在Haskell

Question

錯誤的單子井字遊戲，我開始在Haskell實現井字遊戲：

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]] 

當我執行「測試」我收到以下錯誤：

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 

這是什麼問題的原因，我該如何解決它？

Answer 1

其實不是一個太神祕的信息。如果沒有Show情況下，添加一個：

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

Answer 2

你根本都忘了把它加，這樣

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]]