Add structs.merge

Author: mattyclarkson

docs/structs_doc.md

@@ -2,6 +2,31 @@
 
 Skylib module containing functions that operate on structs.
 
+<a id="structs.merge"></a>
+
+## structs.merge
+
+<pre>
+load("@bazel_skylib//lib:structs.bzl", "structs")
+
+structs.merge(<a href="#structs.merge-first">first</a>, <a href="#structs.merge-rest">*rest</a>)
+</pre>
+
+Merges multiple `struct` instances together. Later `struct` keys overwrite early `struct` keys.
+
+**PARAMETERS**
+
+
+| Name  | Description | Default Value |
+| :------------- | :------------- | :------------- |
+| <a id="structs.merge-first"></a>first |  The initial `struct` to merge keys/values into.   |  none |
+| <a id="structs.merge-rest"></a>rest |  Other `struct` instances to merge.   |  none |
+
+**RETURNS**
+
+A merged `struct`.
+
+
 <a id="structs.to_dict"></a>
 
 ## structs.to_dict

lib/structs.bzl

@@ -34,6 +34,22 @@ def _to_dict(s):
         if key != "to_json" and key != "to_proto"
     }
 
+def _merge(first, *rest):
+    """Merges multiple `struct` instances together. Later `struct` keys overwrite early `struct` keys.
+
+    Args:
+      first: The initial `struct` to merge keys/values into.
+      *rest: Other `struct` instances to merge.
+
+    Returns:
+      A merged `struct`.
+    """
+    map = _to_dict(first)
+    for r in rest:
+        map |= _to_dict(r)
+    return struct(**map)
+
 structs = struct(
     to_dict = _to_dict,
+    merge = _merge,
 )

tests/structs_tests.bzl

@@ -46,9 +46,33 @@ def _add_test(ctx):
 
 add_test = unittest.make(_add_test)
 
+def _merge_test(ctx):
+    """Unit tests for structs.merge."""
+    env = unittest.begin(ctx)
+
+    # Fixtures
+    a = struct(a = 1)
+    b = struct(b = 2)
+    c = struct(a = 3)
+
+    # Test one argument
+    asserts.equals(env, {"a": 1}, structs.to_dict(structs.merge(a)))
+
+    # Test two arguments
+    asserts.equals(env, {"a": 1}, structs.to_dict(structs.merge(a, a)))
+    asserts.equals(env, {"a": 1, "b": 2}, structs.to_dict(structs.merge(a, b)))
+
+    # Test overwrite
+    asserts.equals(env, {"a": 3}, structs.to_dict(structs.merge(a, c)))
+
+    return unittest.end(env)
+
+merge_test = unittest.make(_merge_test)
+
 def structs_test_suite():
     """Creates the test targets and test suite for structs.bzl tests."""
     unittest.suite(
         "structs_tests",
         add_test,
+        merge_test,
     )