Text and HTML templates

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Imagine you’re working on a web application and need to return HTML that lists most recent tweets. You need to load list of tweets from a database and create HTML based on that information.

Building that HTML string by building smaller strings and concatenating them with + would be tedious.

Packages text/template and html/template in Go standard library make implement data-driven templates for generating textual output:

var tmplStr = `User {{.User}} has {{.TotalTweets}} tweets.
{{- $tweetCount := len .RecentTweets }}
Recent tweets:
{{range $idx, $tweet := .RecentTweets}}Tweet {{$idx}} of {{$tweetCount}}: '{{.}}'
{{end -}}
Most recent tweet: '{{index .RecentTweets 0}}'

t := template.New("tweets")
t, err := t.Parse(tmplStr)
if err != nil {
	log.Fatalf("template.Parse() failed with '%s'\n", err)
data := struct {
	User         string
	TotalTweets  int
	RecentTweets []string
	User:         "kjk",
	TotalTweets:  124,
	RecentTweets: []string{"hello", "there"},
err = t.Execute(os.Stdout, data)
if err != nil {
	log.Fatalf("t.Execute() failed with '%s'\n", err)
User kjk has 124 tweets.
Recent tweets:
Tweet 0 of 2: 'hello'
Tweet 1 of 2: 'there'
Most recent tweet: 'hello'

Each template has a name. template.New("tweets") creates an empty template with name tweets.

t.Parse(s string) parses the template.

t.Execute(w io.Writer, v interface{}) executes the template with a given value and writing the result to an io.Writer.

{{ ... }} is an action are instructions for templating engine.

{{.TweetCount}} means printing the value of TweetCount in current context.

Data passed to a template can be hierarchical (i.e. a struct withing a struct within a struct…).

Current context . refers to current scope within the data.

Initial . refers to top-level scope:

tmplStr := "Data: {{.}}\n"
t := template.Must(template.New("simple").Parse(tmplStr))
execWithData := func(data interface{}) {
	err := t.Execute(os.Stdout, data)
	if err != nil {
		log.Fatalf("t.Execute() failed with '%s'\n", err)

st := struct {
	Number int
	Str    string
	Number: 3,
	Str:    "hello",
Data: 5
Data: foo
Data: {3 hello}

Values that don’t have pre-defined formatting are printed using Stringer interface. For custom formatting of your type in a template implement String() string method.

{{range .Tweets}}{{end}} evaluates inner part for every element of []string slice Tweets and sets current context . within the inner part to elements of Tweets slice.

{{index .RecentTweets 0}} is equivalent to RecentTweets[0] in Go code.

Text in a template is copied verbatim. Having to preserve whitespace can lead to ugly templates.

To help write more readable templates We can add - at the beginning or end of action as seen in {{end -}}.

This remove whitespace before or after the action.

{{range .RecentTweets}} changes variable scope and we don’t have access to data outside. If we need to access data from upper scope, we can define variables like {{ $tweetCount := len .RecentTweets }}.

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