class AClassThatUsesSend
  def action
    # here's where the action happens
    @helper_type = "foo"

    # this is equivalent to self.foo_helper or just foo_helper
    send("#{@helper_type}_helper")
  end

  def foo_helper
    #more fun here
  end
end

# of course you don't want to just take user input unfiltered and feed it
# into send() for possible safety concerns, so consider something like this if
# @helper_type comes from a param

valid_helpers = ['foo', 'bar', 'quux']
send("#{@helper_type}_helper"} if valid_helpers.include? @helper_type

# There are many ways you could sanitize the value of @helper_type.
# This is just one approach.