Define udf using by-delegate?

[Jolanrensen]

UDFs are currently defined for example like:

val stringArrayMerger: UDFWrapper1 = udf.register<WrappedArray<String>, String>("stringArrayMerger") {
    it.asIterable().joinToString(" ")
}

They can then be invoked from SQL or directly in code like: stringArrayMerger(testData.col("value")).

Obviously the thing I notice is that stringArrayMerger is typed twice. This can of course be rewritten using delegates :)

This is possible

val stringArrayMerger: UDFWrapper1 by udf.register<WrappedArray<String>, String> {
    it.toIterable().asKotlinIterable().joinToString(" ")
}

when adding something like:

class UDFWrapperBuilderDelegate<UDF>(val getValue: (name: String) -> UDF) {
    operator fun getValue(
        thisRef: Any?,
        property: KProperty<*>
    ): UDF = getValue(property.name)
}

and for each UDF register function:

inline fun <reified T0, reified R> UDFRegistration.register(noinline func: (T0) -> R): UDFWrapperBuilderDelegate<UDFWrapper1> =
    UDFWrapperBuilderDelegate { name -> register(name, func) }

inline fun <reified T0, reified T1, reified R> UDFRegistration.register(noinline func: (T0, T1) -> R): UDFWrapperBuilderDelegate<UDFWrapper2> =
    UDFWrapperBuilderDelegate { name -> register(name, func) }

...

Just a thought :)

[Jolanrensen]

The compiler is even smart enough to only have to type the bare minimum:

// UDFWrapper1
val stringArrayMerger by udf.register { it: WrappedArray<String> ->
    it.toIterable().asKotlinIterable().joinToString(" ")
}

// UDFWrapper2
val stringArrayMerger2 by udf.register { a: WrappedArray<String>, b: Int ->
    a.toIterable().asKotlinIterable().joinToString(" $b ")
}

[Jolanrensen]

:Okay, maybe it's better to not use delegates after all. The function is only registered after the delegate value is accessed, which does not happen when it's called from SQL for example... Better look for another notation :)

[Jolanrensen]

https://github.com/Kotlin/kotlin-spark-api/pull/152 handles it :)