Csongor Kiss bio photo

Csongor Kiss

I write here once every 2 years

Twitter Google+ LinkedIn Github Youtube


I’m happy to announce a new library, generic-optics, accompanied by version of generic-lens.


A few months ago, the folks at Well-Typed announced the optics library, which aims to improve on the user experience compared to the lens library. Oleg Grenrus has written an excellent migration guide from lens to optics, so please have a look there for some more background.

generic-optics is essentially a port of generic-lens that is compatible with optics, and is designed to be a drop-in replacement for generic-lens. This means that if you’re already using generic-lens with lens and decide to migrate to optics, you should be able to replace the generic-lens dependency with generic-optics and expect things to just work.


To explain why I’m so excited about optics, I’m going to compare a real-life workflow between generic-lens and generic-optics.

First, language pragmas and imports:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE DeriveGeneric #-}

import Data.Generics.Product
import GHC.Generics

Note that the module Data.Generics.Product is shared between generic-lens and generic-optics.

When using generic-lens with the lens library, we would import

import Control.Lens

When using generic-optics with optics, the import becomes

import Optics.Core

Now we define a simple record:

data MyRecord = MyRecord { a :: Int, b :: Int, c :: (Bool, Int) }
  deriving (Generic, Show)

myRecord1 :: MyRecord
myRecord1 = MyRecord 0 1 (False, 2)

With either library, we can view the a field using the field lens:

lens|optics> myRecord1 ^. field @"a"

If we ask what the type of field @"a" is in GHCi, we already see the advantage of optics’s opaque representation.


lens> :t field @"a"
field @"a"
  :: (HasField "a" s t a b, Functor f) => (a -> f b) -> s -> f t


optics> :t field @"a"
field @"a" :: HasField "a" s t a b => Lens s t a b

Now let us use the typed lens, which performs a type-directed lookup in a product type, as long as there is a unique field with that type:

lens|optics> myRecord1 ^. typed @(Bool, Int)

When the type of the field is not unique (such as if we tried to retrieve a field of type Int), both generic-optics and generic-lens provides a helpful type error:

lens|optics> myRecord1 ^. typed @Int

<interactive> error:
    • The type MyRecord contains multiple values of type Int.
      The choice of value is thus ambiguous. The offending constructors are:
      • MyRecord

For situation likes this, both libraries provide a traversal called types that focuses on all values of the given type.

Let’s see what happens if we replace typed with types in the above example when using lens:

lens> myRecord1 ^. types @Int

<interactive>:43:14-23: error:
    • No instance for (Monoid Int) arising from a use of ‘types’

This error is rather puzzling. Unless we know what’s going on under the hood, it’s not obvious where the Monoid constraint is coming from.

Compare this with generic-optics:

optics> myRecord1 ^. types @Int

<interactive>:32:1-23: error:
    • A_Traversal cannot be used as A_Getter

Right! types @Int is a traversal, but ^. takes a getter! Arguably this is a more helpful message. Consulting the documentation of optics, we find the combinator we’re looking for: ^.., which returns all the values focused on by a traversal:

lens|optics> myRecord1 ^.. types @Int

This now of course works in both libraries.

To summarise, using the two libraries should be nearly identical as long as everything goes well and we’re not hitting type errors. Where generic-optics (but really, optics itself) shines is when things do not go all that well, in which case the resulting error messages are a lot more comprehensible.


The above was just to give a little taste of using generic-optics. The interface of generic-optics is intended to be largely identical to that of generic-lens.


At the time of writing, the main difference is the support for overloaded labels in generic-lens, which allows writing

lens> import Data.Generics.Labels ()
lens> myRecord1 ^. #a

I intend to add support for this for generic-optics too, but it isn’t implemented yet.

Changes in generic-lens

To support this new interface, generic-lens itself has undergone a major reorganisation. I thought this was a good opportunity to clean some things up and change the interface at places, which ultimately resulted in a new major version bump.

Most notably, GHC versions below 8.4 are no longer supported. generic-lens (and generic-optics too) promises good performance by making sure that the generic overhead is eliminated at compile time. Doing so requires really careful coding practices, and GHC’s optimiser changes between every version, which meant that certain tricks that worked for 8.2 didn’t work for 8.6 and vice versa. The result was horrible CPP macros to enable certain hacks on certain versions of GHC. In the end, I decided it wasn’t worth the effort to maintain these hacks for older versions of the compiler.

I intend to write a blog post in the near future describing some of these hacks, as they are quite interesting and potentially educational.

For a more comprehensive list of changes, refer to the changelog.


Thanks for reading this blog post, and I’m hope you’re as excited about generic-optics as I am! Since this release required a major refactoring and moving things around, it is possible that some documentation is out of date, or certain functions are not exported from where you would expect. If you find anything that looks off, please either open a pull request or let me know on the issue tracker!

Finally, if you find generic-lens or generic-optics useful, consider buying me a coffee!