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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.

Example

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.refine) //Explicitly refine your external values at runtime.

runtimeValue.refineEither.map(log) //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:

log(-1.0)

-- 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 = ???
log(runtimeValue)

-- 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

SBT:

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

Mill:

ivy"io.github.iltotore::iron:version"

Note: replace version with the version of Iron.

Platform support

Module	JVM	JS	Native
iron	✔️	✔️	✔️
iron-cats	✔️	✔️	✔️
iron-circe	✔️	✔️	✔️
iron-jsoniter	✔️	✔️	✔️
iron-scalacheck	✔️	✔️	❌
iron-zio	✔️	✔️	❌
iron-zio-json	✔️	✔️	❌

Adopters

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

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

• gvolpe/trading (book)
• cheleb/laminar-form-derivation

Useful links

• Website
• Scaladoc
• Code of Conduct
• Contributing to Iron