From Java 8 to Kotlin A series of small bang conversions Nat Pryce info@natpryce.com | @natpryce | github.com/npryce | speakerdeck.com/npryce

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Coming up... What we did Overenthusiasm Culture Shock You can gradually convert Java to Kotlin without slowing delivery. We went too far so that you don't have to. Kotlin is a foreign country; they do things differently there.

Conversion

Culture Shock

Code layout and organisation The Kotlin style guide is not very extensive It seems trivial, but inconsistent conventions became annoying ● Code layout ● Code organisation ● Visibility modifiers Our project developed its own conventions … eventually.

It took us time to accept using null data class SubmissionDraft( override val id: DraftId, override val submissionStatus: SubmissionStatus, override val createdTimestamp: Instant, override val submittedTimestamp: Instant? = null, override val submissionId: SubmissionId?, override val decisions: UserDecisions )

Extension functions on nullable types "Elvis has left the building!" fun String?.orEmpty() = this ?: "" fun List?.orEmpty() = this ?: emptyList() fun T?.discardIf(p: (T)->Boolean) = this?.let { if (p(it)) null else this } But beware… val maybeCount: Int? = … maybeCount?.toString() // returns null or the count maybeCount.toString() // returns "null" or the count The type checker won't help: String is a subtype of String? Impossible to spot the difference in test diagnostics!

Exceptions are not type checked! sealed class Result data class Failure(val value: ErrorCode) : Result() data class Success(val value: T) : Result() inline fun Result.map(f: (T) -> U): Result = ... inline fun Result.flatMap(f: ( T) -> Result): Result = ... inline fun Result.forEach(f: (T) -> Unit): Unit = ... inline fun Result.orElse(f: (ErrorCode) -> T) = ... inline fun T?.asResultOr(errorCodeFn: () -> ErrorCode): Result< T> = ... ... etc.

Exceptions are not type checked! Technica l error? Further action? Catch as close as possible to failing call. Return error code in Result Translate to HTTP response in the web layer Let the exception propagate – the web layer will send a 500 response Yes No Yes No

Result instead of exceptions fun sendToEjp(draft: SubmissionDraft): Result = journals.getByPCode(draft. pCode) .flatMap { journal -> journal.ejpJournalId .asResultOr { PeerReviewSystemNotSupported() } .flatMap { ejpJournalId -> payloadConverter.convertForCreate(draft) .flatMap { requestJson -> val uri = EjpUrlRoutes. create.path(ejpJournalId) sendCreate(uri, payload) .flatMap { responseJson -> responseJson.toSubmissionId() } } } } Type safe, but...

Accept early returns in functional code inline fun Result.onFailure(handler: (Failure) -> Nothing): T = when (this) { is Failure -> handler( this) is Success -> value } fun create(draft: SubmissionDraft): Result { val journal = journals.getByPCode(draft. pCode) ?: return Failure(JournalNotFound(draft. pCode)) val journalId = journal. ejpJournalId ?: return Failure(PeerReviewSystemMisconfiguration()) val ejpPayload = ejpPayloadConverter.convertForCreate(draft) .onFailure { return it } val uri = EjpUrlRoutes. createPath.path(journalId) val responseJson = sendCreate(uri, payload) .onFailure { return it } return responseJson.toSubmissionId() }

Overenthusiasm

Infixation val config = EnvironmentVariables overriding ConfigurationProperties.fromResource("default.conf") infix fun Configuration.overriding(defaults: Configuration) = Override(this, defaults) (n) An unhealthy obsession with infix functions. fun Author.toJson(): JsonNode = obj( "name" of name, "affiliation" of affiliation.toJson(), "email" of emailAddress ) Unnecessary (but published, so can't be changed without breaking code) Useful

Operator punning Prefer conventional operators or clearly named functions val submissionId = PathElement(::SubmissionId) val fileId = PathElement(::FileId) val submissionPath = "submission"/submissionId val submissionFilePath = submissionPath/"file"/fileId WTF?!?

Spaces in Identifiers private fun String.`fromCamelCaseTo space separated`() = this.replace(wordBoundaryRegex, "$1 $2") private val wordBoundaryRegex = Regex("(\\p{Ll})(\\p{Lu})") @Test fun `an author cannot edit another author's submission`() … Too jokey & makes client code hard to read Ok – test names act as documentation in IDE views & no client code

Nesting apply, with, extension functions, extension function literals, extension functions as instance methods, ...

The beginning... info@natpryce.com | @natpryce | github.com/npryce | speakerdeck.com/npryce (who are hiring in London & Berlin) https://www.springernature.com/gp/group/careers Thanks to