Careful to be sure to include the exact name of one of the files parsed otherwise the t.Execute won’t find the template.
custom := template.FuncMap{
`foo`: func() string { return `bar` },
}
data := map[string]any{`this`: `that`}
t, err := template.New(`tutorial.txt`).Funcs(custom).ParseGlob(`files/*.txt`)
if err != nil {
log.Fatal(err)
}
if err := t.Execute(tutfile, data); err != nil {
log.Fatal(err)
}