Introduces Nullable validator

lib/ex_json_schema/validator.ex

@@ -79,6 +79,8 @@ defmodule ExJsonSchema.Validator do
 
   def do_validation_errors(root = %Root{}, schema = %{}, data, path) do
     schema
+    # TODO
+    # |> Map.put_new("nullable", false)
     |> Enum.flat_map(fn {propertyName, _} = property ->
       case validator_for(propertyName) do
         nil -> []
@@ -156,6 +158,7 @@ defmodule ExJsonSchema.Validator do
   defp validator_for("$ref"), do: ExJsonSchema.Validator.Ref
   defp validator_for("required"), do: ExJsonSchema.Validator.Required
   defp validator_for("type"), do: ExJsonSchema.Validator.Type
+  defp validator_for("nullable"), do: ExJsonSchema.Validator.Nullable
   defp validator_for("uniqueItems"), do: ExJsonSchema.Validator.UniqueItems
   defp validator_for(_), do: nil
 end

lib/ex_json_schema/validator/error.ex

@@ -117,6 +117,10 @@ defmodule ExJsonSchema.Validator.Error do
     defstruct([:missing])
   end
 
+  defmodule Nullable do
+    defstruct([:allowed])
+  end
+
   defmodule Type do
     defstruct([:expected, :actual])
   end

lib/ex_json_schema/validator/error/string_formatter.ex

@@ -194,6 +194,12 @@ defmodule ExJsonSchema.Validator.Error.StringFormatter do
     end
   end
 
+  defimpl String.Chars, for: Error.Nullable do
+    def to_string(%Error.Nullable{allowed: false}) do
+      "Nullable value is not allowed."
+    end
+  end
+
   defimpl String.Chars, for: Error.Type do
     def to_string(%Error.Type{expected: expected, actual: actual}) do
       "Type mismatch. Expected #{type_names(expected)} but got #{type_names(actual)}."

lib/ex_json_schema/validator/nullable.ex

@@ -0,0 +1,35 @@
+defmodule ExJsonSchema.Validator.Nullable do
+  @moduledoc """
+  `ExJsonSchema.Validator` implementation for `"nullable"` attributes.
+
+  See:
+  https://tools.ietf.org/html/draft-fge-json-schema-validation-00#section-5.5.2
+  https://tools.ietf.org/html/draft-wright-json-schema-validation-01#section-6.25
+  https://tools.ietf.org/html/draft-handrews-json-schema-validation-01#section-6.1.1
+  """
+
+  alias ExJsonSchema.Validator.Error
+
+  @behaviour ExJsonSchema.Validator
+
+  @impl ExJsonSchema.Validator
+  def validate(_, _, {"nullable", nullable}, data, _) do
+    do_validate(nullable, data)
+  end
+
+  def validate(_, _, _, data, _) do
+    # by default nullable is not allowed
+    nullable = false
+    do_validate(nullable, data)
+  end
+
+  defp do_validate(nullable, data) do
+    if !nil_allowed?(nullable) && is_nil(data) do
+      [%Error{error: %Error.Nullable{allowed: false}}]
+    else
+      []
+    end
+  end
+
+  defp nil_allowed?(nullable), do: nullable == true
+end

lib/ex_json_schema/validator/type.ex

@@ -35,6 +35,7 @@ defmodule ExJsonSchema.Validator.Type do
     end
   end
 
+  defp valid?(_, _, nil), do: true
   defp valid?(_, "number", data), do: is_number(data)
   defp valid?(_, "array", data), do: is_list(data)
   defp valid?(_, "object", data), do: is_map(data)

test/ex_json_schema/validator_test.exs

@@ -764,6 +764,19 @@ defmodule ExJsonSchema.ValidatorTest do
              validate(%{"type" => "string"}, 666, error_formatter: Error.StringFormatter)
   end
 
+  test "allows attibute to have value nil if nullable is not specified" do
+    assert :ok = validate(%{"type" => "string"}, nil, error_formatter: Error.StringFormatter)
+  end
+
+  test "allows attibute to have value nil if nullable is allowed" do
+    assert :ok = validate(%{"type" => "string", "nullable" => true}, nil, error_formatter: Error.StringFormatter)
+  end
+
+  test "does not allowe nil values" do
+    assert {:error, [{"Nullable value is not allowed.", "#"}]} =
+             validate(%{"type" => "string", "nullable" => false}, nil, error_formatter: Error.StringFormatter)
+  end
+
   test "using the string formatter by default" do
     assert {:error, [{"Type mismatch. Expected String but got Integer.", "#"}]} = validate(%{"type" => "string"}, 666)
   end