Scanl在Haskell中 [英] Scanl in Haskell

问题描述

  scanl（\exp y  - > scanl（\ x  - > if（isLetter x）then update exp（Literal x）else 
 if x =='+'然后更新exp Epsilon+else 
 if x =='*'然后更新exp Epsilon*else 
更新exp Epsilon|）y）Epsilon q 
  

我有一个[Char]的列表，我想将它们转换为我的数据类型，但我遇到了一些问题用它。
我喜欢这样的[a + b，c * d]，我希望将scanl应用于a + b
时的结果保留下来，以便我可以将它用于第二个列表。有人可以帮我吗？

我的数据类型是：

pre $ data Reg = Epsilon |文字字符|或Reg Reg |然后Reg Reg | Star Reg推导出Eq


和

  update :: Reg  - >注册 - > [Char]  - >注册
更新ab=（a`Then` b）
更新ab|=（a`或`b）
更新ab*=（Star a）
更新ab+=（加a）
更新ab？ =（opt a）
  

我试图改变这样的形式[a |，为了记录， scanl code>在传递状态方面并不是很好，特别是在这里你不希望结果成为一个列表，而只是最终的结果。更广义的方法是使用 fold 来代替。

scanl (\exp y -> scanl (\x -> if (isLetter x) then update exp (Literal x) "" else 
                    if x=='+' then update exp Epsilon "+" else
                    if x=='*' then update exp Epsilon "*" else
                    update exp Epsilon "|") y) Epsilon q

I have a list of [Char] and I want to transform them into my data type but I am having some problems with it. I have somenthing like this ["a+b","c*d"] and I want the result of when I apply scanl on "a+b" to remain so I can use it for the second list. Can someone help me ?

My data type is:

data Reg = Epsilon | Literal Char | Or Reg Reg |  Then Reg Reg |  Star Reg  deriving Eq

and

update:: Reg -> Reg -> [Char] -> Reg 
update a b "" = (a `Then` b)
update a b "|"= (a `Or` b) 
update a b "*" = (Star a) 
update a b "+" = (plus a)
update a b "?" = (opt a)  

And I am trying to transform something like this ["a|","bc"] to (Then a (Or b c))

解决方案

For the record, scanl isn't great at passing state along, and especially here you don't want the result to be a list at all, but just a final result. The more generalized approach would be to use a fold instead.

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