“No instance for” trait that's already implemented

问题

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?

回答1:

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

来源：https://stackoverflow.com/questions/50439760/no-instance-for-trait-thats-already-implemented