This is a good example of what the OP is asking for. However, you really have to see an example of it to understand the benefits. The writers of the "Physically Based Rendering: From Theory to Implementation" did this for their book, and it's pretty amazing.

They would write text, then a code block:

    func main() {
       <main body variable declarations>
       <main body functions>
    }

followed by another block of text, and then an expansion of the code

    <main body variable declarations> = 
        var foo int
        var bar int64

And so on.

    <main body variable declarations> +=
        var zed rune

Which, when run through the "tangler", would produce:

    func main () {
        var foo int
        var bar int64
        var zed rune
    }

The first really interesting implementation I've seen of literate programming, and it makes me want to implement it myself.