Try T type in go

functools.go

package main

import (
	"fmt"
)

// filter ...
func filter[T any](slice []T, filter func(i int, x T) bool) (o []T) {
	for i, x := range slice {
		if filter(i, x) {
			o = append(o, x)
		}
	}
	return o
}

// mapInto ...
func mapInto[T comparable](slice []T, mappingFunc func(i int, x T) map[T]T) (o []map[T]T) {
	for i, x := range slice {
		mappedX := mappingFunc(i, x)
		o = append(o, mappedX)
	}
	return o
}

// mapIntoSlice ...
func mapIntoSlice[T any](slice []T, mapper func(i int, x T) T) (o []T) {
	for i, x := range slice {
		mappedX := mapper(i, x)
		o = append(o, mappedX)
	}
	return o
}

// concat concatenates the first slice into the second slice
func concat[T any](from []T, into []T) []T {
	for _, x := range from {
		into = append(into, x)
	}
	return into
}

// reduce  ...
// TODO: make it work on maps and nD slices
func reduce[T any](slice []T, reducer func(acc []T, curr []T, index int, orig []T) []T) (o []T) {
	var x T
	var i T
	for i, x = range slice {
		o = reducer(o, x, i, slice)
		// o = append(o, reduced)
	}
	return o
}

func main() {
	// Filtering
	// Ints --
	sameInts := []int{1, -1, 10, 1000}
	// _ the index is really cool but just works for ints ;((
	x := filter(sameInts, func(_, x int) bool { return x >= 0 })
	fmt.Println("x >= 0", x)

	i := filter(sameInts, func(i int, x int) bool { return i >= 1 })
	fmt.Println("i >= 1", i)

	ineg := filter(sameInts, func(_, x int) bool { return x < 0 })
	fmt.Println("i < 0", ineg)

	ix := filter(sameInts, func(i int, x int) bool { return i >= 1 && x%2 == 0 })
	fmt.Println("i >= 1 && x%2 == 0", ix)

	// Floats --
	sameFloats := []float64{1.0, 2.0, 3.125, 3.141596}
	fx := filter(sameFloats, func(i int, x float64) bool { return x > 1.0 && i == 0 })
	fmt.Println("x >= 1.0", fx)

	// Strings, Runes -
	sameRunes := []rune{'a', 'e', 'i', 'o', 'z'}
	notv := filter(sameRunes, func(i int, x rune) bool { return x == 'z' })
	fmt.Println("x == 'z'", string(notv))

	v := filter(sameRunes, func(i int, x rune) bool { return x == 'a' || x == 'e' || x == 'i' || x == 'o' || x == 'u' })
	fmt.Println("x == 'a' || x == 'e' || x == 'i' || x == 'o' || x == 'u' ", string(v))

	// Mapping
	orig := []int{1, 1, 1}

	// [1,1,1].map((i, x) => i+x) -> [1, 2, 3]
	fmt.Println("mapIntoSlice => ", orig, mapIntoSlice(orig, func(i int, x int) int { return i + x }))

	// [0, 1, 2].map((_, x) => ({[_]: _+x}))
	fmt.Println("mapInto => ", orig, mapInto(orig, func(i int, x int) map[int]int { return map[int]int{i: i + x} }))

	// Reducing
	// var flattened = [[0, 1], [2, 3], [4, 5]].reduce((acc, curr) => acc.concat(curr), [])

	nested := [][]int{[]int{0, 1}, []int{2, 3}, []int{4, 5}}
	// n := []int{1, 2, 3}
	// a := reduce(nested [][]int, func(acc []int, curr []int, i int, a[]int) []int { return concat(curr, acc)})
	a := reduce(nested, func(acc []int, curr []int, i int, a [][]int) []int {
		//acc = append(acc, curr)
		acc = concat(acc, curr)
		return acc
	})
	fmt.Println("reduce =>", a)

}