Kotlin 関数定義の方法に悩む…

gistfile1.kt

// 拡張関数で定義。
// たぶん一番Kotlinらしい書き方
// kotlin friendly
fun String.isFourCharacters() = this.size == 4

// static関数として定義。
// Javaっぽい書き方？
// like java?
//fun isFourCharacters(str : String) = str.isFourCharacters()

// 関数リテラルを変数に代入
// 関数型言語っぽい？
// like functional language?
val isFourCharacters = {
  (str : String) -> str.isFourCharacters()
}

fun main(args : Array<String>) {
  val strs = array("java", "kotlin", "groovy")
  
  val result1 = strs filter {
    it.isFourCharacters()
    // or
    // isFourCharacters(it)
  }
  
  // like functional language
  val result2 = strs filter isFourCharacters
}

You seem to be looking for member literals:

http://youtrack.jetbrains.com/issue/KT-1183