I'd like to sort a List using a function reference...

# getting-started

[davec]

I'd like to sort a List using a function reference looked up from a Map. If all function references returned the same type (String, for example) this would work fine. However in the case below where the return type is Any, it doesn't work.

data class Foo(val name:String, val age:Int)        
        val sortModes = mapOf("name" to Foo::name, "age" to Foo::age)
        val myList = listOf(Foo("Dog", 17), Foo("Cat", 12), Foo("Mouse", 3))
        val sortFunc = sortModes["name"] ?: error("Not found") //  returns KProperty1<Foo, Any>
        println(myList.sortedWith(compareBy(sortFunc)) // <-- compiler error

What magic is required to get Kotlin to sort by a function reference that can return Any?

[davec]

This works, but yuk:

println(myList.sortedWith(compareBy {
            val value = sortFunc(it)
            when(value) {
                is String -> value
                is Int -> value
                else -> error("unsupported type")
            }
        }))

[hudsonb]

Could you sort from a Comparator looked up from a map instead? Something like:

data class Foo(val name:String, val age:Int)
    val sortModes = mapOf("name" to Comparator { o1, o2 -> o1.name.compareTo(o2.name) }, 
                                          "age" to Comparator<Foo> { o1, o2 -> o1.age - o2.age })
    val myList = listOf(Foo("Dog", 17), Foo("Cat", 12), Foo("Mouse", 3))
    val sortFunc = sortModes["name"] ?: error("Not found")
    println(myList.sortedWith(sortFunc))

[davec]

Thanks -- that does work, but I still feel like there must be an even simpler way, I'm just struggling to figure out what that would be.

[davec]

Here's a slight improvement based on Brian's idea:

val sortModes = mapOf(
            "name" to compareBy(Foo::name),
            "age"  to compareBy(Foo::age)
        )
        val myList = listOf(Foo("Dog", 17), Foo("Cat", 12), Foo("Mouse", 3))

        val sortFunc = sortModes["age"] ?: error("Not found")
        println("by name: ${myList.sortedWith(sortFunc)}")

I'm going with that unless anyone has any other suggestions.

[hudsonb]

I knew there was something like compareBy but couldnt think of it

[hudsonb]

I think that's an ideal solution really, you end up with a Map<String, Comparator<Foo>> which is much nicer than Map<String, Any>

[davec]

Agreed. Thanks again for your help!