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如何訪問我的查詢[VarcharIntDate]中的數據?haskell postgresql-simple,如何從更廣泛的查詢中訪問特定數據
data VarcharIntDate = VarcharIntDate {
vc :: Maybe String,
i :: Maybe Int,
d :: Maybe Date
} deriving (Show)
instance FromRow VarcharIntDate where
fromRow = VarcharIntDate <$> field <*> field <*> field
我知道如何打印出來,但我不能做很多其他事情,因爲我不`噸明白如何對待「IO [VarcharIntDate]」
print =<< (query_ conn "select vc,i,d from varintdate" :: IO [VarcharIntDate])
,但我想做什麼是從所有行返回的[VarcharIntDate]或'i'的第n行訪問'd',所以我可以開始從查詢中提取數據並開始使用它。