Friday

Friday is built off of a parser that is the topic of a chapter in Function Programming in Scala (2nd Ed) of parser combinators.

The library provides an AST to convert JSON to/from, as well as type-classes for Encoders, Decoders, and Codecs.

There is also an emphasis on Json AST helper methods to easily work with JSON, without having to convert to/from an explicit schema.

Working with the AST

The Json AST is described fully as

enum Json {
  case JsonNull
  case JsonBool(value: Boolean)
  case JsonNumber(value: Double)
  case JsonString(value: String)
  case JsonArray(value: IndexedSeq[Json])
  case JsonObject(value: Map[String, Json])
}

A json string can be parsed to Json, using the parse method on the Json companion object.

def parse(json: String): Either[ParseError, Json]

Accessing Values

There are two ways to access values from a Json instance:

1. Direct Access (Unsafe) - These methods will throw an exception if the value is not of the expected type:
   • strVal - Get the underlying String
   • numVal - Get the underlying Double
   • boolVal - Get the underlying Boolean
   • arrVal - Get the underlying IndexedSeq[Json]
   • objVal - Get the underlying Map[String, Json]

For example:

JsonString("Some Str").strVal // Works fine
JsonString("Some Str").numVal // Throws exception

2. Safe Access - These methods return an Option of the underlying value:
   • strOpt - Try to get String
   • numOpt - Try to get Double
   • boolOpt - Try to get Boolean
   • arrOpt - Try to get IndexedSeq[Json]
   • objOpt - Try to get Map[String, Json]

Working with Objects

To quickly parse through possible sub-fields on a JsonObject, there is a ? extension method on both Json and Option[Json] that takes a field name as an argument:

def ?(field: String): Option[Json]

This allows for safe traversal of nested JSON structures. For example:

{
  "name": "Branch",
  "some": {
    "nested": {
      "key": "value"
    }
  }
}

We can safely traverse this structure:

val js: Json = ???

val maybeName: Option[Json] = js ? "name" // Some(JsonString("Branch"))
val deepField: Option[Json] = js ? "some" ? "nested" ? "key" // Some(JsonString("value"))

// Missing fields return None without throwing exceptions
val probablyNot: Option[Json] = js ? "totally" ? "not" ? "there" // None

Type Classes

Friday provides three main type classes for working with JSON:

1. JsonEncoder[A] - For converting Scala types to JSON
2. JsonDecoder[A] - For converting JSON to Scala types
3. JsonCodec[A] - Combines both encoding and decoding capabilities

Decoders

A JsonDecoder[A] converts Json to type A by implementing:

def decode(json: Json): Try[A]

For example:

given JsonDecoder[String] with {
  def decode(json: Json): Try[String] =
    Try(json.strVal)
}

Common decoders are provided and can be imported with:

import dev.wishingtree.branch.friday.JsonDecoder.given

Auto derivation is supported for Product types (case classes):

case class Person(name: String, age: Int) derives JsonDecoder

// Usage
val personJson = """{"name": "Mark", "age": 42}"""
val person: Try[Person] = Json.decode[Person](personJson)

Encoders

A JsonEncoder[A] converts type A to Json by implementing:

def encode(a: A): Json

For example:

given JsonEncoder[String] with {
  def encode(a: String): Json = Json.JsonString(a)
}

Common encoders are provided and can be imported with:

import dev.wishingtree.branch.friday.JsonEncoder.given

Auto derivation works the same as with decoders:

case class Person(name: String, age: Int) derives JsonEncoder

// Usage
val person = Person("Mark", 42)
val json: Json = person.toJson  // Using extension method
// or
val json: Json = Json.encode(person)  // Using companion object

Codecs

When you need both encoding and decoding, use JsonCodec[A]:

trait JsonCodec[A] { self =>
  given encoder: JsonEncoder[A]
  given decoder: JsonDecoder[A]

  def encode(a: A): Json = encoder.encode(a)
  def decode(json: Json): Try[A] = decoder.decode(json)
}

Codecs can be created in several ways:

1. Auto derivation for case classes:

case class Person(name: String, age: Int) derives JsonCodec

2. Combining existing encoder and decoder from the companion object JsonCodec.apply:

val codec: JsonCodec[Person] = JsonCodec[Person]  // If encoder and decoder are in scope

3. From explicit encode/decode functions:

val codec = JsonCodec.from[Person](
  decode = json => Try(/* decode logic */),
  encode = person => /* encode logic */
)

4. Transforming existing codecs:

// Transform a String codec into an Instant codec
val instantCodec: JsonCodec[Instant] = JsonCodec[String].transform(
  Instant.parse    // String => Instant
)(_.toString)      // Instant => String

The codec provides extension methods for convenient usage:

// Encoding
person.toJson         // Convert to Json
person.toJsonString   // Convert directly to JSON string

// Decoding
json.decodeAs[Person]       // Json => Try[Person]
jsonString.decodeAs[Person] // String => Try[Person]

You can also transform codecs to work with different types while preserving type safety:

// Transform with bimap
val longCodec: JsonCodec[Long] = JsonCodec[String].bimap(_.toLong)(_.toString)

// Transform with map
val intCodec: JsonCodec[Int] = JsonCodec[Long].map(_.toInt)(_.toLong)

Other Libraries

If you like Friday, you should check out uPickle for a more comprehensive JSON library.