Chapter 6 Functions, Defer, and Panic

• 6.1 Function Calls
• 6.2 The Defer Statement
• 6.3 The panic and recover Builtins

A function is a first-class citizen when it can be assigned like any other value, passed as an argument, returned as a result, and can capture variables from the environment in which it is defined, thereby becoming a closure. Strachey drew the distinction between first-class and second-class objects in his lecture notes for the International Summer School in Computer Programming, held in Copenhagen in August 1967, and used ALGOL procedures as his example of a second-class citizen; the notes were not formally published until 2000.

This chapter examines the implementation of three function-related mechanisms in Go: how function values and closures are represented in memory and how they are called (6.1), how defer guarantees that cleanup runs no matter which path leaves a function (6.2), and how panic and recover propagate along the call stack and are intercepted (6.3). The three share one runtime mechanism. Deferred calls rely on closures, panic handling relies on deferred calls, and the same code runs deferred calls on a normal return and during a panic. Understanding this mechanism makes clear the trade-offs Go makes among convenience, run-time performance, and explicit control flow.