YAML and JSON.mapping presence: true option (#4843)

Allows to distinguish absence of key-value pairs vs null values.

spec/std/json/mapping_spec.cr

@@ -146,6 +146,13 @@ private class JSONWithNilableUnion2
   })
 end
 
+private class JSONWithPresence
+  JSON.mapping({
+    first_name: {type: String?, presence: true, nilable: true},
+    last_name:  {type: String?, presence: true, nilable: true},
+  })
+end
+
 describe "JSON mapping" do
   it "parses person" do
     person = JSONPerson.from_json(%({"name": "John", "age": 30}))
@@ -427,4 +434,14 @@ describe "JSON mapping" do
     obj.value.should be_nil
     obj.to_json.should eq(%({}))
   end
+
+  describe "parses JSON with presence markers" do
+    it "parses person with absent attributes" do
+      json = JSONWithPresence.from_json(%({"first_name": null}))
+      json.first_name.should be_nil
+      json.first_name_present?.should be_true
+      json.last_name.should be_nil
+      json.last_name_present?.should be_false
+    end
+  end
 end

spec/std/yaml/mapping_spec.cr

@@ -89,6 +89,13 @@ private class YAMLWithTimeEpochMillis
   })
 end
 
+private class YAMLWithPresence
+  YAML.mapping({
+    first_name: {type: String?, presence: true, nilable: true},
+    last_name:  {type: String?, presence: true, nilable: true},
+  })
+end
+
 describe "YAML mapping" do
   it "parses person" do
     person = YAMLPerson.from_yaml("---\nname: John\nage: 30\n")
@@ -288,4 +295,14 @@ describe "YAML mapping" do
     yaml.value.should eq(Time.epoch_ms(1459860483856))
     yaml.to_yaml.should eq("---\nvalue: 1459860483856\n")
   end
+
+  describe "parses YAML with presence markers" do
+    it "parses person with absent attributes" do
+      yaml = YAMLWithPresence.from_yaml("---\nfirst_name:\n")
+      yaml.first_name.should be_nil
+      yaml.first_name_present?.should be_true
+      yaml.last_name.should be_nil
+      yaml.last_name_present?.should be_false
+    end
+  end
 end

src/json/mapping.cr

@@ -45,6 +45,7 @@ module JSON
   # * **root**: assume the value is inside a JSON object with a given key (see `Object.from_json(string_or_io, root)`)
   # * **setter**: if `true`, will generate a setter for the variable, `true` by default
   # * **getter**: if `true`, will generate a getter for the variable, `true` by default
+  # * **presence**: if `true`, a `{{key}}_present?` method will be generated when the key was present (even if it has a `null` value), `false` by default
   #
   # This macro by default defines getters and setters for each variable (this can be overrided with *setter* and *getter*).
   # The mapping doesn't define a constructor accepting these variables as arguments, but you can provide an overload.
@@ -78,6 +79,14 @@ module JSON
           @{{key.id}}
         end
       {% end %}
+
+      {% if value[:presence] %}
+        @{{key.id}}_present : Bool = false
+
+        def {{key.id}}_present?
+          @{{key.id}}_present
+        end
+      {% end %}
     {% end %}
 
     def initialize(%pull : ::JSON::PullParser)
@@ -144,6 +153,12 @@ module JSON
           @{{key.id}} = (%var{key.id}).as({{value[:type]}})
         {% end %}
       {% end %}
+
+      {% for key, value in properties %}
+        {% if value[:presence] %}
+          @{{key.id}}_present = %found{key.id}
+        {% end %}
+      {% end %}
     end
 
     def to_json(json : ::JSON::Builder)

src/yaml/mapping.cr

@@ -58,6 +58,7 @@ module YAML
   # * *converter* takes an alternate type for parsing. It requires a `#from_yaml` method in that class, and returns an instance of the given type. Examples of converters are `Time::Format` and `Time::EpochConverter` for `Time`.
   # * **setter**: if `true`, will generate a setter for the variable, `true` by default
   # * **getter**: if `true`, will generate a getter for the variable, `true` by default
+  # * **presence**: if `true`, a `{{key}}_present?` method will be generated when the key was present (even if it has a `null` value), `false` by default
   #
   # This macro by default defines getters and setters for each variable (this can be overrided with *setter* and *getter*).
   # The mapping doesn't define a constructor accepting these variables as arguments, but you can provide an overload.
@@ -85,6 +86,14 @@ module YAML
           @{{key.id}}
         end
       {% end %}
+
+      {% if value[:presence] %}
+        @{{key.id}}_present : Bool = false
+
+        def {{key.id}}_present?
+          @{{key.id}}_present
+        end
+      {% end %}
     {% end %}
 
     def initialize(%pull : ::YAML::PullParser)
@@ -100,6 +109,7 @@ module YAML
         {% for key, value in properties %}
           when {{value[:key] || key.id.stringify}}
             %found{key.id} = true
+
             %var{key.id} =
               {% if value[:nilable] || value[:default] != nil %} %pull.read_null_or { {% end %}
 
@@ -144,6 +154,12 @@ module YAML
           @{{key.id}} = %var{key.id}.as({{value[:type]}})
         {% end %}
       {% end %}
+
+      {% for key, value in properties %}
+        {% if value[:presence] %}
+          @{{key.id}}_present = %found{key.id}
+        {% end %}
+      {% end %}
     end
 
     def to_yaml(%yaml : ::YAML::Builder)