Implement #zip for Promise

Author: obrok

lib/concurrent/promise.rb

@@ -134,6 +134,22 @@ def flat_map(&block)
       child
     end
 
+    # Builds a promise that produces the result of self and others in an Array
+    # and fails if any of them fails.
+    #
+    # @param [Array<Promise>] others
+    #
+    # @return [Promise]
+    def zip(*others)
+      others.reduce(self) do |p1, p2|
+        p1.flat_map do |result1|
+          p2.then do |result2|
+            [result1, result2].flatten
+          end
+        end
+      end
+    end
+
     protected
 
     def set_pending

spec/concurrent/promise_spec.rb

@@ -291,6 +291,24 @@ module Concurrent
 
     end
 
+    describe '#zip' do
+      let(:promise1) { Promise.new(executor: executor) { 1 } }
+      let(:promise2) { Promise.new(executor: executor) { 2 } }
+      let(:promise3) { Promise.new(executor: executor) { 3 } }
+
+      it 'yields the results as an array' do
+        composite = promise1.zip(promise2, promise3).execute
+        sleep 0.1
+        expect(composite.value).to eq([1,2,3])
+      end
+
+      it 'fails if one component fails' do
+        composite = promise1.zip(promise2, rejected_subject, promise3).execute
+        sleep 0.1
+        expect(composite).to be_rejected
+      end
+    end
+
     context 'fulfillment' do
 
       it 'passes the result of each block to all its children' do