Adds toEither :: a -> b? -> Either a b

A function for converting a possibly null-y value to an Either
Closes #83

index.js

@@ -1943,6 +1943,31 @@
         return either.isRight ? [either.value] : [];
       }));
 
+  //# toEither :: a -> b? -> Either a b
+  //.
+  //. Converts an arbitrary value to an Either: a Left if the value is `null`
+  //. or `undefined`; a Right otherwise. The first argument specifies the
+  //. value of the Left in the "failure" case.
+  //.
+  //. ```javascript
+  //. > S.toEither("XYZ", null)
+  //. Left("XYZ")
+  //.
+  //. > S.toEither("XYZ", "ABC")
+  //. Right("ABC")
+  //.
+  //. > R.map(R.head, S.toEither("Invalid protocol", "https://example.com/".match(/^https?:\/\//)))
+  //. Right("https://")
+  //.
+  //. > R.map(R.head, S.toEither("Invalid protocol", "ftp://example.com/".match(/^https?:\/\//)))
+  //. Left("Invalid protocol")
+  //. ```
+  S.toEither =
+  def('toEither',
+      {},
+      [a, b, $Either(a, b)],
+      function(x, y) { return y == null ? Left(x) : Right(y); });
+
   //# encaseEither :: (Error -> l) -> (a -> r) -> a -> Either l r
   //.
   //. Takes two unary functions, `f` and `g`, the second of which may throw,

test/toEither.js

@@ -0,0 +1,23 @@
+'use strict';
+
+var eq = require('./utils').eq;
+var S = require('..');
+
+
+describe('toEither', function() {
+
+  it('is a binary function', function() {
+    eq(typeof S.toEither, 'function');
+    eq(S.toEither.length, 2);
+  });
+
+  it('returns Left of the first argument when second argument is `null`-y', function() {
+    eq(S.toEither('a', null), S.Left('a'));
+    eq(S.toEither('a', undefined), S.Left('a'));
+  });
+
+  it('returns a Right of the second argument when it is not `null`-y', function() {
+    eq(S.toEither('a', 42), S.Right(42));
+  });
+
+});