consider adding a merge mode deep_merge_patch

lib/travis/yml/parts/part.rb

@@ -9,6 +9,7 @@ class Part
           deep_merge
           deep_merge_append
           deep_merge_prepend
+          deep_merge_patch
           replace
         )
 

lib/travis/yml/support/merge.rb

@@ -66,6 +66,21 @@ def deep_merge_prepend(lft, rgt)
           end
         end
 
+        def deep_merge_patch(lft, rgt)
+          keys(lft, rgt).inject(lft) do |hash, key|
+            hash[key] = if hashes?(lft[key], rgt[key])
+              send(mode(lft[key]) || :deep_merge_patch, lft[key], rgt[key])
+            elsif arrays?(lft[key], rgt[key])
+              send(mode(lft[key]) || :patch, lft[key], rgt[key])
+            elsif lft.key?(key)
+              lft[key]
+            else
+              rgt[key]
+            end
+            hash
+          end
+        end
+
         def prepend(lft, rgt)
           lft.replace(rgt + lft)
         end
@@ -74,6 +89,17 @@ def append(lft, rgt)
           lft.replace(lft + rgt)
         end
 
+        def patch(lft, rgt)
+          lft.each.with_index do |_, ix|
+            rgt[ix] = if hashes?(rgt[ix], lft[ix])
+              deep_merge_patch(lft[ix], rgt[ix])
+            else
+              lft[ix] || rgt[ix]
+            end
+          end
+          rgt
+        end
+
         def mode(obj)
           obj.merge_mode if obj.respond_to?(:merge_mode)
         end

spec/travis/yml/support/merge_spec.rb

@@ -57,6 +57,38 @@
     it { expect(subject.keys.first.line).to eq 2 }
   end
 
+  describe 'deep_merge_append' do
+    let(:mode) { :deep_merge_append }
+    it { should eq 'foo' => { 'one' => ['one', 'two'], 'two' => 'two' }, 'bar' => 'bar' }
+    it { expect(subject.keys.first).to be_a Key }
+    it { expect(subject.keys.first.line).to eq 2 }
+  end
+
+  describe 'deep_merge_patch' do
+    let(:mode) { :deep_merge_patch }
+
+    let(:lft) do
+      parse %(
+        foo:
+          bar:
+          - one: two
+          -
+          - three: three
+      )
+    end
+
+    let(:rgt) do
+      parse %(
+        foo:
+          bar:
+          - one: one
+          - two: two
+      )
+    end
+
+    it { should eq 'foo' => { 'bar' => [{ 'one' => 'two' }, { 'two' => 'two' }, { 'three' => 'three'}] } }
+  end
+
   describe 'merge tags (1)' do
     let(:lft) do
       parse %(