Function Programs

Learn function programs through an executable example, then connect the small demonstration to production code.

Functions make repeated calculations reusable and testable. They can accept parameters, return multiple values and report errors without relying on globals.

The example implements a safe integer division function that returns both the result and an error.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func divide(a, b int) (int, error) {
	if b == 0 {
		return 0, fmt.Errorf("division by zero")
	}
	return a / b, nil
}
func main() {
	for _, pair := range [][2]int{{20, 4}, {20, 0}} {
		result, err := divide(pair[0], pair[1])
		fmt.Println("Result:", result, "Error:", err)
	}
}

Output

Result: 5 Error: <nil>
Result: 0 Error: division by zero

The first call yields 5 with no error. The second call detects a zero divisor and returns an error rather than panicking.

• Integer division truncates toward zero; it does not compute a fractional float result.
• Do not ignore an error return from a function just because an example happens to succeed.

Change divide to return a float64 result while still reporting a zero-divisor error.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Function Programs in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.