This is a tiny framework for modelling success/failure of operations
in Kotlin. In short, it is a model in type
of Result<V: Any?, E : Throwable>.
Result<V: Any?, E: Throwable> is to provide higher abstraction of operation that can be ended with
result either success or failure. Result.Success represents value in case of success,
and Result.Failure represents error in case of failure which is upper bounded with Throwable
type.
repositories {
mavenCentral()
}
dependencies {
// if you are working on JVM or Android only project
implementation("com.github.kittinunf.result:result-jvm:«version»") //for JVM support
// if you are working in KMM project
implementation("com.github.kittinunf.result:result:«version»") //for Kotlin Multiplatform support
}
This model is highly inspired by "Railway Oriented Programming" concept.
Result allows one to express series of success/failure operations in Kotlin as;
Result.of<T, Throwable>(doOperation())
.flatMap { normalizedData(it) }
.map { createRequestFromData(it) }
.flatMap { database.updateFromRequest(it) }Work with Result is easy
//multi-declaration
val (value, error) = result
//get
val value: Int = result.get() // throw exception if error
//terminal operator
//success
result.success {
}
//failure
result.failure {
}
//fold is there, if you want to handle both success and failure
result.fold({ value ->
//do something with value
}, { err ->
//do something with err
})Result is suitable whenever there is a need to represent an operation that has the possibility of
failure. Error handling can be cumbersome to work with.
Result helps process the operations in a nice, functional way, while maintaining readability to
your code.
Let's consider a need to read data from foo, and to perform some further validation
fun process(): String {
try {
val foo = File("/path/to/file/foo.txt").readText()
val isSuccessful = processData(foo)
if (!isSuccessful) {
return "Data is corrupted and cannot be processed"
}
} catch (e: Throwable) {
//do something if error
Logger.log(ERROR, e.message())
}
}However, things start getting ugly when we have chain of operations being run sequentially, such as
fun process(): String {
try {
val foo = File("/path/to/file/foo.txt").readText()
val isSuccessful = normalizedData(foo)
if (!isSuccessful) {
return "Data cannot be processable"
}
val request = createRequestFromData(foo)
try {
val result = database.updateFromRequest(request)
if (!result) {
return "Record in DB is not found"
}
} catch (dbEx: DBThrowable) {
return "DB error, cannot update"
}
} catch (e: Throwable) {
//do something if error
Logger.log(ERROR, e.message())
}
}Ouch, it looks pretty bleak.
Let's see how Result can help us.
First, we break things down into a small set of model in Result.
- Read a file
val operation = { File("/path/to/file/foo.txt").readText() }
Result.of { operation() } // Result<String, FileThrowable>- Normalize a data
fun normalizedData(foo): Result<Boolean, NormalizedThrowable> {
Result.of { foo.normalize() }
}- Create a request from data
fun createRequestFromData(foo): Request {
return createRequest(foo)
}- Update DB with Request
fun database.updateFromRequest(request): Result<Boolean, DBThrowable> {
val transaction = request.transaction
return Result.of {
db.openTransaction {
val success = db.execute(transaction)
if (!success) {
throw DBThrowable("Error")
}
return success
}
}
}The whole operation can be chained by the following;
Result.of { doOperation() }
.flatMap { normalizedData(it) }
.map { createRequestFromData(it) }
.flatMap { database.updateFromRequest(it) }The creates a nice "happy path" of the whole chain, also handle error as appropriate. It looks better and cleaner, right?.
In some case, one wants to model an always successful operation. Result<V: Any?, NoException> is a
good idea for that.
NoException is to indicate that there is no exception to throw. E.g.
// Add operation can never be failure
fun add(i: Int, j: Int) : Result<Int, NoException>Nice thing about modelling in this way is to be able to compose it with others "fail-able"
operations in Result.
map and flatMap
map transforms Result with given transformation (V) -> U. As a result, we are able to
transform V into a new V in the case where Result is Result.Success. When Result
is Result.Failure, error is re-wrapped into a new Result.
flatMap is similar to map, however it requires transformation in type of (V) -> Result<U, ...>
.
mapError and flatMapError
mapError ((E) -> E2) and flatMapError ((E) -> Result<E2, ...>) are counterpart of map
and flatMap. However, they are operate on Result.Failure. It is quite handy when one needs to do
some transformation on given Throwable into a custom type of Throwable that suits ones' need.
Please check out more features in the ResultTest
If interested, here are more articles that one might enjoy.
Credit to Scott Wlaschin
Result is brought to you by contributors.
Result is released under the MIT license.
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