Use eval to clear up a lot of conditionals.

Generate four positions by TYPE: a, b, c, and d.

class ShainFactory
 def create(type, alphabet)
  shain = nil
  if type == 'a'
    shain = a.new(alphabet)
  elsif type == 'b'
    shain = b.new(alphabet)
  elsif type == 'c'
    shain = c.new(alphabet)
  elsif type == 'd'
    shain = d.new(alphabet)
  end
  shain
 end
end

Since it is too long, use eval to make it clear. eval executes the specified string as Ruby program code and returns the result. If the content of type is a, a.new is returned.

class ShainFactory
 def create(type, alphabet)
  eval "#{type}.new(alphabet)"
 end
end

The code is now clear.