Structured data is the foundation of software. Different components of a system may need the same information but may have different demands on its structure for reasons of performance, resource efficiency, technical constraints, convenience, and so on. For instance, transmitting data over a network requires a format that is suitable for serialization, while persisting data requires a format that is more suitable for storage. Thus, programmers need to translate data between several data structures and formats all the time. Authoring these translations manually is a lot of work because programmers need to implement the logic twice, once for each direction. This is redundant, tedious, and error-prone, and a case of low coherence. Bidirectional data transformations that use functional optics like lenses and projections simplify these conversions and foster understanding of the relationship between data structures by explicitly describing their connections in a composable manner.
See the paper by Crestani et al. for an introduction to Bidirectional Data Transformations.
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The mechanical parts of this pattern can be expressed in library code. All functional programming languages come with well-engineered libraries for lenses, prisms etc.
The active-clojure library comes with active.clojure.lens namespace for lens primitives and combinators. These lenses are well integrated with active.clojure.record. Any record field accessor defined with define-record-type is also a valid lens. The following listing defines two record types URL and Link and a lens link-host that composes the lenses link-url and url-host.
(define-record-type URL
make-url
url?
[protocol url-protocol
host url-host
port url-port
path url-path])
(define-record-type Link
{:projection-lens-constructor make-link-lens}
make-link
link?
[description link-description
url link-url])
(def link-host (lens/>> link-url url-host))
For Haskell there's the widely used lens package. An alternative is Optics which is based on different design choices that are best described by the section Comparison with lens.
The de-facto standard lens library in Scala is called Monocle.
Aether is an Optics library for F#, similar to the Haskell lens package.
OCamlverse mentions two libraries for lenses in OCaml:
Accessor by JaneStreet.
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