module Goliath::Rack::Validation::Coerce

def call(env)
  begin
    coerce_value(env['params']) if @coerce_instance
    nil
  rescue FailedCoerce => e
    return e.error unless @optional
  end
  super if defined?(super)
end

def coerce_setup!(opts={})
  as = opts.delete(:as)
  if as
    unless Class === as
      raise Exception.new(NOT_CLASS_ERROR)
    end
    @coerce_instance = as.new
    unless @coerce_instance.respond_to?(:coerce)
      raise Exception.new(INVALID_COERCE_TYPE % @coerce_instance)
    end
  end
end

def coerce_value(params)
  opts = {:default => @default, :message => @message}
  value_before_coerce = fetch_key(params)
  value_after_coerce = @coerce_instance.coerce(value_before_coerce, opts)
  fetch_key(params, value_after_coerce)
end