Golang templates (and passing funcs to template)

Question

I'm getting an error when I try and access a function I'm passing to my template:

Error: template: struct.tpl:3: function "makeGoName" not defined

Can anyone please let me know what I'm doing wrong?

Template file (struct.tpl):

type {{.data.tableName}} struct {
  {{range $key, $value := .data.tableData}}
  {{makeGoName $value.colName}} {{$value.colType}} `db:"{{makeDBName $value.dbColName}},json:"{{$value.dbColName}}"`
  {{end}}
}

Calling file:

type tplData struct {
    tableName string
    tableData interface{}
}

func doStuff() {
    t, err := template.ParseFiles("templates/struct.tpl")
    if err != nil {
        errorQuit(err)
    }

    t = t.Funcs(template.FuncMap{
        "makeGoName": makeGoName,
        "makeDBName": makeDBName,
    })

    data := tplData{
        tableName: tableName,
        tableData: tableInfo,
    }

    t.Execute(os.Stdout, data)
}

func makeGoName(name string) string {
    return name
}

func makeDBName(name string) string {
    return name
}

This is for a program that generates struct boilerplate code (in case anyone is wondering why I'm doing that in my template).

Answer 1

When registering custom functions for your template and using ParseFiles() you need to specify the name of the template when instantiating it and executing it. You also need to call ParseFiles() after you call Funcs().

// Create a named template with custom functions
t, err := template.New("struct.tpl").Funcs(template.FuncMap{
    "makeGoName": makeGoName,
    "makeDBName": makeDBName,
}).ParseFiles("templates/struct.tpl") // Parse the template file
if err != nil {
    errorQuit(err)
}

// Execute the named template
err = t.ExecuteTemplate(os.Stdout, "struct.tpl", data)
if err != nil {
    errorQuit(err)
}

When working with named templates the name is the filename without the directory path e.g. struct.tpl not templates/struct.tpl. So the name in New() and ExecuteTemplate() should be the string struct.tpl.

Answer 2

Custom functions need to be registered before parsing the templates, else the parser would not be able to tell whether an identifier is a valid function name or not. Templates are designed to be statically analyzable, and this is a requirement to that.

You can first create a new, undefined template with template.New(), and besides the template.ParseFiles() function, the template.Template type (returned by New()) also has a Template.ParseFiles() method, you can call that.

Something like this:

t, err := template.New("").Funcs(template.FuncMap{
    "makeGoName": makeGoName,
    "makeDBName": makeDBName,
}).ParseFiles("templates/struct.tpl")

Note that the template.ParseFiles() function also calls template.New() under the hood, passing the name of the first file as the template name.

Also Template.Execute() returns an error, print that to see if no output is generated, e.g.:

if err := t.Execute(os.Stdout, data); err != nil {
    fmt.Println(err)
}