“无实例”已经实施的特质 [英] "No instance for" trait that's already implemented
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问题描述
我想使用仆人的 ClientM monad with finally :: MonadBaseControl IO m = > m a - > m b - > ma ，但遇到错误没有（MonadBaseControl IO ClientM）实例。
奇怪的是，这个确切的实例似乎是已定义，可以从 internal 。
我需要以某种方式显式导入这些实例吗？
正如丹尼尔所说，如果你导入定义所需实例的模块，那么它工作得很好：
<$
p $ p>
前奏> 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> ：最后输入@ ClientM
finally @ ClientM :: ClientM a - > ClientM b - > ClientM a
I'd like to use Servant's ClientM monad with finally:: MonadBaseControl IO m => m a -> m b -> m a, but am faced with error No instance for (MonadBaseControl IO ClientM).
Oddly, this exact instance appears to be defined already, be it from an internal module.
Do I need to explicitly import such instances somehow?
解决方案
It works fine if, as Daniel said, you import the module that defines the desired instance:
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
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