📐 Controlling Whitespace in Go Templates: Nested Loops 🔗

When working with Go’s text/template package, formatting nested data structures often results in unwanted blank lines. Because Go templates preserve all whitespace—including the newlines used to format the template code itself—standard loops produce bloated output.

Here is how to solve the whitespace problem using Go’s template trim markers.

📑 Table of Contents 🔗

⚠️ The Problem: Preserved Newlines 🔗

Say we want to format this nested data structure:

var data = map[string]map[string][]string{
	"1": {
		"1.1": {"1.1.1", "1.1.2", "1.1.3"},
	},
	"2": {
		"2.1": {"2.1.1", "2.1.2", "2.1.3"},
	},
}

A natural way to write the template is to use standard {{ range }} blocks:

{{ range $name, $map := . }}
{{ $name }}:
  {{ range $name, $values := $map }}
  {{ $name }}:
    {{ range $value := $values }}
    {{ $value }}
    {{ end }}
  {{ end }}
{{ end }}

Because every newline and indentation space in the template is printed exactly as written, the output is littered with blank spaces:

1:
  
  1.1:
    
    1.1.1
    
    1.1.2
    
    1.1.3
    
  

2:
...

✂️ The Solution: The Trim Marker (-) 🔗

You can instruct the Go template engine to consume adjacent whitespace by adding a minus sign (-) inside the action tags:

• {{- ... }} removes all preceding whitespace (spaces, tabs, and newlines) up to the previous non-whitespace character.
• {{ ... -}} removes all following whitespace up to the next non-whitespace character.

To fix our nested loop, we strategically place the - marker to eat the template’s formatting newlines without removing our intended indentation:

{{- range $name, $map := . -}}
{{ $name }}:
  {{- range $name, $values := $map }}
  {{ $name }}:
    {{- range $value := $values }}
    {{ $value }}
    {{- end }}
  {{- end }}
{{ end -}}

By stripping the newlines immediately following the range declarations and the end blocks, we get clean, tightly packed output:

1:
  1.1:
    1.1.1
    1.1.2
    1.1.3
2:
  2.1:
    2.1.1
    2.1.2
    2.1.3

💻 Full Runnable Example 🔗

You can copy and run this code to test the behavior directly.

package main

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

var data = map[string]map[string][]string{
	"1": {
		"1.1": {"1.1.1", "1.1.2", "1.1.3"},
	},
	"2": {
		"2.1": {"2.1.1", "2.1.2", "2.1.3"},
	},
}

func main() {
	const tpl = `
{{- range $name, $map := . -}}
{{ $name }}:
  {{- range $name, $values := $map }}
  {{ $name }}:
    {{- range $value := $values }}
    {{ $value }}
    {{- end }}
  {{- end }}
{{ end -}}
`
	fmt.Println("Output:")
	fmt.Println("'''")
	
	t := template.Must(template.New("nested").Parse(tpl))
	t.Execute(os.Stdout, data)
	
	fmt.Println("'''")
}

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