Add simple validation DSL

lib/primalize/single.rb

@@ -85,6 +85,10 @@ def primalize primalizer, &coerce
         Primalizer.new(primalizer, &coerce)
       end
 
+      def validate type, &validation
+        Validation.new(type, validation)
+      end
+
       def type_mismatch_handler= handler
         @type_mismatch_handler = handler
       end
@@ -400,5 +404,28 @@ def inspect
         "any#{@types.empty? ? nil : params}"
       end
     end
+
+    class Validation
+      def initialize type, validation
+        @type = type
+        @validation = validation
+      end
+
+      def coerce value
+        if @type.respond_to? :coerce
+          @type.coerce value
+        else
+          value
+        end
+      end
+
+      def === value
+        @type === value && @validation.(coerce(value))
+      end
+
+      def inspect
+        "validate(#{@type.inspect})"
+      end
+    end
   end
 end

spec/primalize/single_spec.rb

@@ -368,5 +368,18 @@ def order_serializer_class.inspect
         CSV
       end
     end
+
+    it 'validates attributes' do
+      my_serializer = Class.new(Single) do
+        attributes(
+          number: validate(integer(&:to_i)) { |value| value.between? 0, 10 },
+        )
+      end
+
+      expect(my_serializer.new(double(number: 10)).call).to eq(number: 10)
+      expect(my_serializer.new(double(number: nil)).call).to eq(number: 0)
+      expect(my_serializer.new(double(number: '0')).call).to eq(number: 0)
+      expect { my_serializer.new(double(number: 12)).call }.to raise_error TypeError
+    end
   end
 end