我一直试图想出一种简单直观的方法来使用
Haskell数据库.我从
Yesod book中获取了这些代码,并尝试将其清理干净,以便更容易理解和使用.
- {-# LANGUAGE QuasiQuotes,TemplateHaskell,TypeFamilies,OverloadedStrings #-}
- {-# LANGUAGE GADTs,FlexibleContexts #-}
- import Database.Persist
- import Database.Persist.sqlite (withsqliteConn,runsqlConn,runMigration)
- import Database.Persist.TH (share,mkPersist,mkMigrate,sqlSettings,persist)
- share [mkPersist sqlSettings,mkMigrate "migrateAll"] [persist|
- Person -- Table name
- name String -- String value
- age Int Maybe -- Numerical value
- |]
- updateDB x y = withsqliteConn "data.db" $runsqlConn $do
- runMigration migrateAll -- Creates "Person" table if one doesn't exist
- insert $Person x $Just y -- Inserts values into .db file
- main = do
- updateDB "Frank Silver" 40 -- adds name "Frank Silver" and age "40" to data.db file
- No instance for (Control.Monad.Trans.Resource.MonadResource IO)
- arising from a use of `updateDB'
- Possible fix:
- add an instance declaration for
- (Control.Monad.Trans.Resource.MonadResource IO)
- In a stmt of a 'do' block: updateDB "Frank Silver" 40
- In the expression: do { updateDB "Frank Silver" 40 }
- In an equation for `main': main = do { updateDB "Frank Silver" 40 }
任何建议指向我正确的方向将不胜感激.
同
- main = do
- updateDB "Frank Silver" 40
updateDB“Frank Silver”40的类型被推断为IO(),因为这是main的默认类型(对于某些a,它必须具有类型IO a).但是从定义来看,它的类型被推断为MonadRescource m => m a for some a(可能是a =(),但我不确定),并且没有实例MonadResource IO.因此,您需要将updateDB转换为IO操作,正常的方法是runResourceT,它将ResourceT m a转换为m a(此处为m = IO),因此
- main = runResourceT $updateDB "Frank Silver" 40
作品.
