How to Detect the Last Element in a Golang Template Range?

Detecting Last Element in a Template Range

In a Golang text template, the range directive iterates over elements in a collection and provides access to the current element and its index. While this mechanism facilitates efficient looping, it presents a challenge when identifying the last element in the sequence.

Issue:

Consider a template:

{{range $i, $e := .SomeField}}
    {{if $i}}, {{end}}
    $e.TheString
{{end}}

By default, this template will output a comma-separated list of elements:

one, two, three

However, to generate a human-readable output with "and" preceding the final element, we need to determine the range's last index.

Solution:

While arithmetic operations are not directly supported in templates, a workaround involves using Go's reflect package:

package main

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

var fns = template.FuncMap{
    "last": func(x int, a interface{}) bool {
        return x == reflect.ValueOf(a).Len() - 1
    },
}


func main() {
    t := template.Must(template.New("abc").Funcs(fns).Parse(`{{range  $i, $e := .}}{{if $i}}, {{end}}{{if last $i $}}and {{end}}{{$e}}{{end}}.`))
    a := []string{"one", "two", "three"}
    t.Execute(os.Stdout, a)
}

This solution introduces a Go function last to the template's function map. The function takes the current index and a collection as arguments and returns true if the index is equal to the collection's length minus one.

By incorporating this function into the template, we can distinguish the last element and output the desired result:

one, two, and three

Additional Notes:

An alternative approach utilizes the len function to calculate the length of the collection within the template without the need for reflection:

{{range  $i, $e := .}}{{if $i}}, {{end}}{{if eq $i (len $)}}and {{end}}{{$e}}{{end}}.`

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