The Go Template ParseFiles func parsing multiple files

What would it happen if I pass two or more files to Go Template's ParseFiles func?

func (*Template) ParseFiles

It helps says:

> ParseFiles parses the named files and associates the resulting templates with t. If an error occurs, parsing stops and the returned template is nil; otherwise it is t. There must be at least one file. Since the templates created by ParseFiles are named by the base names of the argument files, t should usually have the name of one of the (base) names of the files. If it does not, depending on t's contents before calling ParseFiles, t.Execute may fail. In that case use t.ExecuteTemplate to execute a valid template.
>
> When parsing multiple files with the same name in different directories, the last one mentioned will be the one that results.

But I'm still not sure what would the differences be that affects the output, for

MyTempl.ParseFiles(tf1)

vs.

MyTempl.ParseFiles(tf1, tf2)

Will the content of tf2 be appended to that of tf1?

First a little about the "template" concept:

A template.Template value is "the representation of a parsed template". But the wording here is a little "imperfect". A template.Template value may be (and usually is) a collection of multiple, associated templates. template.Template has an unexported field:

tmpl   map[string]*Template // Map from name to defined templates.

This tmpl field holds all other associated templates, templates that are visible to the template, and which can be referred to by their names.

You can read more about this in this answer: https://stackoverflow.com/questions/41176355/go-template-name/41187671#41187671

Back to the Template.ParseFiles() method. This method parses multiple templates from the files passed to it as parameters. The templates parsed form the files will be named after the file names (without folders, just the file name), and they will be added to the internal, associated templates map of the t template designated by the method receiver.

The parsed templates will not be appended. Multiple, separate template.Template values will be created for them, but they will be associated (so they can refer to each other, e.g. they can include each other).

Let's see an example. Let's assume we have these 2 template files:

a.html is:

I'm a.

And b.html:

I'm b.

And an example code:

t := template.New("a.html")
if _, err := t.ParseFiles("a.html", "b.html"); err != nil {
	panic(err)
}
if err := t.Execute(os.Stdout, nil); err != nil {
	panic(err)
}

This example creates a new, empty template named a.html, then parses 2 files: a.html and b.html.

What will be the result? t will denote the a.html template, because we created it prior with that specific name. Running the code, the output will be:

I'm a.

Now if we change the first line to:

t := template.New("x.html")

And leave the rest unchanged, running it we see something similar:

panic: template: "x.html" is an incomplete or empty template

The reason is that t denotes a template named x.html but it's empty, as we didn't parse anything "into" it, and the parsed files also didn't match the name x.html. So attempting to execute it results in an error.

Now if we try to execute one of its associated, named template:

if err := t.ExecuteTemplate(os.Stdout, "a.html", nil); err != nil {
	panic(err)
}

It succeeds, and again gives:

I'm a.