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Haskell将Maybe转换为定义的类型

haskell

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VirtualProdigy · 技术社区 · 7 年前

Maybe find 功能。我定义了一个函数 FindNextState 包含一个元组和两个字符串。此函数调用 getListOfNextStates 它使用lambda和模式匹配从传入的元组中获取列表,然后使用函数的谓词我在该列表中找到匹配项。问题是查找返回值 getToState 函数,因为它期望在转换时。有没有什么方法可以转换 也许吧 由find函数返回?

代码

type State = String
type Symbol = String
type Transisition = (State, Symbol, State)

states = ["s0","s1"]
initState = "s0"
finalStates = ["s3"]
transisitionList = [("s0", "0", "s1"), ("s0", "1", "s1")]
dfa = (states, initState, finalStates, transisitionList)

getToState :: Transisition -> State
getToState  (_, _, toState) = toState


findNextState :: DFA -> State -> Symbol -> Maybe Transisition --Maybe Transisition is the issue, I need it to be my type of Transisition otherwise Haskell throws an error when I load my program 
findNextState (_,_,_,tList) state symbol =  getListOfNextStates tList state symbol

getListOfNextStates :: [Transisition] -> State -> Symbol -> Maybe Transisition
getListOfNextStates tList state symbol = find(\(sState,sym,eState) -> matchTransition state symbol (sState,sym,eState)) tList

样本输入

findNextState dfa "s2" "0"
Just ("s2","0","s3")

*Main> :t findNextState dfa "s2" "0"
findNextState dfa "s2" "0" :: Maybe Transisition

**所需代码**

findNextState :: DFA -> State -> Symbol -> State
findNextState (_,_,_,tList) state symbol = getToState( (getListOfNextStates tList state symbol) )

1 回复 | 直到 7 年前

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7

DarthFennec 7 年前

我建议你保持 Maybe ,因为它允许代码在没有找到匹配项时正常地失败。如果你那样做,你就会离开 getListOfNextStates 返回 Maybe Transition ,然后更改 findNextState 返回 Maybe State . 现在可以这样定义:

findNextState :: DFA -> State -> Symbol -> Maybe State
findNextState (_,_,_,tList) state symbol = case newstate of
    Just s -> Just (getToState s)
    Nothing -> Nothing
  where newstate = getListOfNextStates tList state symbol

或者更简洁地说,您可以使用 fmap :: (a -> b) -> Maybe a -> Maybe b (或其中缀版本, <$> )像这样:

findNextState :: DFA -> State -> Symbol -> Maybe State
findNextState (_,_,_,tList) state symbol = getToState <$> newstate
  where newstate = getListOfNextStates tList state symbol

如果你真的不认为会有一个失败的发现,或者你只是不在乎,你可以使用 fromJust :: Maybe Transition -> Transition

import Data.Maybe

getListOfNextStates :: [Transisition] -> State -> Symbol -> Transisition
getListOfNextStates tList state symbol = fromJust (find (\(sState,sym,eState) -> matchTransition state symbol (sState,sym,eState)) tList)

退货 Nothing ,有效地破坏了程序。我不会在真正的代码中这样做,除非你能绝对确保它永远不会发生。