Add rational typecaster

fixes #3

lib/attr_typecastable/types.rb

@@ -8,6 +8,7 @@
 require 'attr_typecastable/types/date'
 require 'attr_typecastable/types/date_time'
 require 'attr_typecastable/types/float'
+require 'attr_typecastable/types/rational'
 require 'attr_typecastable/types/boolean'
 require 'attr_typecastable/types/object'
 
@@ -25,6 +26,7 @@ module Types
       ::Date => AttrTypecastable::Types::Date,
       ::DateTime => AttrTypecastable::Types::DateTime,
       ::Float => AttrTypecastable::Types::Float,
+      ::Rational => AttrTypecastable::Types::Rational,
       ::Boolean => AttrTypecastable::Types::Boolean,
     }.freeze
 

lib/attr_typecastable/types/rational.rb

@@ -0,0 +1,18 @@
+require 'attr_typecastable/types/exception'
+require 'attr_typecastable/types/base'
+
+module AttrTypecastable
+  module Types
+    class Rational < Base
+      def do_typecast(value)
+        return value if value.is_a?(::Rational)
+
+        if value.respond_to?(:to_r)
+          value.to_r
+        else
+          raise CastError, "value does not have `to_r` method"
+        end
+      end
+    end
+  end
+end

spec/attr_typecastable_spec.rb

@@ -41,6 +41,7 @@ class User
     typed_attr_accessor :birthday_datetime, DateTime
     typed_attr_accessor :age, Integer
     typed_attr_accessor :bmi, Float
+    typed_attr_accessor :bmi_r, Rational
     typed_attr_accessor :property, CastToMoney
     typed_attr_accessor :adult, Boolean
     typed_attr_accessor :admin, Boolean, true_value: ["yes"]
@@ -105,6 +106,9 @@ def initialize(name: nil, default_name: nil)
       user.bmi = "20.1"
       assert { user.bmi > 20.09 && user.bmi < 20.11 }
 
+      user.bmi = "20.1"
+      assert { user.bmi == "20.1".to_r }
+
       user.property = 10000
       assert { user.property == Money.new(10000) }