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https://github.com/propensive/panopticon

Versatile and composable lenses for Scala
https://github.com/propensive/panopticon

lens lenses lenses-library optics scala

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Versatile and composable lenses for Scala

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# Panopticon



__Versatile and composable lenses for Scala__



A _lens_ is an object which is able to access and modify specific parts of a

(potentially) complex immutable data structure. It consists of a _getter_ for

access, and a _setter_ for modification. For example, a lens could focus on the

`id` field of a `User` case class; its getter would take an instance of `User`

and return its `id`, while the setter would take an existing `User` instance

and a new `id` value, and return a new instance of `User` with the new `id`,

and all other fields unchanged.



Lenses are notable for their composability. A single lens can focus on a field

inside a case class nested inside another case class, or more deeply nested

fields. In such an example, a single lens, composed from simpler lenses, could

create a new instance of the outermost case class with just one of its

innermost field modified, in a single operation, avoiding potentially very

complex syntax involving making copies of each intermediate case class.



Panopticon provides concise and elegant syntax for working with lenses.



## Features



- create lenses for accessing and modifying deeply-nested fields in case classes

- no performance penalty at runtime over handwritten modification code

- compose lenses with the `++` operator



## Availability Plan



Panopticon has not yet been published. The medium-term plan is to build Panopticon

with [Fury](https://github.com/propensive/fury) and to publish it as a source build on

[Vent](https://github.com/propensive/vent). This will enable ordinary users to write and build

software which depends on Panopticon.



Subsequently, Panopticon will also be made available as a binary in the Maven

Central repository. This will enable users of other build tools to use it.



For the overeager, curious and impatient, see [building](#building).



## Getting Started



All terms and types are defined in the `panopticon` package, which can be imported with,

```scala

import panopticon.*

```



### Creating a Lens



Imagine we have two case classes,

```scala

import anticipation.*



case class User(id: Int, name: Text, birthday: Date)

case class Date(day: Int, month: Int, year: Int)

```

describing users of a hypothetical system, and numerical calendar dates.



We can construct a `Lens` for modifying a user's name with:

```scala

val userName = Lens[User](_.name)

```



Step-by-step, the expression is a reference to the `Lens` factory object, the

type upon which the lens will operate (`User`), and a lambda from that type to

the field the lens will focus on. The `User` type cannot be inferred, but the

type of `name`, which is `Text`, will be inferred.



Similarly, we could construct a lens which accesses the month field of a `Date`

with `Lens[Date](_.month)`. Or we could construct a lens which directly access

the year of birth of a user, like so:

```scala

val userBirthYear = Lens[User](_.birthday.year)

```



### Applying Lenses



If we construct an instance of a `User` with,

```scala

val user = User(38295, t"Bob Mason", Date(12, 7, 1997))

```

then we can always access the user's birth year with `user.birthday.year`, but

we can also use the lens created above, `userBirthYear`, like so:

```scala

val birthYear: Int = userBirthYear.get(user)

```



This is unspectacular. But it becomes more useful for updating a user.



We can change that user's birth year with,

```scala

val user2: User = userBirthYear.set(user, 1995)

```

which compares favorably with the equivalent code written using the case

classes' `copy` methods:

```scala

val user2: User = user.copy(birthday = user.birthday.copy(year = 1995))

```



We can also specify the lens inline in the expression, like so:

```scala

val user2: User = Lens[User](_.birthday.year).set(user, 1995)

```



The saving on syntactic verbosity improves the deeper the nesting.



### Composing Lenses



Although the `userBirthYear` lens above was created in a single expression, it

is also possible to construct it by composing a lens for `User#birthday` and a

lens for `Date#year`, using the `++` operator:

```scala

val userBirthday = Lens[User](_.birthday)

val dateYear = Lens[Date](_.year)

val userBirthYear = userBirthday ++ dateYear

```



## Status



Panopticon is classified as __embryotic__. For reference, Scala One projects are

categorized into one of the following five stability levels:



- _embryonic_: for experimental or demonstrative purposes only, without any guarantees of longevity

- _fledgling_: of proven utility, seeking contributions, but liable to significant redesigns

- _maturescent_: major design decisions broady settled, seeking probatory adoption and refinement

- _dependable_: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version `1.0.0` or later

- _adamantine_: proven, reliable and production-ready, with no further breaking changes ever anticipated



Projects at any stability level, even _embryonic_ projects, can still be used,

as long as caution is taken to avoid a mismatch between the project's stability

level and the required stability and maintainability of your own project.



Panopticon is designed to be _small_. Its entire source code currently consists

of 164 lines of code.



## Building



Panopticon will ultimately be built by Fury, when it is published. In the

meantime, two possibilities are offered, however they are acknowledged to be

fragile, inadequately tested, and unsuitable for anything more than

experimentation. They are provided only for the necessity of providing _some_

answer to the question, "how can I try Panopticon?".



1. *Copy the sources into your own project*

   

   Read the `fury` file in the repository root to understand Panopticon's build

   structure, dependencies and source location; the file format should be short

   and quite intuitive. Copy the sources into a source directory in your own

   project, then repeat (recursively) for each of the dependencies.



   The sources are compiled against the latest nightly release of Scala 3.

   There should be no problem to compile the project together with all of its

   dependencies in a single compilation.



2. *Build with [Wrath](https://github.com/propensive/wrath/)*



   Wrath is a bootstrapping script for building Panopticon and other projects in

   the absence of a fully-featured build tool. It is designed to read the `fury`

   file in the project directory, and produce a collection of JAR files which can

   be added to a classpath, by compiling the project and all of its dependencies,

   including the Scala compiler itself.

   

   Download the latest version of

   [`wrath`](https://github.com/propensive/wrath/releases/latest), make it

   executable, and add it to your path, for example by copying it to

   `/usr/local/bin/`.



   Clone this repository inside an empty directory, so that the build can

   safely make clones of repositories it depends on as _peers_ of `panopticon`.

   Run `wrath -F` in the repository root. This will download and compile the

   latest version of Scala, as well as all of Panopticon's dependencies.



   If the build was successful, the compiled JAR files can be found in the

   `.wrath/dist` directory.



## Contributing



Contributors to Panopticon are welcome and encouraged. New contributors may like

to look for issues marked

[beginner](https://github.com/propensive/panopticon/labels/beginner).



We suggest that all contributors read the [Contributing

Guide](/contributing.md) to make the process of contributing to Panopticon

easier.



Please __do not__ contact project maintainers privately with questions unless

there is a good reason to keep them private. While it can be tempting to

repsond to such questions, private answers cannot be shared with a wider

audience, and it can result in duplication of effort.



## Author



Panopticon was designed and developed by Jon Pretty, and commercial support and

training on all aspects of Scala 3 is available from [Propensive

OÜ](https://propensive.com/).



## Name



The _Panopticon_, meaning "all-seeing", was a concept for the design of

prisons, created by Jeremy Bentham, a philosopher and social reformer. Its name

is given to this library for the _seeing_ capabilities of lenses.



In general, Scala One project names are always chosen with some rationale,

however it is usually frivolous. Each name is chosen for more for its

_uniqueness_ and _intrigue_ than its concision or catchiness, and there is no

bias towards names with positive or "nice" meanings—since many of the libraries

perform some quite unpleasant tasks.



Names should be English words, though many are obscure or archaic, and it

should be noted how willingly English adopts foreign words. Names are generally

of Greek or Latin origin, and have often arrived in English via a romance

language.



## Logo



The logo shows an optical lens.



## License



Panopticon is copyright © 2024 Jon Pretty & Propensive OÜ, and

is made available under the [Apache 2.0 License](/license.md).