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
http://fvisser.nl/post/2013/okt/1/fclabels-2.0.html
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.
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.
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.
- Add `for` as a synonym for `>-` to avoid a clash with the Arrows extension.
