neotype

A friendly newtype library for Scala 3.

"io.github.kitlangton" %% "neotype" % "0.3.0"

Features

Compile-time Checked Values
Write validations as plain, old Scala expressions
Helpful compilation errors (see below)
No runtime allocations (Thanks to inline and opaque type)
Integrates with other libraries (e.g. zio-json, circe, tapir, etc.)

5 Minute Video Tour

Example

Here is how to define a compile-time validated Newtype.

import neotype.*

// 1. Define a newtype.
object NonEmptyString extends Newtype[String]:

  // 2. Optionally, define a validate method.
  override inline def validate(input: String): Boolean =
    input.nonEmpty

// 3. Construct values.
NonEmptyString("Hello") // OK
NonEmptyString("")      // Compile Error

Attempting to call NonEmptyString("") would result in the following compilation error:

Error: /src/main/scala/examples/Main.scala:9:16
  NonEmptyString("")
  ^^^^^^^^^^^^^^^^^^
  —— Newtype Error ——————————————————————————————————————————————————————————
  NonEmptyString was called with an INVALID String.
  input: ""
  check: input.nonEmpty
  ———————————————————————————————————————————————————————————————————————————

Integrations

Neotype integrates with the following libraries:

JSON
- zio-json
- play-json
- jsoniter
- circe
DATABASE
- doobie
- quill
MISCELLANEOUS
- zio-test DeriveGen
- zio-config
- zio-schema
- tapir
- chimney
- caliban

ZIO Json Example

import neotype.*

type NonEmptyString = NonEmptyString.Type
object NonEmptyString extends Newtype[String]:
  override inline def validate(value: String): Result =
    if value.nonEmpty then true else "String must not be empty"

import neotype.interop.ziojson.given
import zio.json.*

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

val parsed = """{"name": "Kit", "age": 30}""".fromJson[Person]
// Right(Person(NonEmptyString("Kit"), 30))

val failed = """{"name": "", "age": 30}""".fromJson[Person]
// Left(".name(String must not be empty)")