如何將系統時間傳遞給Haskell中的變數

我正在嘗試使用 Haskell 從系統時鐘獲取當前 POSIX 時間，但出現以下錯誤：

    ? No instance for (Show (IO POSIXTime))
        arising from the sixth field of ‘Block’ (type ‘IO POSIXTime’)
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    ? When deriving the instance for (Show Block)
   |
16 |                    , nonce          :: Maybe Integer } deriving Show
   |                                                                 ^^^^

這是背景關系的代碼：

import Data.Time
import Data.Time.Clock.POSIX

data Transaction = Transaction { from  :: String
                               , to    :: String
                               , value :: Float } deriving Show

data Block = Block { version        :: Int
                   , index          :: Int
                   , transactions   :: [Transaction]
                   , prevBlockHash  :: String
                   , merkleRootHash :: String
                   , time           :: IO POSIXTime
                   , nonce          :: Maybe Integer } deriving Show

genesis :: Block
genesis = Block version index transactions prevBlockHash merkleRootHash time Nothing
    where version = 1
          index = 0
          transactions = []
          prevBlockHash = "000000000000000000000000000000000"
          merkleRootHash = ""
          time = getPOSIXTime
          nonce = Nothing

我已經參考了許多關于堆疊溢位的類似問題的解決方案，但幾天來一直無法解決該錯誤。

uj5u.com熱心網友回復：

讓我們從使用您擁有的型別的插圖開始

data Block = Block { 
                     version        :: Int
                   -- removed a few field for illustrative purposes 
                   , time           :: IO POSIXTime
                   , nonce          :: Maybe Integer } deriving Show

正如所評論的那樣，IO POSIXTime它并不是真正的 a POSIXTime，而是獲得 a 的動作的表示POSIXTime。讓我們將型別更改為實際的 POSIXTime。

data Block = Block { 
                     version        :: Int
                   , time           :: POSIXTime
                   , nonce          :: Maybe Integer } deriving Show

現在我們可以調整 genesis 來處理我們的新塊： import Data.Time import Data.Time.Clock.POSIX

data Block = Block { 
                     version        :: Int
                   , time           :: POSIXTime
                   , nonce          :: Maybe Integer } deriving Show


genesis :: IO Block
genesis = 
  do
     time <- getPOSIXTime
     let version = 1
     let nonce = Nothing
     return (Block version time nonce)

blockTimeAsUTCString :: Block -> String -- No need for IO
blockTimeAsUTCString (Block {time = p}) = show $ posixSecondsToUTCTime p

main :: IO ()
main = ioBlock >>= print >>
       ioBlock >>= putStrLn . blockTimeAsUTCString
       where ioBlock = genesis
            

請注意我們如何將 IO 從 Block 內部移到外部，以及它如何與 Haskell 可執行檔案始終為 an 的事實很好地配合，IO ()并blockTimeAsUTCString展示了如何在不依賴 IO 的情況下處理記錄。

uj5u.com熱心網友回復：

如果您真的堅持使用這種型別簽名，Block我將繼續實施Show (IO POSIXTIme)：

instance Show (IO POSIXTime) where 
  show _ = "Time shower"  

你可能會去做更一般的Show (IO a)例子

instance Show (IO a) where 
  show _ = "I can't Show what is inside this IO monad" 

最終，您可以撰寫自己的Show實體Block而不是派生它：

instance Show Block where 
  show block = "Block " <> show (version block) <> " .... " 

否則將Block型別簽名更改為POSIXTime不包含在IO？

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