How do functions and multiple return values work in Go?

Question

Accepted Answer

Go functions are declared with `func`, and a distinctive feature is **multiple return values** — a function can return several values at once. This is used pervasively, especially for the `(result, error)` idiom that defines Go's error handling.

## Basic function

```go
func add(a int, b int) int {     // params with types, then the return type
    return a + b
}

func greet(name string) string {
    return "Hello, " + name
}
// shorthand for same-typed params: func add(a, b int) int
```

## Multiple return values — the defining feature

```go
func divide(a, b int) (int, error) {     // returns BOTH a result AND an error
    if b == 0 {
        return 0, errors.New("division by zero")
    }
    return a / b, nil    // nil error = success
}

// callers handle both — the ubiquitous Go error pattern
result, err := divide(10, 2)
if err != nil {
    log.Fatal(err)       // handle the error
}
fmt.Println(result)
```

Returning `(value, error)` is THE idiom for error handling in Go — instead of exceptions, functions return errors as values that callers explicitly check. This makes error handling visible and impossible to silently ignore.

## The comma-ok pattern

```go
// many built-ins return (value, ok) — a boolean for "did it work?"
value, ok := myMap["key"]   // ok = false if the key is missing
if ok { ... }

v, ok := someInterface.(string)   // type assertion with ok
```

## Named return values

```go
func split(sum int) (x, y int) {   // named returns
    x = sum * 4 / 9
    y = sum - x
    return   // "naked return" — returns the named values
}
```

## Variadic functions

```go
func sum(nums ...int) int {     // accepts any number of ints
    total := 0
    for _, n := range nums {
        total += n
    }
    return total
}
sum(1, 2, 3)          // 6
sum(nums...)          // spread a slice into args
```

## Functions are first-class

```go
var op func(int, int) int = add   // assign functions to variables
func apply(fn func(int) int, x int) int { return fn(x) }  // pass as arguments
```

## Why it matters

Go's functions are central, and **multiple return values** are one of the language's defining features — they enable the `(result, error)` idiom that *is* Go's approach to error handling (explicit error-as-value checking instead of exceptions).

This pattern, used in virtually every Go program, makes error handling visible and deliberate.

The related comma-ok idiom (`value, ok :=`) appears throughout (map access, type assertions, channel receives).

Understanding multiple returns, the error pattern, named returns, variadic functions, and first-class functions is essential for reading and writing idiomatic Go — and the error-as-value philosophy is one of the most important conceptual differences between Go and exception-based languages.