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https://github.com/propensive/jacinta

Simple interfaces for reading, processing and writing JSON in Scala
https://github.com/propensive/jacinta

ast json json-api scala serializer

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Simple interfaces for reading, processing and writing JSON in Scala

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# Jacinta



__Simple interfaces for reading, processing and writing JSON__



_Jacinta_ is a fully-featured JSON library built upon the JSON parser,

[Merino](https://github.com/propensive/merino/), and designed to make it easy

and safe to work with JSON in Scala.



## Features



- parse and represent JSON in Scala

- intuitive dynamic API for field access, without compromising typesafety

- typeclass-based conversion to and from JSON

- generic derivation of typeclass interfaces for reading and writing product and coproduct types to JSON



## Availability



## Getting Started



### Parsing



A `Json` value may be obtained from any readable value by passing it to the `Json.parse` method. This could be a

string value, for example,

```scala

import jacinta.*

Json.parse(t"""{ "name": "Alfred", "age": 83 }""")

```

but could also be a file or any other data stream with an appropirate `Readable` typeclass instance in scope:

```scala

import galilei.*

val input = (dir / t"source.json").file

Json.parse(input)

```



If parsing fails, a `JsonParseError` is thrown. Otherwise, an instance of `Json` representing a JSON abstract

syntax tree is returned.



### Serialization



Many types may be serialized to JSON, i.e. converted into instances of `Json`, by calling the `.json` extension

method upon them. `42.json` will produce a `Json` value of the integer `42` represented as a JSON number type.



Other primitive types may be converted in obvious ways, for example, `t"Hello World".json`. Case class instances

may be converted into `Json` instances of objects provided the type of every parameter of the case class can be.

This applies recursively, so a case class composed of other case classes may be serialized to JSON. For example:

```scala

case class Person(firstName: Text, lastName: Text)

case class Recipient(person: Person, emailAddress: Text)



val recipient = Recipient(Person(t"Mike", t"Smith"), t"[email protected]").json

```



Given these definitions, the `recipient` instance would serialize to the JSON,

```json

{

  "person": {

    "firstName": "Mike",

    "lastName": "Smith"

  },

  "emailAddress": "[email protected]"

}

```



#### Coproducts



Sealed traits of two or more case class subtypes will be serialized to JSON objects, exactly as each of the

subtypes would be, but with an additional field called `_type`, whose value will be set to the unqualified type

name, e.g. `"_type": "Leaf"`.



Although this encoding of coproduct types is non-standard, it is a reasonable default, and can always be

overridden with specific typeclass instances for the sealed trait type.



#### Collections



Furthermore, all traversable standard collection types can be serialized to JSON arrays, provided the elements

of the collection can be.



### Acessing values



Instances of `Json` are dynamically-typed which means that members with arbitrary names may be accessed as if

they were methods. Taking the `recipient` example above, it would be valid to access `recipient.person`, as if

the method `person` existed on the `Json` type. It doesn't, but since `Json` instances inherit from the special

`Dynamic` trait, the code will be transformed into `recipient.selectDynamic("person")` at compiletime, which

will return a new `Json` instance representing the JSON:

```json

{

  "firstName": "Mike",

  "lastName": "Smith"

}

```



It is therefore possible to call `recipient.person.firstName` directly and get a `Json` value representing the

JSON string, `"Mike"`.



As dynamic values with little known statically about them, instances of type `Json` are not particularly useful

_directly_, and should be converted to other types like `Text`, `Int` or `Person` before being used elsewhere in

a program. This is achieved with the `Json#as` method which takes the destination type as a parameter, for

example,

```scala

val addressee: Text = recipient.person.firstName.as[Text]

```

or,

```scala

val person: Person = recipient.person.as[Person]

```



As well as accessing arbitrary fields in a JSON object, elements of an array may be accessed by simply applying

the integer index to a `Json` value representing an array, for example, `json.organisation.users(2).as[User]`.



#### Errors



Since dynamic field access is unchecked at compiletime, it's possible that a JSON object would not contain the

requested field, or a JSON array would not contain the requsted index. This will throw an exception only when

attempting to convert the value to a static type. So the expression, `recipient.user.firstName`, (noting that

`user` is not a valid field of `recipient`) would not produce an error in itself. Only when invoking,

`recipient.user.firstname.as[Text]` would an exception be thrown, of type `JsonAccessError`.



Similarly, if the expression, `recipient.person.firstName.as[Int]` were evaluated, a `JsonAccessError` would be

thrown due to the field, `firstName` being a JSON string and not a JSON number.



All methods which throw exceptions are annotated with `throws` clauses, and if `saferExceptions` is enabled,

these must be handled.



### Typeclasses



While all Java primitive types and `String`s, collection types and case class types can be serialized and

deserialized automatically, it's possible to support other types or to replace existing default implementations

by providing contextual instances of the typeclasses, `Json.Writer` and `Json.Reader`.



For example, assuming the existence of an `Email` type (which simply wraps a `Text` instance), a `Reader` and

`Writer` for `Email` could be provided in `Email`'s companion object, like so:

```scala

case class Email(value: Text)



object Email:

  given Json.Reader[Email] = json => Email(json.as[Text])

  given Json.Writer[Email] = _.value.json

```

Note that `Email` is a case class, so default instances of `Json.Reader[Email]` and `Json.Writer[Email]` would

exist already, but would be replaced by these new definitions. (If `Email` were instead a non-`case` `class`,

these would be chosen unambiguously as the only contextual instances.)



#### Functor and Cofunctor



`Json.Reader`s are functors, and the `Reader#map` method is provided to transform a reader of one type into a

reader of another. Likewise, `Json.Writer`s are cofunctors with `Writer#contramap` methods. Given these

definitions, an alternative way to write the definitions for `Email` by transforming the existing instances for

the `Text` type would be:

```scala

object Email:

  given Json.Reader[Email] = summon[Json.Reader[Text]].map(Email(_))

  given Json.Writer[Email] = summon[Json.Writer[Text]].contramap(_.value)

```



## Status



Jacinta is classified as __fledgling__. For reference, Soundness 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.



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

of 610 lines of code.



## Building



Jacinta 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 Jacinta?".



1. *Copy the sources into your own project*

   

   Read the `fury` file in the repository root to understand Jacinta'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 Jacinta 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 `jacinta`.

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

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



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

   `.wrath/dist` directory.



## Contributing



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

to look for issues marked

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



We suggest that all contributors read the [Contributing

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

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



Jacinta 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



_Jacinta_ is one of the feminine forms of the given name Jason, which is homophonous to JSON.



In general, Soundness 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 a pair of braces—important syntax in JSON—positioned to look like two people face-to-face, alluding to the concept of _communication_.



## License



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

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