如何编写Haskell函数来查询数据库？

Question

所以我正在尝试在Haskell中编写一个数据库层,以与SQLite DB进行交互.我在关于数据库的章节中遵循了Real World Haskell一书中的指示.这是我到目前为止:

{-# LANGUAGE FlexibleContexts #-}

import Database.HDBC
import Database.HDBC.Sqlite3

db = "dev.db"
conn = connectSqlite3 db

getPerson person =
  quickQuery' conn "select name from person where name like ?" [toSql person]

main :: IO ()
main = do
  print $ getPerson "Michael"

但我得到错误:

Could not deduce (IConnection (IO Connection))
     arising from a use of ‘quickQuery'’
   from the context: convertible-1.1.1.0:Data.Convertible.Base.Convertible
                       a SqlValue
     bound by the inferred type of
              getPerson :: convertible-1.1.1.0:Data.Convertible.Base.Convertible
                             a SqlValue =>
                           a -> IO [[SqlValue]]

作为Haskell初学者,我真的不明白.我可以在getPerson上添加一个类型注释,但这似乎并没有解决它.

Answer 1

此代码已编译

{-# LANGUAGE FlexibleContexts #-}

import Database.HDBC
import Database.HDBC.Sqlite3
import Data.Convertible.Base

db :: String
db = "dev.db"

getPerson :: (Convertible a SqlValue, IConnection conn) => conn -> a -> IO [[SqlValue]]
getPerson conn person =
  quickQuery' conn "select name from person where name like ?" [toSql person]

main :: IO ()
main = do
    conn <- connectSqlite3 db
    person <- getPerson conn "Michael"
    print person

connectSqlite3 :: FilePath -> IO Connection所以conn应该提取的价值IO monad.

也许有更好的方法来写这个使用bind传递连接quickQuery但我不知道,我从来没有使用过这个库,只是从类型中猜测ghci