Improve checked json casting (#10087)

This introduces new utility functions to get elements from JSON — in an ergonomic way and with nice error messages if the expected type does not match.

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>

src/libutil/json-utils.cc

@@ -1,5 +1,8 @@
 #include "json-utils.hh"
 #include "error.hh"
+#include "types.hh"
+#include <nlohmann/json_fwd.hpp>
+#include <iostream>
 
 namespace nix {
 
@@ -18,26 +21,115 @@ nlohmann::json * get(nlohmann::json & map, const std::string & key)
 }
 
 const nlohmann::json & valueAt(
-    const nlohmann::json & map,
+    const nlohmann::json::object_t & map,
     const std::string & key)
 {
     if (!map.contains(key))
-        throw Error("Expected JSON object to contain key '%s' but it doesn't", key);
+        throw Error("Expected JSON object to contain key '%s' but it doesn't: %s", key, nlohmann::json(map).dump());
 
-    return map[key];
+    return map.at(key);
 }
 
-const nlohmann::json & ensureType(
+std::optional<nlohmann::json> optionalValueAt(const nlohmann::json & value, const std::string & key)
+{
+    try {
+        auto & v = valueAt(value, key);
+        return v.get<nlohmann::json>();
+    } catch (...) {
+        return std::nullopt;
+    }
+}
+
+
+std::optional<nlohmann::json> getNullable(const nlohmann::json & value)
+{
+    if (value.is_null())
+        return std::nullopt;
+
+    return value.get<nlohmann::json>();
+}
+
+/**
+ * Ensure the type of a JSON object is what you expect, failing with a
+ * ensure type if it isn't.
+ *
+ * Use before type conversions and element access to avoid ugly
+ * exceptions, but only part of this module to define the other `get*`
+ * functions. It is too cumbersome and easy to forget to expect regular
+ * JSON code to use it directly.
+ */
+static const nlohmann::json & ensureType(
     const nlohmann::json & value,
     nlohmann::json::value_type expectedType
     )
 {
     if (value.type() != expectedType)
         throw Error(
-            "Expected JSON value to be of type '%s' but it is of type '%s'",
+            "Expected JSON value to be of type '%s' but it is of type '%s': %s",
             nlohmann::json(expectedType).type_name(),
-            value.type_name());
+            value.type_name(), value.dump());
 
     return value;
 }
+
+const nlohmann::json::object_t & getObject(const nlohmann::json & value)
+{
+    return ensureType(value, nlohmann::json::value_t::object).get_ref<const nlohmann::json::object_t &>();
+}
+
+const nlohmann::json::array_t & getArray(const nlohmann::json & value)
+{
+    return ensureType(value, nlohmann::json::value_t::array).get_ref<const nlohmann::json::array_t &>();
+}
+
+const nlohmann::json::string_t & getString(const nlohmann::json & value)
+{
+    return ensureType(value, nlohmann::json::value_t::string).get_ref<const nlohmann::json::string_t &>();
+}
+
+const nlohmann::json::number_integer_t & getInteger(const nlohmann::json & value)
+{
+    return ensureType(value, nlohmann::json::value_t::number_integer).get_ref<const nlohmann::json::number_integer_t &>();
+}
+
+const nlohmann::json::boolean_t & getBoolean(const nlohmann::json & value)
+{
+    return ensureType(value, nlohmann::json::value_t::boolean).get_ref<const nlohmann::json::boolean_t &>();
+}
+
+Strings getStringList(const nlohmann::json & value)
+{
+    auto & jsonArray = getArray(value);
+
+    Strings stringList;
+
+    for (const auto & elem : jsonArray)
+        stringList.push_back(getString(elem));
+
+    return stringList;
+}
+
+StringMap getStringMap(const nlohmann::json & value)
+{
+    auto & jsonObject = getObject(value);
+
+    StringMap stringMap;
+
+    for (const auto & [key, value] : jsonObject)
+        stringMap[getString(key)] = getString(value);
+
+    return stringMap;
+}
+
+StringSet getStringSet(const nlohmann::json & value)
+{
+    auto & jsonArray = getArray(value);
+
+    StringSet stringSet;
+
+    for (const auto & elem : jsonArray)
+        stringSet.insert(getString(elem));
+
+    return stringSet;
+}
 }