# fused-effects-lens
This package provides an interface to the [`lens`](https://github.com/ekmett/lens) library (and other such libraries such as `microlens` and `generic-lens`) that is compatible with [`fused-effects`](https://github.com/robrix/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:
``` haskell
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:
``` haskell
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:
``` haskell
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`.