"No instance for" 已经实施的特征

Question

我想使用 Servant 的 ClientM monad with finally:: MonadBaseControl IO m => m a -> m b -> m a，但遇到错误 No instance for (MonadBaseControl IO ClientM)。

奇怪的是，这个实例似乎是 defined already, be it from an internal 模块。

我是否需要以某种方式显式导入此类实例？

Answer 1

如果像 Daniel 所说的那样导入定义所需实例的模块，它就可以正常工作：

Prelude> import Control.Exception.Lifted
Prelude Control.Exception.Lifted> import Servant.Client
Prelude Control.Exception.Lifted Servant.Client> :set -XTypeApplications
Prelude Control.Exception.Lifted Servant.Client> :type finally @ ClientM
finally @ ClientM :: ClientM a -> ClientM b -> ClientM a

"No instance for" 已经实施的特征

"No instance for" trait that's already implemented

haskell

servant