How can I get the nearest city to geo-coordinates with Go?

How do I get a geolocation (e.g. the nearest city) from coordinates (e.g. 49.014,8.4043) with Go?

I tried to use golang-geo:

package main

import (
	"log"

	"github.com/kellydunn/golang-geo"
)

func main() {
	p := geo.NewPoint(49.014, 8.4043)
	geocoder := new(geo.GoogleGeocoder)
	geo.HandleWithSQL()
	res, err := geocoder.ReverseGeocode(p)
	if err != nil {
		log.Println(err)
	}
	log.Println(string(res))
}

but it gives Schloßplatz 23, 76131 Karlsruhe, Germany. I would like
Karlsruhe (so: only the city).

How do I get only the city?

The data you are looking to extract is not returned directly from the library. You can, however, perform a request and parse the JSON response yourself to extract the city, rather than the full address:

package main

import (
	"encoding/json"
	"fmt"
	"log"

	"github.com/kellydunn/golang-geo"
)

type googleGeocodeResponse struct {
	Results []struct {
		AddressComponents []struct {
			LongName  string   `json:"long_name"`
			Types     []string `json:"types"`
		} `json:"address_components"`
	}
}

func main() {
	p := geo.NewPoint(49.014, 8.4043)
	geocoder := new(geo.GoogleGeocoder)
	geo.HandleWithSQL()
	data, err := geocoder.Request(fmt.Sprintf("latlng=%f,%f", p.Lat(), p.Lng()))
	if err != nil {
		log.Println(err)
	}
	var res googleGeocodeResponse
	if err := json.Unmarshal(data, &res); err != nil {
		log.Println(err)
	}
	var city string
	if len(res.Results) > 0 {
		r := res.Results[0]
	outer:
		for _, comp := range r.AddressComponents {
            // See https://developers.google.com/maps/documentation/geocoding/#Types
            // for address types
			for _, compType := range comp.Types {
				if compType == "locality" {
					city = comp.LongName
					break outer
				}
			}
		}
	}
	fmt.Printf("City: %s\n", city)
}

The documentation for the Geocoder interface from that library says (emphasis mine):

> ... Reverse geocoding should accept a pointer to a Point, and return the street address that most closely represents it.

So you'll have to either parse the city name from the street address (which is its own challenge) or find a different geocoder library that provides a city explicitly.

Author of golang-geo here.

For those following along with this stack overflow question, I've answered @moose 's primary question in issue #31 here.

The tl;dr answer to this question is that while the Google Geocoding APIs do support some fuzzy notion of getting different levels of precision, it has yet to be implemented in golang-geo to date.