iltotore / iron   2.4.0

Apache License 2.0 Website GitHub

Strong type constraints for Scala

Scala versions: 3.x
Scala.js versions: 1.x
Scala Native versions: 0.4


Scala version support CI

Iron is a lightweight library for refined types in Scala 3.

It enables attaching constraints/assertions to types, to enforce properties and forbid invalid values.

  • Catch bugs. In the spirit of static typing, use more specific types to avoid invalid values.
  • Compile-time and runtime. Evaluate constraints at compile time, or explicitly check them at runtime (e.g. for a form).
  • Seamless. Iron types are subtypes of their unrefined versions, meaning you can easily add or remove them.
  • No black magic. Use Scala 3's powerful inline, types and restricted macros for consistent behaviour and rules. No unexpected behaviour.
  • Extendable. Easily create your own constraints or integrations using classic typeclasses.

To learn more about Iron, see the microsite.


import io.github.iltotore.iron.*
import io.github.iltotore.iron.constraint.numeric.*

def log(x: Double :| Positive): Double =
  Math.log(x) //Used like a normal `Double`

log(1.0) //Automatically verified at compile time.
log(-1.0) //Compile-time error: Should be strictly positive

val runtimeValue: Double = ???
log(runtimeValue.refineUnsafe) //Explicitly refine your external values at runtime. //Use monadic style for functional validation
runtimeValue.refineEither[Positive].map(log) //More explicitly

Helpful error messages

Iron provides useful errors when a constraint does not pass:

-- Constraint Error --------------------------------------------------------
Could not satisfy a constraint for type scala.Double.

Value: -1.0
Message: Should be strictly positive

Or when it cannot be verified:

val runtimeValue: Double = ???
-- Constraint Error --------------------------------------------------------
Cannot refine non full inlined input at compile-time.
To test a constraint at runtime, use the `refine` extension method.

Note: Due to a Scala limitation, already-refined types cannot be tested at compile-time (unless proven by an `Implication`).

Inlined input: runtimeValue

Import in your project


libraryDependencies += "io.github.iltotore" %% "iron" % "version"



Note: replace version with the version of Iron.

Platform support

Module JVM JS Native
iron ✔️ ✔️ ✔️
iron-borer ✔️ ✔️
iron-cats ✔️ ✔️ ✔️
iron-circe ✔️ ✔️ ✔️
iron-ciris ✔️ ✔️ ✔️
iron-decline ✔️ ✔️ ✔️
iron-doobie ✔️
iron-jsoniter ✔️ ✔️ ✔️
iron-scalacheck ✔️ ✔️
iron-skunk ✔️ ✔️ ✔️
iron-upickle ✔️ ✔️ ✔️
iron-zio ✔️ ✔️
iron-zio-json ✔️ ✔️


Here is a non-exhaustive list of projects using Iron.

Submit a PR to add your project or company to the list.

Useful links