array.at

Summary: This diff adds a function to the standard library to access items in an array by index. Basic stuff. Will come in handy in future diffs where arrays are used like tuples.

Reviewed By: iahs

Differential Revision: D68004374

fbshipit-source-id: e66e9925d81119f7497d18ca63ec1fae1771160d

thrift/compiler/whisker/standard_library.cc

@@ -121,6 +121,41 @@ map::value_type create_array_functions() {
     array_functions["empty?"] = w::make_native_function<array_empty>();
   }
 
+  {
+    /**
+     * Gets the object from an array at a given index. If the index is negative,
+     * or larger than the size of the array, then an error is thrown.
+     *
+     * Name: array.at
+     *
+     * Arguments:
+     *   - [whisker::array] — The array to get from
+     *   - [i64] — The index of the item to get
+     *
+     * Returns:
+     *   [object] the item at the given index.
+     */
+    class array_at : public named_native_function {
+     public:
+      array_at() : named_native_function("array.at") {}
+
+      object::ptr invoke(context ctx) override {
+        ctx.declare_named_arguments({});
+        ctx.declare_arity(2);
+
+        auto a = ctx.argument<array>(0);
+        auto index = ctx.argument<i64>(1);
+
+        if (index < 0 || std::size_t(index) >= a.size()) {
+          ctx.error(
+              "Index '{}' is out of bounds (size is {}).", index, a.size());
+        }
+        return a.at(index);
+      }
+    };
+    array_functions["at"] = w::make_native_function<array_at>();
+  }
+
   return map::value_type{"array", std::move(array_functions)};
 }
 

thrift/compiler/whisker/test/standard_library_test.cc

@@ -102,6 +102,48 @@ TEST_F(StandardLibraryTest, array_empty) {
   }
 }
 
+TEST_F(StandardLibraryTest, array_at) {
+  {
+    auto result = render(
+        "{{ (array.at (array.of 1 \"foo\" 2) 0) }}\n"
+        "{{ (array.at (array.of 1 \"foo\" 2) 1) }}\n"
+        "{{ (array.at (array.of 1 \"foo\" 2) 2) }}\n",
+        w::null);
+    EXPECT_THAT(diagnostics(), testing::IsEmpty());
+    EXPECT_EQ(
+        *result,
+        "1\n"
+        "foo\n"
+        "2\n");
+  }
+
+  {
+    auto result = render("{{ (array.at (array.of) 0) }}\n", w::null);
+    EXPECT_FALSE(result.has_value());
+    EXPECT_THAT(
+        diagnostics(),
+        testing::ElementsAre(diagnostic(
+            diagnostic_level::error,
+            "Function 'array.at' threw an error:\n"
+            "Index '0' is out of bounds (size is 0).",
+            path_to_file,
+            1)));
+  }
+
+  {
+    auto result = render("{{ (array.at (array.of 0 1 2) -1) }}\n", w::null);
+    EXPECT_FALSE(result.has_value());
+    EXPECT_THAT(
+        diagnostics(),
+        testing::ElementsAre(diagnostic(
+            diagnostic_level::error,
+            "Function 'array.at' threw an error:\n"
+            "Index '-1' is out of bounds (size is 3).",
+            path_to_file,
+            1)));
+  }
+}
+
 TEST_F(StandardLibraryTest, string_len) {
   {
     auto result = render(