https://github.com/kcsongor/generic-lens

Generically derive traversals, lenses, and prisms.
https://github.com/kcsongor/generic-lens

generics lenses optics traversals type-level-programming

Last synced: about 1 month ago
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Generically derive traversals, lenses, and prisms.

• Host: GitHub
• URL: https://github.com/kcsongor/generic-lens
• Owner: kcsongor
• Created: 2016-09-05T20:22:28.000Z (almost 9 years ago)
• Default Branch: master
• Last Pushed: 2025-04-07T09:20:49.000Z (3 months ago)
• Last Synced: 2025-04-14T02:16:08.403Z (2 months ago)
• Topics: generics, lenses, optics, traversals, type-level-programming
• Language: Haskell
• Homepage:
• Size: 1020 KB
• Stars: 441
• Watchers: 19
• Forks: 55
• Open Issues: 40
• Metadata Files:
  • Readme: README.md
  • Funding: .github/FUNDING.yml

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README

        
# generic-lens

[![Build Status](https://travis-ci.org/kcsongor/generic-lens.svg?branch=master)](https://travis-ci.org/kcsongor/generic-lens)

[![Hackage](https://img.shields.io/hackage/v/generic-lens.svg)](https://hackage.haskell.org/package/generic-lens)

Generically derive isos, traversals, lenses and prisms.

This library uses `GHC.Generics` to derive efficient optics (traversals, lenses

and prisms) for algebraic data types in a type-directed way, with a focus on

good type inference and error messages when possible.

The library is described in the paper:

> Csongor Kiss, Matthew Pickering, and Nicolas Wu. 2018. Generic deriving of generic traversals. Proc. ACM Program. Lang. 2, ICFP, Article 85 (July 2018), 30 pages. DOI: https://doi.org/10.1145/3236780

## Package structure

* [`generic-lens`](https://hackage.haskell.org/package/generic-lens):

  Generic derivation of van Laarhoven optics, compatible with the

  [`lens`](https://hackage.haskell.org/package/lens) library. There

  is no dependency on `lens`, however, and other mainstream lens libraries

  sharing the same interface are supported just as well.

  

* [`generic-optics`](https://hackage.haskell.org/package/generic-optics):

  Generic derivation of optics compatible with the [`optics`](https://hackage.haskell.org/package/optics)

  library.

* [`generic-lens-core`](https://hackage.haskell.org/package/generic-lens-core):

  The bulk of the work happens here. Indeed, `generic-lens` and

  `generic-optics` are just thin layers on top of `generic-lens-core`.

  

`generic-lens` are `generic-optics` are designed to be drop-in

replacements for each other. This means that they share the same module names. If, for

whatever reason, they are both used in the same project, the module imports can be disambiguated

using the `-XPackageImports` extension.

## Getting started

A typical module using `generic-lens` or `generic-optics` will usually have the following

extensions turned on:

```haskell

{-# LANGUAGE AllowAmbiguousTypes       #-}

{-# LANGUAGE DataKinds                 #-}

{-# LANGUAGE DeriveGeneric             #-}

{-# LANGUAGE DuplicateRecordFields     #-}

{-# LANGUAGE FlexibleContexts          #-}

{-# LANGUAGE NoMonomorphismRestriction #-}

{-# LANGUAGE TypeApplications          #-}

```

In particular, `NoMonomorphismRestriction` can be helpful as it

enables proper type inference for top-level functions without a type

signature.

For usage examples, visit the documentation on hackage, or have a look

in the `examples` and `tests` folders in `generic-lens` or

`generic-optics`.

## Performance

The runtime characteristics of the derived optics is in most cases identical at

`-O1`, in some cases only slightly slower than the hand-written version. This

is thanks to GHC's optimiser eliminating the generic overhead.

The

[inspection-testing](https://hackage.haskell.org/package/inspection-testing)

library is used to ensure (see [here](test/Spec.hs)) that everything gets

inlined away.

There is also a [benchmark suite](https://github.com/mpickering/generic-lens-benchmarks) with larger, real-world examples.