substack-ac-6.go

// you want to lookup the weather for multiple cities at once, and fail if any of the lookups fails.

func Cities(cities ...string) ([]*Info, error) {
    var group errgroup.Group
    var mu sync.Mutex
    res := make([]*Info, len(cities)) // res[i] corresponds to cities[i]

    for i, city := range cities {
        i, city := i, city // create locals for closure below
        group.Go(func() error {
            info, err := City(city)
            mu.Lock()
            res[i] = info
            mu.Unlock()
            return err
        })
    }
    if err := group.Wait(); err != nil {
        return nil, err
    }
    return res, nil
}