Package Info


First class accessor labels implemented as lenses


This package provides first class labels that can act as bidirectional record fields. The labels can be derived automatically using Template Haskell which means you don't have to write any boilerplate yourself. The labels are implemented as /lenses/ and are fully composable. Lenses can be used to /get/, /set/ and /modify/ parts of a data type in a consistent way.

See "Data.Label" for an introductory explanation or see the introductory blog post at <>

  • /Total and partial lenses/

Internally lenses do not used Haskell functions directly, but are implemented as categories. Categories allow the lenses to be run in custom computational contexts. This approach allows us to make partial lenses that point to fields of multi-constructor datatypes in an elegant way.

See "Data.Label.Partial" for the use of partial labels.

  • /Monomorphic and polymorphic lenses/

We have both polymorphic and monomorphic lenses. Polymorphic lenses allow updates that change the type. The types of polymorphic lenses are slightly more verbose than their monomorphic counterparts, but their usage is similar. Because monomorphic lenses are built by restricting the types of polymorphic lenses they are essentially the same and can be freely composed with eachother.

See "Data.Label.Mono" and "Data.Label.Poly" for the difference between polymorphic and monomorphic lenses.

  • /Using fclabels/

To simplify working with labels we supply both a set of labels for Haskell's base types, like lists, tuples, Maybe and Either, and we supply a set of combinators for working with labels for values in the Reader and State monad.

See "Data.Label.Base" and "Data.Label.Monadic" for more information.

  • /Changelog from to

> - Allow HUnit 1.5.*.

License: BSD-3-Clause



Package Version Update ID Released Package Hub Version Platforms Subpackages info GA Release 2018-08-01 15
  • AArch64
  • ghc-fclabels
  • ghc-fclabels-devel info GA Release 2018-07-30 15
  • ppc64le
  • x86-64
  • ghc-fclabels
  • ghc-fclabels-devel