Introduce the idea of syntax sugar lint rules

Signed-off-by: Juan Cruz Viotti <[email protected]>

src/linter/CMakeLists.txt

@@ -19,6 +19,9 @@ noa_library(NAMESPACE sourcemeta PROJECT alterschema NAME linter
     simplify/minimum_real_for_integer.h
     simplify/single_type_array.h
 
+    # Desugar
+    desugar/type_boolean_as_enum.h
+
     # Redundant
     redundant/additional_properties_default.h
     redundant/content_media_type_without_encoding.h

src/linter/desugar/type_boolean_as_enum.h

@@ -0,0 +1,35 @@
+class TypeBooleanAsEnum final : public Rule {
+public:
+  TypeBooleanAsEnum()
+      : Rule{"type_boolean_as_enum",
+             "Setting `type` to `boolean` is syntax sugar for an enumeration "
+             "of two values: `false` and `true`"} {};
+
+  [[nodiscard]] auto
+  condition(const sourcemeta::jsontoolkit::JSON &schema, const std::string &,
+            const std::set<std::string> &vocabularies,
+            const sourcemeta::jsontoolkit::Pointer &) const -> bool override {
+    return contains_any(
+               vocabularies,
+               {"https://json-schema.org/draft/2020-12/vocab/validation",
+                "https://json-schema.org/draft/2019-09/vocab/validation",
+                "http://json-schema.org/draft-07/schema#",
+                "http://json-schema.org/draft-06/schema#",
+                "http://json-schema.org/draft-04/schema#",
+                "http://json-schema.org/draft-03/schema#",
+                "http://json-schema.org/draft-02/hyper-schema#",
+                "http://json-schema.org/draft-01/hyper-schema#"}) &&
+           schema.is_object() && schema.defines("type") &&
+           schema.at("type").is_string() &&
+           schema.at("type").to_string() == "boolean" &&
+           !schema.defines("enum") && !schema.defines("const");
+  }
+
+  auto transform(Transformer &transformer) const -> void override {
+    auto choices = sourcemeta::jsontoolkit::JSON::make_array();
+    choices.push_back(sourcemeta::jsontoolkit::JSON{false});
+    choices.push_back(sourcemeta::jsontoolkit::JSON{true});
+    transformer.assign("enum", choices);
+    transformer.erase("type");
+  }
+};

src/linter/include/sourcemeta/alterschema/linter.h

@@ -28,6 +28,9 @@ enum class LinterCategory {
   /// Rules that simplify the given schema
   Simplify,
 
+  /// Rules that simplify keywords that are syntax sugar to other keywords
+  Desugar,
+
   /// Rules that remove schema redundancies
   Redundant
 };

src/linter/linter.cc

@@ -39,6 +39,8 @@ template <typename T> auto every_item_is_boolean(const T &container) -> bool {
 #include "simplify/maximum_real_for_integer.h"
 #include "simplify/minimum_real_for_integer.h"
 #include "simplify/single_type_array.h"
+// Desugar
+#include "desugar/type_boolean_as_enum.h"
 // Redundant
 #include "redundant/additional_properties_default.h"
 #include "redundant/content_media_type_without_encoding.h"
@@ -176,6 +178,9 @@ auto add(Bundle &bundle, const LinterCategory category) -> void {
       bundle.add<MinimumRealForInteger>();
       bundle.add<SingleTypeArray>();
       break;
+    case LinterCategory::Desugar:
+      bundle.add<TypeBooleanAsEnum>();
+      break;
     case LinterCategory::Redundant:
       bundle.add<AdditionalPropertiesDefault>();
       bundle.add<ContentMediaTypeWithoutEncoding>();