aaronallen8455 / optician

Generate optics with a GHC plugin

Optician 👓

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.

Example

{-# 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")

Features

• 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

Quick-start guide

1. Add the optician package as a dependency of your project.
2. Pass the -fplugin Optician argument to GHC to load the plugin. This can be done project wide by adding ghc-options: -fplugin Optician to your *.cabal or project.yaml file, or on a per module basis using the {-# OPTIONS_GHC -fplugin Optician #-} pragma.
3. Import the Optician module and use field 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 the TypeApplications extension (see example above).

Support for OverloadedLabels

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

Limitations

• Currently only GHC 9.4.x and 9.6.x are supported