Separate class and instance in different modules in Haskell?

In one module, I define a class:

class Monad m => MonadTime m where
    currentTime :: m UTCTime

The MonadTime is exported.

In the different module, I wanted to create an instance:

-- MonadTime is imported

instance MonadTime IO where
    currentTime :: IO UTCTime
    currentTime = Data.Time.Clock.getCurrentTime

However, the issue is:

‘currentTime’ is not a (visible) method of class ‘MonadTime’

If I move the instance to the first module, where class is defined, everything works. But my idea was to provide the implementation in main module.

How should I resolve this?

>Solution :

You need to export it with the method, so:

module MyModule(MonadTime(currentTime)) where

class Monad m => MonadTime m where
    currentTime :: m UTCTime

and in the other module:

module MyOtherModule where

import MyModule(MonadTime(currentTime))

instance MonadTime IO where
    currentTime :: IO UTCTime
    currentTime = Data.Time.Clock.getCurrentTime