Type class for accessing Verbosity.

Lets define simple data type that looks something like:

data Config = Config
    { _appVerbosity :: Verbosity
    , ...
    }
  deriving (Show, ...)

Now we can define instance of HasVerbosity by hand:

instance HasVerbosity Config where
    verbosity f c@Config{_appVerbosity = a} =
        (b -> c{_appVerbosity = b}) <$> f a

Package lens has TemplateHaskell functions that can define lenses for you:

import Control.Lens.TH (makeLenses)

data Config = Config
    { _appVerbosity :: Verbosity
    , ...
    }
  deriving (Show, ...)

makeLenses ''Config

Don't forget to to turn on TemplateHaskell by putting following pragma at the beginning of your module:

{-# LANGUAGE TemplateHaskell #-}

Now definition of HasVerbosity instance will look like:

instance HasVerbosity Config where
    verbosity = appVerbosity   -- Lens generated by makeLenses.