Introduce string.elem() method.

RELNOTES[NEW]:
  Strings have a new .elems() method, that provides an iterator on the characters of the string.

PiperOrigin-RevId: 189980269

2 files changed, 34 insertions, 0 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java b/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
index 59135375e4..9856c7a3a3 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
@@ -894,6 +894,27 @@ public class MethodLibrary {
     }
   };
 
+  @SkylarkSignature(
+    name = "elems",
+    objectType = StringModule.class,
+    returnType = SkylarkList.class,
+    doc =
+        "Returns an iterable value containing successive 1-element substrings of the string. "
+            + "Equivalent to <code>[s[i] for i in range(len(s))]</code>, except that the "
+            + "returned value might not be a list.",
+    parameters = {@Param(name = "self", type = String.class, doc = "This string.")}
+  )
+  private static final BuiltinFunction elems =
+      new BuiltinFunction("elems") {
+        public SkylarkList<String> invoke(String self) throws ConversionException {
+          ImmutableList.Builder<String> builder = new ImmutableList.Builder<>();
+          for (char c : self.toCharArray()) {
+            builder.add(String.valueOf(c));
+          }
+          return SkylarkList.createImmutable(builder.build());
+        }
+      };
+
   @SkylarkSignature(name = "endswith", objectType = StringModule.class, returnType = Boolean.class,
       doc = "Returns True if the string ends with <code>sub</code>, "
           + "otherwise False, optionally restricting to [<code>start</code>:<code>end</code>], "
diff --git a/src/test/skylark/testdata/string_elems.sky b/src/test/skylark/testdata/string_elems.sky
new file mode 100644
index 0000000000..3dbe724995
--- /dev/null
+++ b/src/test/skylark/testdata/string_elems.sky
@@ -0,0 +1,13 @@
+# string.elems
+assert_eq(list("abcd".elems()), ["a", "b", "c", "d"])
+
+assert_eq(len("aaa".elems()), 3)
+
+def test_iter():
+  i = 0
+  for c in "abcd".elems():
+    assert_eq(c, "abcd"[i])
+    i += 1
+  assert_eq(i, 4)
+
+test_iter()