03-constraints.md

Constraints

In the example on the previous page we tried to sum a series of 64-bit integers with the generic Sum[T int](...T) T function (Go playground):

package main

import (
	"fmt"
)

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

func main() {
	fmt.Println(Sum([]int64{1, 2, 3}...))
}

However, when trying to run this example the compiler balks with the following error:

./prog.go:17:17: int64 does not implement int

This occurs because while the function Sum[T int](...T) T is written generically, the type of T is constrained to be only an int. The function needs to be rewritten so it can accept an int or an int64:

func Sum[T int | int64](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([]int64{1, 2, 3}...))
}

Using the | operator, the constraints for T can be expressed such that T can be satisfied by an int or int64. Now when the example is run, it produces the expected output (Golang playground):

6
6

But what about wanting a Sum function that can handle any numeric value? While it is possible to just list them all in Sum[T int | int32 | int64 |..., this would result in difficult-to-read code. Luckily there is a better way...

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Next: The any constraint