How to update field value in a Go template

I have the following case, where I am passing a struct containing a map to a template:

package main

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

var fns = template.FuncMap{
	"plus1": func(x int) int {
		return x + 1
	},
}

type codec struct {
	Names map[string]string
	Count int
}

func main() {
	a := map[string]string{"one": "1",
		"two":   "2",
		"three": "3"}

	t := template.Must(template.New("abc").Funcs(fns).Parse(`{{$l := len .Names}}{{range $k, $v := .Names}}{{if ne (plus1 $.Count) $l}}{{$k}} {{$v}} {{end}}{{end}}.`))

	err := t.Execute(os.Stdout, codec{a, 0})
	if err != nil {
		log.Println(err)
	}
}

I would like to increment the Count field of codec so that I can know how many items of the map I've seen.

One solution is to make the plus1 function a closure that acts directly on the value of the codec:

// first create a codec instance
c := codec {a, 0}

// now define the function as a closure with a reference to c
fns := template.FuncMap{
  "plus1": func() int {
      c.Count++
      return c.Count
   },
}

// now we don't need to pass anything to it in the template
t := template.Must(template.New("abc").Funcs(fns).Parse(`{{$l := len .Names}}{{range $k, $v := .Names}}{{if ne (plus1) $l}}{{$k}} {{$v}} {{end}}{{end}}.`))

The output was:

one 1 three 3

which I'm guessing is what you were aiming for? And the value is retained in c at the end of execution.

You can simply define a method on your struct:

type codec struct {
    Names map[string]string
    Count int
}

func (c *codec) IncAndGet() int {
    c.Count++
    return c.Count
}

Calling it from a template:

c := &codec{Count: 2}
t := template.Must(template.New("").Parse(`{{.IncAndGet}} {{.IncAndGet}}`))
t.Execute(os.Stdout, c)

Output (try it on the <kbd>Go Playground</kbd>):

3 4

Note that for this to work, the method needs a pointer receiver (func (c *codec) IncAndGet()) and you have to pass a pointer to Template.Execute() (c is a pointer in our example: c := &codec{Count: 2}).

If you don't want any result just counting, define it to have a string return type and return the empty string "":

func (c *codec) Inc() string {
    c.Count++
    return ""
}