使用约束构建通用函数

发表于 分类于 ，

函数

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handleDBException :: SomeException -> HandlerFor site (Either DatabaseException a)
handleDBException e = 
    let eMsg = T.pack $ show e
        in
        if "Duplicate" `T.isInfixOf` eMsg
            then return $ Left DuplicateEntryException
            else return $ Left $ GeneralDatabaseException eMsg

tryDbAction :: YesodDB site a -> HandlerFor site (Either DatabaseException a)
tryDbAction dbAction = ( Right <$> dbAction) `catch` handleDBException

上述函数并没有使用到HandlerFor这个类型相关的函数，完全可以用Monad约束代替。

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handleDBException :: Monad m => SomeException -> m (Either DatabaseException a)
handleDBException e = 
    let eMsg = T.pack $ show e
        in
        if "Duplicate" `T.isInfixOf` eMsg
            then return $ Left DuplicateEntryException
            else return $ Left $ GeneralDatabaseException eMsg

tryDbAction :: (Functor m, Monad m, MonadUnliftIO m) => m a -> m (Either DatabaseException a)
tryDbAction dbAction = ( Right <$> dbAction) `catch` handleDBException

newtype

在构建newtype的时候也是一样，例如构建一个ReaderT

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newtype ServeFor site param a = ServeFor (ReaderT (ServiceContext param) (HandlerFor site) a)
    deriving (Functor, Applicative, Monad, MonadIO, MonadResource, MonadLogger)

也可以修改如下

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newtype Monad m => ServeFor param m a = ServeFor (ReaderT (ServiceContext param) m a)