BSD-3-Clause licensed by Csongor Kiss
Maintained by [email protected]
This version can be pinned in stack with:generic-lens-,3962

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 exposes a van Laarhoven interface. For an alternative interface, supporting an opaque optic type, see generic-optics.


generic-lens- (2023-04-15)

  • Support unprefixed constructor prisms on GHC 9.6 (#152)

generic-lens- (2022-01-22)

  • GHC 9.2 compatibility

generic-lens- (2021-07-13)

  • GHC 9.0 compatibility

generic-lens- (2021-01-25)

  • Bump to generic-lens-core-

generic-lens- (2020-02-11)

  • Drop support for GHC < 8.4
  • Better inference for field'
  • Param traversal now properly recurses deeply (#88)
  • Reorganise internals (see generic-lens-core)

Breaking API changes:

  • HasTypesUsing now takes 4 params
  • Removed HasConstraints traversal


  • Give HasAny/AsAny the same VTA behavior on 8.6 and 8.8 (Ryan Scott)


  • Add HasTypesUsing and HasTypesCustom for custom traversals (Lysxia)
  • Improve type errors when no Generic instance is defined
  • types now supports Text by default

Breaking API changes

  • HasType now includes a reflexive case so that every type ‘contains’ itself (Matt Parsons)
  • AsSubtype and Subtype now include a reflexive case so that every type is a subtype of itself


  • Fix regression in type inference for polymorphic optics
  • Add HasField0, HasPosition0, AsConstructor0, HasField_, HasPositon_, and AsConstructor_ (Lysxia)
  • types now supports Data.Word and Data.Int (Lysxia)
  • Add Wrapped iso for newtypes (Isaac Elliott)
  • Expose internals through Data.GenericLens.Internal
  • Add labels for prisms (Daniel Winograd-Cort)


  • Fix compile-time performance regression


  • Remove dump-core dependency
  • Relax upper bound on criterion (#42)


  • Traversals (types, param, constraints)
  • Prisms are now optimal too
  • Monomorphic versions of lenses and prisms also included

Breaking API changes

  • projectSub now returns Maybe sub instead of Either sup sub (#21)


  • Infer input type from result type (#25)
  • Allow changing of multiple type parameters (#24)
  • Allow changing of type parameters that have kinds other than * (#23)
  • Fix error message in subtype lens


  • Lenses and prisms are now type-changing.
  • More informative error messages
  • More readable type signatures in type errors and when using :t
  • Use doctest
  • Include examples in Haddock

Breaking API changes

  • The type parameters of the classes have been changed to accommodate the type-changing update:

    class HasField name a s -> class HasField name s t a b etc.

    Accordingly, field :: Lens' s a -> field :: Lens s t a b