fused-effects-lens

This package provides an interface to the lens library (and other such libraries such as microlens and generic-lens) that is compatible with fused-effects. The standard formulation of lens combinators for operating in MonadState contexts—use, .=, et al—rely on mtl for MonadState and MonadReader, which is not applicable to Reader and State effects.

This package is meant to be used alongside lens, like so:

import Control.Lens hiding (view, use, assign)
import Control.Effect.Lens (view, use, assign)

Example

Given a Context type that we will use in a State effect:

data Context = Context
  { _amount :: Int
  , _disabled :: Bool
  } deriving (Eq, Show)

makeLenses ''Context

We can can use the use combinators to extract a lens target from the current state, and assign to write to a field of that state:

stateTest :: (Member (State Context) sig, Carrier sig m, Monad m) => m Int
stateTest = do
  initial <- use amount
  amount .= (initial + 1)
  disabled .= True
  use amount

You can find a more complete example, including one that works with multiple State constraints in a single computation, in the test directory.

License

BSD3, like fused-effects.