04-the-any-constraint.md

The any constraint

Previously we illustrated how to write a function that can sum both int and int64 types (Go playground):

// Sum returns the sum of the provided arguments.
func Sum[T int | int64](args ...T) T {
	var sum T
	for i := 0; i < len(args); i++ {
		sum += args[i]
	}
	return sum
}

But why limit the function only int and int64? One way to increase the number of supported types is by continuing to use the | operator:

func Sum[T int | int8 | int32 | int64](args ...T) T

However, this would become cumbersome if we wanted to include any numeric types. Huh, any? Is there not some new any identifier in Go? Can we use it to rewrite Sum[T](...T) T? Let's try (Go playground):

package main

import (
	"fmt"
)

// Sum returns the sum of the provided arguments.
func Sum[T any](args ...T) T {
	var sum T
	for i := 0; i < len(args); i++ {
		sum += args[i]
	}
	return sum
}

func main() {
	fmt.Println(Sum([]int{1, 2, 3}...))
	fmt.Println(Sum([]int8{1, 2, 3}...))
	fmt.Println(Sum([]uint32{1, 2, 3}...))
	fmt.Println(Sum([]float64{1.1, 2.2, 3.3}...))
	fmt.Println(Sum([]complex128{1.1i, 2.2i, 3.3i}...))
}

Unfortunately the above program will fail to compile with the following error:

./prog.go:11:3: invalid operation: operator + not defined on sum (variable of type T constrained by any)

The any identifier is equivalent to the empty interface in all ways. Thus T in the above example might not represent a type for which the addition operator is valid.

Instead we need a type that represents all, possible numeric types...

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Next: Composite constraints