创建PersistBackend的自定义实例 [英] Making Custom Instances of PersistBackend
问题描述
newtype T ma = T {unT :: StateT State(SqlPersist) m)a)
导出(Monad,MonadState State)
并且想要使用persistent insert
和 lookup
调用,所以我需要做一个 PersistBackend
实例为 T
。但是,幻像类型将特定的后端编码到 Key
返回类型中 - 这会导致一些额外的麻烦。为了解决幻像类型问题,我的实例具有如下形式:
pre $实例(Monad m,MonadIO m,MonadBaseControl IO m)= > PersistBackend T m其中
insert = T。电梯 。 liftM(Key。unKey)。插入
...以及大约十几种这样的方法...
我是否会盲目地俯瞰更简单的方法?除了通过手动提升来调用函数之外的其他方法: insertT = T。电梯 。插入
我有两个不同的建议:
- 翻转所有内容,并将SqlPersist放在您的StateT中,而不是反过来。 使用提升版本的功能创建模块,类似到
MonadState
。 。
我会选择(1)。如果有很多人遇到这种情况,我想我们可以有一个专门的软件包来提供所有功能的提升版本。
I have a monad transformer stack of the form:
newtype T m a = T { unT :: StateT State (SqlPersist m) a }
deriving (Monad, MonadState State)
And want to use the persistent insert
and lookup
calls, so I need to make a PersistBackend
instance for T
. However, a phantom type encodes the specific backend into the Key
return type - this causes some extra headaches. To solve the phantom type issue, my instance has the form:
instance (Monad m, MonadIO m, MonadBaseControl IO m) => PersistBackend T m where
insert = T . lift . liftM (Key . unKey) . insert
... and about a dozen such methods ...
Am I blindly overlooking an easier way? A way other than calling the function by manually lifting: insertT = T . lift . insert
I have two different recommendations:
- Flip things around, and put the SqlPersist around your StateT instead of vice-versa.
- Create a module with lifted versions of the functions, similar to what
MonadState
does.
I'd go for (1). If a lot of people are running into this, I suppose we could have a specialized package providing the lifted versions of all the functions.
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