Mail templates

Every mail is one plain text file. The first line is the subject and the rest is the body. Placeholders use Go’s text/template syntax:

You are invited, {{.Name}}
Hello {{.Name}},

Follow the link below to set your password.

Your application ships its templates inside the binary with embed, and one setting names a directory whose files replace them. That is the whole override story: an operator who wants different wording drops a file with the same name into that directory, with no rebuild and no restart, because files are read again on every send.

Wiring it up

//go:embed templates
var files embed.FS

defaults, err := fs.Sub(files, "templates")
if err != nil {
}
tpls, err := mailkit.NewTemplates(defaults, overrideDir)
if err != nil {
}

fs.Sub matters. Template names are plain file names like invite.tmpl, with no directories in them, and Render refuses a name carrying a path separator. fs.Sub strips the templates/ prefix the embedding adds.

overrideDir may be empty, which means embedded templates only. A directory that does not exist is refused at construction, so a typo in the setting fails at startup instead of silently ignoring every override.

Rendering

m, err := tpls.Render("invite.tmpl", map[string]string{"Name": "Maria Perez"})

The answer is a Message with the subject and body filled and To empty. Any value works as data, a struct or a map alike.

Three details of the split:

• The subject is the first non blank line of the rendered output, so a leading {{/* comment */}} line is harmless.
• A file with no newline at all is a subject with an empty body.
• A template that renders to nothing answers ErrNoSubject.

Windows line endings and a leading byte order mark, which some editors add invisibly, are both tolerated.