如何为go文本/模板添加默认值？

Question

I want to create a golang template with a default value that is used if a parameter is not supplied, but if I try to use the or function in my template, it gives me this error:

template: t2:2:20: executing "t2" at <index .table_name 0>: error calling index: index of untyped nil

Here's an example of the code: https://play.golang.org/p/BwlpROrhm6

// text/template is a useful text generating tool.
// Related examples: http://golang.org/pkg/text/template/#pkg-examples
package main

import (
    "fmt"
    "os"
    "text/template"
)

var fullParams = map[string][]string{
    "table_name": []string{"TableNameFromParameters"},
    "min":        []string{"100"},
    "max":        []string{"500"},
}
var minimalParams = map[string][]string{
    "min": []string{"100"},
    "max": []string{"500"},
}

func check(err error) {
    if err != nil {
        fmt.Print(err)
    }
}

func main() {
    // Define Template
    t := template.Must(template.New("t2").Parse(`
        {{$table_name := (index .table_name 0) or "DefaultTableName"}}
        Hello World!
        The table name is {{$table_name}}
    `))
    check(t.Execute(os.Stdout, fullParams))
    check(t.Execute(os.Stdout, minimalParams))
}

Googling has pointed me towards an isset function in hugo's template engine, but I can't figure out how they implemented it, and I am not sure if it would even solve my problem.

Answer 1

Another solution is by changing the template definition to

// Define Template
t := template.Must(template.New("t2").Parse(`
    Hello World!
    The table name is {{with .table_name}}{{index . 0}}{{else}}DefaultTableName{{end}}
`))

But, the value won't be stored in a variable, so if you want to reuse it in other places, you need to write it again. The main purpose of the standard template package is for rendering precomputed value, and logic related operations/functions have limited capability. However, you can define your own function then register it to the template's FuncMap e.g. the default function mentioned by @jeevatkm.