Gemfile

source 'https://rubygems.org'

gem 'dry-types', '0.7.2'
gem 'reform', '2.2.1'
gem 'reform-rails', '0.1.2'

reform_types_issue.rb

require 'rubygems'
require 'bundler/setup'

require 'dry-types'
require 'reform'
require 'reform/form/active_model/validations'

Reform::Form.class_eval do
  include Reform::Form::ActiveModel::Validations
end

class GeoForm < Reform::Form
  include Dry::Types.module

  property :lat, type: Form::Decimal
  property :long, type: Form::Decimal
end

model = Struct.new(:lat, :long).new
form = GeoForm.new(model)
form.validate(lat: '1.2', long: '3.4')

# I except BigDecimal here, but get String
p form.lat.class # => String