Example of functional programming in Golang.

go_functional.go

package main

import (
	"constraints"
	"fmt"
)

func main() {
	fmt.Println(Sum(1, 2, 3, 4))
	fmt.Println(Sum("1", "2", "3"))
}

func Map[T any](operation func(p T) T, is ...T) []T {
	r := make([]T, len(is))
	for index, value := range is {
		r[index] = operation(value)
	}
	return r
}

func Reduce[T any](operation func(a, b T) T, is ...T) T {
	var r T
	for index, value := range is {
		if index == 0 {
			r = value
		} else {
			r = operation(r, value)
		}
	}
	return r
}

func Add[T constraints.Ordered](a, b T) T {
	return a + b
}

func Sum[T constraints.Ordered](i ...T) T {
	return Reduce(Add[T], i...)
}

It looks like some people are stumbling across this (thanks Google!) so I should mention that using interface causes a performance hit. You're probably better off with a for loop for now.

Now that generics are here there's less of a performance hit doing functional programming! I've updated the gist.