Quick Answer

Functions that can be called without dot and parentheses

Understanding the Issue

Infix notation allows calling single-argument member functions or extension functions without the dot and parentheses, enabling more readable DSL-style code.

The Problem

This code demonstrates the issue:

Kotlin Error 
// Problem: Verbose function call
val result = myList.joinToString(separator = "|")

The Solution

Here's the corrected code:

Kotlin Fixed 
// Solution: Infix function
infix fun <T> List<T>.join(separator: String) = 
    this.joinToString(separator)

val result = myList join "|"

Key Takeaways

Use infix notation judiciously to create more readable APIs.