如何修正Haskell蛋糕烘焙状态机的语法错误与逻辑问题？

需求说明 #

已定义如下Haskell类型：

data Event = AddEggs | AddFlour | AddSugar | Mix | Bake
  deriving (Eq, Show)

data State = Start | Error | Finished | HasEggs | HasFlour | HasSugar | HasFlourAndSugar | Mixed 
  deriving (Eq, Show)

状态机规则：

• 初始状态为Start，终止状态为Error或Finished
• 合法事件序列：
  1. [AddEggs, AddFlour, AddSugar, Mix, Bake]
  2. [AddEggs, AddSugar, AddFlour, Mix, Bake]
• AddFlour与AddSugar顺序可互换
• 进入Error或Finished后状态不再变更
• 任何违规操作直接进入Error且无法恢复

原始实现代码 #

data Event = AddEggs | AddFlour | AddSugar | Mix | Bake
  deriving (Eq, Show)

data State = Start | Error | Finished | HasEggs | HasFlour | HasSugar | HasFlourAndSugar | Mixed 
  deriving (Eq, Show)

stepHelper :: State -> Event -> Bool -> Bool -> State

stepHelper Start AddEggs False False = HasEggs
stepHelper Start AddEggs True _ = Error
stepHelper Start AddEggs _ True = Error
stepHelper Start _ _ _ = Error

stepHelper HasEggs AddFlour has_flour has_sugar = if has_flour then Error else HasFlour
stepHelper HasEggs AddSugar has_flour has_sugar = if has_sugar then Error else HasSugar

stepHelper HasEggs AddFlour _ has_sugar = stepHelper HasEggs AddFlour True has_sugar
stepHelper HasEggs AddSugar has_flour _ = stepHelper HasEggs AddSugar has_flour True
stepHelper _ Mix has_flour has_sugar = if has_flour && has_sugar then Mixed else Error
stepHelper Mixed Bake _ _ = Finished

step :: State -> Event -> State
step s e = stepHelper s e False False

-- do not edit this
bake :: [Event] -> State
bake events = go Start events
  where go state [] = state
        go state (e:es) = go (step state e) es

报错信息 #

运行stack runhaskell Set7Test.hs后出现：

Set7.hs:61:1: warning: [-Wtabs]
    Tab character found here, and in 29 further locations.
    Please use spaces instead.
   |
61 |         ------------------------------------------------------------------------------
   | ^^^^^^^^

Set7.hs:119:29: error: parse error on input `='
    |
119 |             go state (e:es) = go (step state e) es

修正后的代码 #

data Event = AddEggs | AddFlour | AddSugar | Mix | Bake
  deriving (Eq, Show)

data State = Start | Error | Finished | HasEggs | HasFlour | HasSugar | HasFlourAndSugar | Mixed 
  deriving (Eq, Show)

-- 核心状态转换函数，处理单个事件的状态变更
step :: State -> Event -> State
-- 终止状态保持不变
step Error _ = Error
step Finished _ = Finished

-- 初始状态Start的转换
step Start AddEggs = HasEggs
step Start _ = Error  -- Start状态下只能加鸡蛋，其他操作直接报错

-- HasEggs状态的转换
step HasEggs AddFlour = HasFlour
step HasEggs AddSugar = HasSugar
step HasEggs _ = Error  -- HasEggs状态下只能加面粉或糖，其他操作报错

-- HasFlour状态的转换
step HasFlour AddSugar = HasFlourAndSugar
step HasFlour _ = Error  -- HasFlour状态下只能加糖，其他操作报错

-- HasSugar状态的转换
step HasSugar AddFlour = HasFlourAndSugar
step HasSugar _ = Error  -- HasSugar状态下只能加面粉，其他操作报错

-- HasFlourAndSugar状态的转换
step HasFlourAndSugar Mix = Mixed
step HasFlourAndSugar _ = Error  -- 该状态下只能搅拌，其他操作报错

-- Mixed状态的转换
step Mixed Bake = Finished
step Mixed _ = Error  -- Mixed状态下只能烘焙，其他操作报错

-- 兜底匹配（理论上不会触发）
step _ _ = Error

-- 遍历事件序列的函数（保持原代码不变）
bake :: [Event] -> State
bake events = go Start events
  where go state [] = state
        go state (e:es) = go (step state e) es

修正说明 #

1. 语法错误修复：

   • 将所有制表符替换为空格，消除-Wtabs警告
   • 移除冗余的stepHelper函数，简化逻辑结构，避免参数传递导致的解析错误

2. 逻辑错误修复：

   • 新增终止状态（Error/Finished）的固定转换规则，确保进入后状态不再变更
   • 完善每个状态的合法事件判断，严格遵循规则：
     • Start仅接受AddEggs
     • HasEggs仅接受AddFlour或AddSugar
     • HasFlour仅接受AddSugar，转换到HasFlourAndSugar
     • HasSugar仅接受AddFlour，转换到HasFlourAndSugar
     • HasFlourAndSugar仅接受Mix，转换到Mixed
     • Mixed仅接受Bake，转换到Finished
   • 所有非法事件直接触发Error状态，符合规则要求

内容的提问来源于stack exchange，提问作者coderodde