generics.go

package main

import "fmt"

type Numeric interface {
	~int | ~float32 | ~float64
}

type Predicate[T any] func(T) bool
type Transform[T any] func(T) T
type Reduction[T any] func(T, T) T

func Filter[T any](items []T, p Predicate[T]) []T {
	filtered := make([]T, 0)
	for _, item := range items {
		if p(item) {
			filtered = append(filtered, item)
		}
	}
	return filtered
}

func Map[T any](items []T, f Transform[T]) []T {
	transformed := make([]T, 0)
	for _, item := range items {
		transformed = append(transformed, f(item))
	}
	return transformed
}

func Reduce[T any](items []T, start T, f Reduction[T]) T {
	reduced := start
	for _, item := range items {
		reduced = f(item, reduced)
	}
	return reduced
}

func main() {
	positive := func(x int) bool {
		return x > 0
	}
	twice := func(x int) int {
		return x * 2
	}
	sumUp := func(x, y int) int {
		return x + y
	}
	numbers := []int{-3, 5, 2, -8, 9}
	positives := Filter(numbers, positive)
	doubled := Map(positives, twice)
	summed := Reduce(doubled, 0, sumUp)
	fmt.Println(summed)
}