Haskell的Monadic TicTacToe错误

Question

我开始在Haskell中实现TicTacToe：

import Control.Monad.State

data Player = X | O
data Field = Player | I deriving (Eq, Show, Read)

data GameField = G [[Field]]

type GameState = State GameField ()

initGame :: GameState
initGame = do
    put $ G [[I,I,I],[I,I,I],[I,I,I]]

action = do
    initGame

test = execState action $ G [[I,I,I],[I,I,I],[I,I,I]]

当我执行“测试”时，出现以下错误：

No instance for (Show GameField) arising from a use of `print'
Possible fix: add an instance declaration for (Show GameField)
In a stmt of an interactive GHCi command: print it

此问题的原因是什么，如何解决？

Answer 1

其实不是太含糊的消息。 如果没有Show实例，请添加一个：

data GameField = G [[Field]] deriving (Show)

Answer 2

您只是忘了在您的GameField牌上添加“衍生节目”。这可以通过添加它来解决，就像这样

import Control.Monad.State

data Player = X | O
data Field = Player | I deriving (Eq, Show, Read)

data GameField = G [[Field]]
  deriving Show

type GameState = State GameField ()

initGame :: GameState
initGame = do
    put $ G [[I,I,I],[I,I,I],[I,I,I]]

action = do
    initGame

test = execState action $ G [[I,I,I],[I,I,I],[I,I,I]]