Kotlin function reference

Let records be stream/collection and extract function which transforms data form an element of such collection.

Is there a way in Kotlin to write

records.map {extract(it)}

without explicitely applying(it) ?

E.g. records.map(extract) or records.map {extract}

回答1:

If extract is a value (local variable, property, parameter) of a functional type (T) -> R or T.() -> R for some T and R, then you can pass it directly to map:

records.map(extract)

^Example:

val upperCaseReverse: (String) -> String = { it.toUpperCase().reversed() }

listOf("abc", "xyz").map(upperCaseReverse) // [CBA, ZYX]

If extract is a top-level single argument function or a local single argument function, you can make a function reference as ::extract and pass it to map:
```
records.map(::extract)
```
^Example:
```
fun rotate(s: String) = s.drop(1) + s.first()

listOf("abc", "xyz").map(::rotate) // [bca, yzx]
```
If it is a member or an extension function of a class SomeClass accepting no arguments or a property of SomeClass, you can use it as SomeClass::extract. In this case, records should contain items of SomeType, which will be used as a receiver for extract.
```
records.map(SomeClass::extract)
```
^Example:
```
fun Int.rem2() = this % 2

listOf("abc", "defg").map(String::length).map(Int::rem2) // [1, 0]
```
Since Kotlin 1.1, if extract is a member or an extension function of a class SomeClass accepting one argument, you can make a bound callable reference with some receiver foo:
```
records.map(foo::extract)
records.map(this::extract) // to call on `this` receiver
```
^Example:
```
listOf("abc", "xyz").map("prefix"::plus) // [prefixabc, prefixxyz]
```