An example snippet for the note https://vladimir.varank.in/notes/2022/05/a-real-life-use-case-for-generics-in-go-api-for-client-side-pagination/

pager.go

package pager

import "errors"

var PagerDone = errors.New("no more pages to iterate")

type pageIter[T any] interface {
	NextPage() ([]T, error)
}

type fetchPageFunc[T any] func(page int) ([]T, PageInfo, error)

type Pager[T any] struct {
	page     int
	pageInfo PageInfo
	err      error

	fetchPage fetchPageFunc[T]
}

func NewPager[T any](page int, fetchPage fetchPageFunc[T]) *Pager[T] {
	if page == 0 {
		page = 1
	}
	p := &Pager[T]{
		page:      page,
		fetchPage: fetchPage,
	}
	// hasNext must be set before the first interation
	p.pageInfo.HasNext = true
	return p
}

var _ pageIter[any] = (*Pager[any])(nil)

func (p *Pager[T]) NextPage() ([]T, error) {
	var list []T
	for len(list) == 0 {
		if p.err != nil {
			return nil, p.err
		}

		if !p.pageInfo.HasNext {
			p.err = PagerDone
			return nil, p.err
		}

		list, p.pageInfo, p.err = p.fetchPage(p.page)
		p.page++
	}

	return list, nil
}

func (p *Pager[T]) PageInfo() PageInfo {
	return p.pageInfo
}

pager_test.go

package pager

import (
	"math/rand"
	"testing"
)

func TestPager(t *testing.T) {
	pager := newTestPager(0)

	n := rand.Intn(50)
	for i := 1; i < n; i++ {
		list, err := pager.NextPage()
		if err != nil {
			t.Fatal(err)
		}
		if len(list) != i {
			t.Fatalf("page info: got list %+v", list)
		}
		pageInfo := pager.PageInfo()
		if !pageInfo.HasNext {
			t.Fatalf("pager: got page info %+v", pageInfo)
		}
	}
}

func newTestPager(init int) *Pager[int] {
	var list []int
	fetch := func(page int) ([]int, PageInfo, error) {
		pageInfo := PageInfo{
			HasNext: true,
		}
		list = append(list, init+page)
		return list, pageInfo, nil
	}
	return NewPager(0, fetch)
}