This is a GHC plugin that generates lenses and prisms without the need for
template haskell or generics. It is intended to be used with the optics
library although support for lens
could be added in the future.
{-# OPTIONS_GHC -fplugin Optician #-}
{-# LANGUAGE DataKinds, TypeApplications #-}
import Optician (field, _Ctor)
import Optics (over, preview)
data Person = MkPerson
{ name :: String
, age :: Int
}
incrementAge :: Person -> Person
incrementAge = over (field @"age") (+1)
data Pet
= Dog String
| Cat String
getDogName :: Pet -> Maybe String
getDogName = preview (_Ctor @"Dog")
- Optics are automatically available for in-scope data constructors
- Improved compilation speed compared to Generics and Template Haskell based optics solutions
- Supports polymorphic updates
- Custom error messages
- Fully compatible with GADTs
- Good type inference
- Add the
optician
package as a dependency of your project. - Pass the
-fplugin Optician
argument to GHC to load the plugin. This can be done project wide by addingghc-options: -fplugin Optician
to your*.cabal
orproject.yaml
file, or on a per module basis using the{-# OPTIONS_GHC -fplugin Optician #-}
pragma. - Import the
Optician
module and usefield
and_Ctor
to generate lenses and prisms respectively. They both require a type-level string argument correspondind to the field or data constructor name which can be given using theTypeApplications
extension (see example above).
For greater terseness, you can use the OverloadedLabels
extension to generate
lenses instead of using field
. To do this, import the Optician.Label
module
and write #fieldName
. For example:
{-# LANGUAGE OverloadedLabels #-}
import Optician.Label
import Optics ((%))
data Foo = Foo { foo :: Bar }
data Bar = Bar { bar :: Bool }
fooBar :: Lens' Foo Bool
fooBar = #foo % #bar
- Currently only GHC 9.4.x and 9.6.x are supported