Variable in template's included template

I am trying to put values into a "header" template, like the title and navigation links but can't access the variables that I sent to the main template from the included one.

Rendering the template:

    ...
    templateName := "index"
    args := map[string]string{
            "Title":       "Main Page",
            "Body":        "This is the content",
    }
    PageTemplates.ExecuteTemplate(w, templateName+".html", args)
    ...

index.html template:

   {{template "header"}} <-- Including the "header.html" template
   {{.Body}}             <-- Variable that works
   {{template "footer"}} <-- Does not matter!

header.html template:

   {{define "header"}}
   <!DOCTYPE html>
   <html lang="en">
   <head>
            <title>{{.Title}}</title> <-- Variable empty :(
   </head>
   <body>
   {{end}}

Apparently, it won't work that way.

Maybe there's a way I could parse/get the template and put my variables into it without putting the whole header file into code? Then I could just send that template as a variable to my main template. But that does not seem like it would be the best way to do it.

Solution

You can pass the context to the template when you call it. In your example, changing {{template "header"}} to {{template "header" .}} should be sufficient.

The relevant parts from the official docs:

{{template "name"}}
The template with the specified name is executed with nil data.

{{template "name" pipeline}}
The template with the specified name is executed with dot set to the value of the pipeline.

PS: It is not relevant to the question, but you should also remove the newline between {{define "header"}} and <!DOCTYPE html>, so that the doctype is really the first thing in your template.