How to Pass Multiple Values to Go Templates' {{template}} Action?

Passing Multiple Values Between Templates

In Go templating, the {{template}} action takes only one optional argument. To pass multiple values, they must be encapsulated within a single value.

Encapsulating Data

One approach is to use a wrapper function that accepts the desired values and returns a single value. For instance, we could create a wrapper function for our City and Region data:

func Wrap(shops []Destination, cityName, regionName string) map[string]interface{} {
    return map[string]interface{}{
        "Shops":      shops,
        "CityName":   cityName,
        "RegionName": regionName,
    }
}

Registering the Wrapper Function

Custom functions can be registered using Template.Funcs(). This must be done before parsing the template:

t := template.Must(template.New("cities.gohtml").Funcs(template.FuncMap{
    "Wrap": Wrap,
}).Parse(src))

Modified Template

The template can then be modified to call the Wrap() function and pass the result to the {{template}} action:

{{define "data"}}
    City: {{.CityName}}, Region: {{.RegionName}}, Shops: {{.Shops}}
{{end}}
{{- range . -}}
        {{$city:=.Name}}
        {{- range .Regions -}}
              {{$region:=.Name}}
              {{- template "data" (Wrap .Shops $city $region) -}}
        {{end}}
{{- end}}

Example

Here's an example using the City and Region structures provided in the question:

t := template.Must(template.New("cities.gohtml").Funcs(template.FuncMap{
    "Wrap": Wrap,
}).Parse(src))
CityWithSomeData := []City{
    {
        Name: "CityA",
        Regions: []Region{
            {Name: "CA-RA", Shops: []Destination{{"CA-RA-SA"}, {"CA-RA-SB"}}},
            {Name: "CA-RB", Shops: []Destination{{"CA-RB-SA"}, {"CA-RB-SB"}}},
        },
    },
    {
        Name: "CityB",
        Regions: []Region{
            {Name: "CB-RA", Shops: []Destination{{"CB-RA-SA"}, {"CB-RA-SB"}}},
            {Name: "CB-RB", Shops: []Destination{{"CB-RB-SA"}, {"CB-RB-SB"}}},
        },
    },
}
if err := t.ExecuteTemplate(os.Stdout, "cities.gohtml", CityWithSomeData); err != nil {
    panic(err)
}

Output:

City: CityA, Region: CA-RA, Shops: [{CA-RA-SA} {CA-RA-SB}]
City: CityA, Region: CA-RB, Shops: [{CA-RB-SA} {CA-RB-SB}]
City: CityB, Region: CB-RA, Shops: [{CB-RA-SA} {CB-RA-SB}]
City: CityB, Region: CB-RB, Shops: [{CB-RB-SA} {CB-RB-SB}]

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