在haskell中计算NFA的？-闭包

Question

我正在 Haskell 中实现一个非确定性有限自动机，我正在尝试实现计算 epsilon 闭包的函数。为此，NFA 实现为：

data Transaction = Transaction {
     start_state :: Int, 
     symbol :: Maybe Char, 
     end_state :: Int
} deriving Show

data Automaton = Automaton {
     initial_state :: Int, 
     states :: Set.Set Int,
     transactions :: [Transaction],
     final_states :: Set.Set Int, 
     language :: Set.Set Char
} deriving Show 

而关闭：

--Perform the computations necessary to eclosure
getClosure :: [Transaction] ->  [Int]
getClosure [] = []
getClosure [tr] = [end_state tr]
getClosure (tr:trs) = end_state tr : getClosure trs 

--Get the ?-closure of a given state 
eclosure :: Automaton -> Int -> [Int]
eclosure a s
  = s : List.concat
         [ eclosure a st
         | st <- getClosure . List.filter (equal_transaction Nothing s)
                     $ transactions a ]

问题是，如果闭包中存在循环，则代码将永远运行。我了解这种行为背后的原因，但我不知道如何解决。请你帮助我好吗？