.js

@@ -7,9 +7,9 @@ const eq = require ('./internal/eq');
 
 test ('fromEither', () => {
 
-  eq (S.show (S.fromEither)) ('fromEither :: b -> Either a b -> b');
+  eq (S.show (S.fromEither)) ('fromEither :: Either a a -> a');
 
-  eq (S.fromEither (0) (S.Left (42))) (0);
-  eq (S.fromEither (0) (S.Right (42))) (42);
+  eq (S.fromEither (S.Left (42))) (42);
+  eq (S.fromEither (S.Right (42))) (42);
 
 });

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);
+
+});