either: implement Foldable

index.js

@@ -973,7 +973,7 @@
       [b, $Maybe(b)],
       Maybe.of);
 
-  //# Maybe#reduce :: Maybe a ~> (b -> a -> b) -> b -> b
+  //# Maybe#reduce :: Maybe a ~> ((b, a) -> b) -> b -> b
   //.
   //. Takes a function and an initial value of any type, and returns:
   //.
@@ -1357,8 +1357,8 @@
   //. `Either a b` is either a Left whose value is of type `a` or a Right whose
   //. value is of type `b`.
   //.
-  //. The Either type satisfies the [Semigroup][], [Monad][], and [Extend][]
-  //. specifications.
+  //. The Either type satisfies the [Semigroup][], [Monad][], [Foldable][], and
+  //. [Extend][] specifications.
 
   //# EitherType :: Type -> Type -> Type
   //.
@@ -1591,6 +1591,30 @@
       [c, $Either(a, c)],
       Either.of);
 
+  //# Either#reduce :: Either a b ~> ((c, b) -> c) -> c -> c
+  //.
+  //. Takes a function and an initial value of any type, and returns:
+  //.
+  //.   - the initial value if `this` is a Left; otherwise
+  //.
+  //.   - the result of applying the function to the initial value and this
+  //.     Right's value.
+  //.
+  //. ```javascript
+  //. > S.Left('Cannot divide by zero').reduce((xs, x) => xs.concat([x]), [42])
+  //. [42]
+  //.
+  //. > S.Right(5).reduce((xs, x) => xs.concat([x]), [42])
+  //. [42, 5]
+  //. ```
+  Either.prototype.reduce =
+  method('Either#reduce',
+         {},
+         [$Either(a, b), $.Function, c, c],
+         function(either, f, x) {
+           return either.isRight ? f(x, either.value) : x;
+         });
+
   //# Either#toBoolean :: Either a b ~> Boolean
   //.
   //. Returns `false` if `this` is a Left; `true` if `this` is a Right.

test/Either/Left.js

@@ -167,6 +167,24 @@ describe('Left', function() {
                    'The value at position 1 is not a member of ‘Function’.\n'));
   });
 
+  it('provides a "reduce" method', function() {
+    eq(S.Left().reduce.length, 2);
+    eq(S.Left().reduce(function(xs, x) { return xs.concat([x]); }, [42]),
+       [42]);
+
+    throws(function() { S.Left().reduce(null, null); },
+           errorEq(TypeError,
+                   'Invalid value\n' +
+                   '\n' +
+                   'Either#reduce :: Either a b -> Function -> c -> c\n' +
+                   '                               ^^^^^^^^\n' +
+                   '                                  1\n' +
+                   '\n' +
+                   '1)  null :: Null\n' +
+                   '\n' +
+                   'The value at position 1 is not a member of ‘Function’.\n'));
+  });
+
   it('provides a "toBoolean" method', function() {
     eq(S.Left('abc').toBoolean.length, 0);
     eq(S.Left('abc').toBoolean(), false);

test/Either/Right.js

@@ -167,6 +167,24 @@ describe('Right', function() {
                    'The value at position 1 is not a member of ‘Function’.\n'));
   });
 
+  it('provides a "reduce" method', function() {
+    eq(S.Right(5).reduce.length, 2);
+    eq(S.Right(5).reduce(function(xs, x) { return xs.concat([x]); }, [42]),
+       [42, 5]);
+
+    throws(function() { S.Right().reduce(null, null); },
+           errorEq(TypeError,
+                   'Invalid value\n' +
+                   '\n' +
+                   'Either#reduce :: Either a b -> Function -> c -> c\n' +
+                   '                               ^^^^^^^^\n' +
+                   '                                  1\n' +
+                   '\n' +
+                   '1)  null :: Null\n' +
+                   '\n' +
+                   'The value at position 1 is not a member of ‘Function’.\n'));
+  });
+
   it('provides a "toBoolean" method', function() {
     eq(S.Right(42).toBoolean.length, 0);
     eq(S.Right(42).toBoolean(), true);