Template

Template

• Package template implements data-driven templates for generating textual output.

• Templates are executed by applying them to a data structure. Annotations in the template refer to elements of the data structure (typically a field of a struct or a key in a map) to control execution and derive values to be displayed. Execution of the template walks the structure and sets the cursor, represented by a period ‘.’ and called “dot”, to the value at the current location in the structure as execution proceeds.

• The input text for a template is UTF-8-encoded text in any format. “Actions”–data evaluations or control structures–are delimited by “{{” and “}}”; all text outside actions is copied to the output unchanged. Except for raw strings, actions may not span newlines, although comments can.

Pipelines

• A pipeline is a possibly chained sequence of “commands”. A command is a simple value (argument) or a function or method call, possibly with multiple arguments.

Example of letter code

func main() {
    // Define a template.
    const letter = `
Dear {{.Name}},
{{if .Attended}}
It was a pleasure to see you at the wedding.
{{- else}}
It is a shame you couldn't make it to the wedding.
{{- end}}
{{with .Gift -}}
Thank you for the lovely {{.}}.
{{end}}
Best wishes,
Josie
`

    // Prepare some data to insert into the template.
    type Recipient struct {
        Name, Gift string
        Attended   bool
    }
    var recipients = []Recipient{
        {"Aunt Mildred", "bone china tea set", true},
        {"Uncle John", "moleskin pants", false},
        {"Cousin Rodney", "", false},
    }

    // Create a new template and parse the letter into it.
    t := template.Must(template.New("letter").Parse(letter))

    // Execute the template for each recipient.
    for _, r := range recipients {
        err := t.Execute(os.Stdout, r)
        if err != nil {
            log.Println("executing template:", err)
        }
    }

}

Glob example

• demonstrate loading a set of templates from a directory

// templateFile defines the contents of a template to be stored in a file, for testing.
type templateFile struct {
    name     string
    contents string
}

func createTestDir(files []templateFile) string {
    dir, err := ioutil.TempDir("", "template")
    if err != nil {
        log.Fatal(err)
    }
    for _, file := range files {
        f, err := os.Create(filepath.Join(dir, file.name))
        if err != nil {
            log.Fatal(err)
        }
        defer f.Close()
        _, err = io.WriteString(f, file.contents)
        if err != nil {
            log.Fatal(err)
        }
    }
    return dir
}

func main() {
    // Here we create a temporary directory and populate it with our sample
    // template definition files; usually the template files would already
    // exist in some location known to the program.
    dir := createTestDir([]templateFile{
        // T0.tmpl is a plain template file that just invokes T1.
        {"T0.tmpl", `T0 invokes T1: ({{template "T1"}})`},
        // T1.tmpl defines a template, T1 that invokes T2.
        {"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
        // T2.tmpl defines a template T2.
        {"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
    })
    // Clean up after the test; another quirk of running as an example.
    defer os.RemoveAll(dir)

    // pattern is the glob pattern used to find all the template files.
    pattern := filepath.Join(dir, "*.tmpl")

    // Here starts the example proper.
    // T0.tmpl is the first name matched, so it becomes the starting template,
    // the value returned by ParseGlob.
    tmpl := template.Must(template.ParseGlob(pattern))

    err := tmpl.Execute(os.Stdout, nil)
    if err != nil {
        log.Fatalf("template execution: %s", err)
    }
}

Helper example

• demonstrates one way to share some templates and use them in different contexts. In this variant we add multiple driver templates by hand to an existing bundle of templates.

// templateFile defines the contents of a template to be stored in a file, for testing.
type templateFile struct {
    name     string
    contents string
}

func createTestDir(files []templateFile) string {
    dir, err := ioutil.TempDir("", "template")
    if err != nil {
        log.Fatal(err)
    }
    for _, file := range files {
        f, err := os.Create(filepath.Join(dir, file.name))
        if err != nil {
            log.Fatal(err)
        }
        defer f.Close()
        _, err = io.WriteString(f, file.contents)
        if err != nil {
            log.Fatal(err)
        }
    }
    return dir
}

func main() {
    // Here we create a temporary directory and populate it with our sample
    // template definition files; usually the template files would already
    // exist in some location known to the program.
    dir := createTestDir([]templateFile{
        // T1.tmpl defines a template, T1 that invokes T2.
        {"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
        // T2.tmpl defines a template T2.
        {"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
    })
    // Clean up after the test; another quirk of running as an example.
    defer os.RemoveAll(dir)

    // pattern is the glob pattern used to find all the template files.
    pattern := filepath.Join(dir, "*.tmpl")

    // Here starts the example proper.
    // Load the helpers.
    templates := template.Must(template.ParseGlob(pattern))
    // Add one driver template to the bunch; we do this with an explicit template definition.
    _, err := templates.Parse("{{define `driver1`}}Driver 1 calls T1: ({{template `T1`}})\n{{end}}")
    if err != nil {
        log.Fatal("parsing driver1: ", err)
    }
    // Add another driver template.
    _, err = templates.Parse("{{define `driver2`}}Driver 2 calls T2: ({{template `T2`}})\n{{end}}")
    if err != nil {
        log.Fatal("parsing driver2: ", err)
    }
    // We load all the templates before execution. This package does not require
    // that behavior but html/template's escaping does, so it's a good habit.
    err = templates.ExecuteTemplate(os.Stdout, "driver1", nil)
    if err != nil {
        log.Fatalf("driver1 execution: %s", err)
    }
    err = templates.ExecuteTemplate(os.Stdout, "driver2", nil)
    if err != nil {
        log.Fatalf("driver2 execution: %s", err)
    }
}