kitlangton / neotype   0.2.0


A friendly newtype library for Scala 3

Scala versions: 3.x
Scala.js versions: 1.x


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A friendly refined/newtype library for Scala 3.

"io.github.kitlangton" %% "neotype" % "0.2.0"


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


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 =

// 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
  —— Newtype Error ——————————————————————————————————————————————————————————
  NonEmptyString was called with an INVALID String.
  input: ""
  check: input.nonEmpty


Neotype integrates with the following libraries.

  • zio-test DeriveGen
  • zio-json
  • zio-config
  • tapir
  • quill
  • circe
  • jsoniter

ZIO Json Example

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

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"

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)")

By importing neotype.ziojson.given, we automatically generate a JsonCodec for NonEmptyString. Custom failure messages are also supported (by overriding def failureMessage in the Newtype definition).