Extension Functions Can Be Utility Functions

I enjoyed Extension functions are not utility functions

I thought that I disagreed, but on more careful reading maybe not. Instead I’ll pull out some extension functions from my current codebase.

We’re finding that extension functions are a simple way to make code more expressive.

Without an extension function:

private fun addressFrom(node: JsonNode): Address = ...

val addresses = nodes.map { addressFrom(it) }

With an extension function:

private fun JsonNode.toAddress(): Address = ...

val addresses = nodes.map { it.toAddress() }

You might the find first example more readable like this:

private fun toAddress(node: JsonNode): Address = ...

val addresses = nodes.map(::toAddress)

in which case fill yer boots, but my mind doesn’t work that way.

Extension functions work well here, but the place where they really shine is method chaining.

Without them:

private fun addressFrom(node: JsonNode): Address = ...

private fun isInUK(address: Address): Boolean = ...

val isInUK = isInUK(addressFrom(node))

Extension functions don’t make you turn your brain inside out:

private fun JsonNode.toAddress(): Address = ...

private fun Address.isInUK(): Boolean = ...

val isInUK = node.toAddress().isInUK()

You may have noticed the private in the examples above - most of the extension functions that we write this way are used locally - just to make a few lines of code more intuitive. But that’s by no means universally true - this is from some library code to extend Jackson in more general ways:

fun JsonNode.getExpected(name: String): JsonNode = get(name) ?: throw MissingPropertyException("property '$name' is missing")

fun JsonNode.getNonBlankText(name: String) = getExpected(name).asText().apply {
    if (isBlank()) throw JsonInterpretationException("property $name is blank")

I had wondered whether this style was best suited to operations on types that we don’t own. But I find we have code that acts on our own types:

fun suggestionsTemplateDataFor(suggestions: List<JournalJson>) = mapOf(
    "results" to suggestions.withOAText()

private fun List<JournalJson>.withOAText(): List<Map<String, Any?>> = this.map { it.withOAText() }

private fun JournalJson.withOAText() = TreeMap(this.asPropertyMap().plus( "openAccessText" to this.accessType.toUIString()))

private fun AccessType.toUIString() = when (this) {
    AccessType.OPEN_ACCESS -> "Yes"
    AccessType.SUBSCRIPTION -> "No"
    AccessType.OPEN_CHOICE -> "Optional"
    else -> "Unknown"

Here JournalJson and AccessType are simple data classes, unencumbered by all the operations that the rest of the system would like them to have; because they can define operations like withOAText() as extension functions when they need them. And if defining an extension on List<JournalJson> isn’t a utility function I don’t know what is, but here it really helps us to at least get the gist of what is going on at the top level.

Hidden in there is a call to

fun Any.asPropertyMap(): Map<String, Any?> = SpaceBlanket(this)

which is the archetypal extension function definition - high-level and fundamental.

There are more thoughts on this subject in a follow up article - More Kotlin Extension Fun

If you like this, Nat Pryce and I are going to be talking about Expressive Kotlin at the Kotlin Night London next Wednesday, 12 October 2016.

Thanks to Springer Nature for allowing me to publish examples from their code. If you’re looking for a Kotlin job in London, they are hiring - please contact me using one of the links in the sidebar.

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