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Biju Kunjummen

Kotlin “Result” type for functional exception handling

In a previous post I had gone over how a “Try” type can be created in Kotlin from scratch to handle exceptions in a functional way. There is no need however to create such a type in Kotlin, a type called “Result” already handles the behavior of “Try” and this post will go over how it works. I will be taking the scenario from my previous post, having two steps:

  1. Parsing a Url
  2. Fetching from the Url

Either of these steps can fail

  • the URL may not be well formed, and 
  • fetching from a remote url may have network issues

So onto the basics of how such a call can be made using the Result type. You can imagine that parsing URL can return this Result type, capturing any exception that may result from such a call:

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fun parseUrl(url: String): Result<URL> =
        kotlin.runCatching { URL(url) }

Kotlin provides the “runCatching” function which accepts the block that can result in an exception and traps the result OR the exception in the “Result” type. Now that a “Result” is available, some basic checks can be made on it, I can check that the call succeeded using the “isSuccess” and “isFailure” properties:

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val urlResult: Result<URL> = parseUrl("http://someurl")
urlResult.isSuccess == true
urlResult.isFailure == false

I can get the value using various “get*” methods:

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urlResult.getOrNull() // Returns null if the block completed with an exception
urlResult.getOrDefault(URL("http://somedefault")) // Returns a default if the block completed with an exception
urlResult.getOrThrow() // Throws an exception if the block completed with an exception

The true power of “Result” type is however in chaining operations on it. So for eg, if you wanted to retrieve the host name given the url:

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val urlResult: Result<URL> = parseUrl("http://someurl")
val hostResult: Result<String> = urlResult.map { url -> url.host }

Or a variant “mapCatching” which can trap any exception when using map operation and capture that as a “Result”:

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val getResult: Result<String> = urlResult.mapCatching { url -> throw RuntimeException("something failed!") }

All very neat! One nit that I have with the current “Result” is a missing “flatMap” operation, so for eg. consider a case where I have these two functions:

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fun parseUrl(url: String): Result<URL> =
    kotlin.runCatching { URL(url) }
     
fun getFromARemoteUrl(url: URL): Result<String> {
    return kotlin.runCatching { "a result" }
}

I would have liked to be able to chain these two operations, along these lines:

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val urlResult: Result<URL> = parseUrl("http://someurl")
val getResult: Result<String> = urlResult.flatMap { url -> getFromARemoteUrl(url)}

but a operator like “flatMap” does not exist (so far, as of Kotlin 1.5.20) 

I can do today is a bit of hack:

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val urlResult: Result<URL> = parseUrl("http://someurl")
val getResult: Result<String> = urlResult.mapCatching { url -> getFromARemoteUrl(url).getOrThrow() }

OR even better, create an extension function which makes flatMap available to “Result” type, this way and use it:

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fun <T, R> Result<T>.flatMap(block: (T) -> (Result<R>)): Result<R> {
    return this.mapCatching {
        block(it).getOrThrow()
    }
}
val urlResult: Result<URL> = parseUrl("http://someurl")
val getResult: Result<String> = urlResult.flatMap { url -> getFromARemoteUrl(url)}

This concludes my exploration of the Result type and the ways to use it. I have found it to be a excellent type to have in my toolbelt.

Published on Java Code Geeks with permission by Biju Kunjummen, partner at our JCG program. See the original article here: Kotlin “Result” type for functional exception handling

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