Swift-Friendly Kotlin Multiplatform Result Class

Wrapper for kotlin.Result with KMM goodness, s.t. it becomes possible to expose a result class to public APIs interfacing with platform-specific code. For Kotlin/Native (read: iOS), this requires a Result equivalent, which is not a value class (a sealed Either type also does not interop well with Swift).

KmmResult comes to the rescue on all KMP targets! → Full documentation.

Using in your Projects

This library is available at maven central.

Gradle

dependencies {
    api("at.asitplus:kmmresult:$version")   //This library was designed to play well with multiplatform APIs
}                                           //and is therefore intended to be exposed through your public API

Quick Start

Creation of Success and Failure objects is provided through a companion:

var intResult = KmmResult.success(3)
intResult = KmmResult.failure (NotImplementedError("Not Implemented"))

Convenience functions:

• map() transforms success types while passing through errors
• transform() transforms a KmmResult of one type to another, leaving failure-cases untouched and preventing nested KmmResults.
• mapFailure transforms error types while passing through success cases
• the more generic fold() is available for conveniently operating on both success and failure branches
• KmmResult sports unwrap() to conveniently map it to the kotlin.Result equivalent
• Result.wrap() extension function goes the opposite way
• mapCatching() does what you'd expect
• wrapping() allows for wrapping the failure branch's exception unless it is of the specified type
• onSuccess() and onFailure() transforming success and error cases, respectively.

Refer to the full documentation for more info.

Java

Works from the JVM as expected:

KmmResult<Boolean> demonstrate() {
    if (new Random().nextBoolean())
        return KmmResult.failure(new NotImplementedError("Not Implemented"));
    else
        return KmmResult.success(true);
}

Swift

Use the initializers:

func funWithKotlin() -> KmmResult<NSString> {
    if 2 != 3 {
        return KmmResult(failure: KotlinThrowable(message: "error!"))
    } else {
        return KmmResult(value: "works!")
    }
}

Kotest Extensions

The kmmresult-test artefact provides first-class Kotest integration:

• catching {…} should suceed
• catching {…} shouldSucceedWith someSuccessValue
• catching {…}.shouldSucceed() returns the value of the success case. Causes a failed assertion otherwise.

Non-Fatal-Only catching

KmmResult comes with catching. This is a non-fatal-only-catching version of stdlib's runCatching, directly returning a KmmResult. It re-throws any fatal exceptions, such as OutOfMemoryError. The underlying logic is borrowed from Arrow's's nonFatalOrThrow.

The only downside of catching is that it incurs instantiation overhead, because it creates a KmmResult instance. Internally, though, only the behaviour is important, not Swift interop. Hence, you don't care for a KmmResult and you certainly don't care for the cost of instantiating an object. Here, the Result.nonFatalOrThrow() extension shipped with KmmResult comes to the rescue. It does exactly what the name suggest: It re-throws any fatal exception and leaves the Result object untouched otherwise. As a convenience shorthand, there's catchingUnwrapped which directly returns an stdlib Result.

Happy folding!

Contributing

External contributions are greatly appreciated! Just be sure to observe the contribution guidelines (see CONTRIBUTING.md).

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