Rust generics: Expected <T> found <Foo>

Question

I'm trying to use generics but I don't master that topic well enough and I get this error:

error: mismatched types:
expected `book::mdbook::MDBook<R>`,
found `book::mdbook::MDBook<renderer::html_handlebars::HtmlHandlebars>`
(expected type parameter,
found struct `renderer::html_handlebars::HtmlHandlebars`) [E0308]

This is the relevant code

pub struct MDBook<R> where R: Renderer {
    title: String,
    author: String,
    config: BookConfig,
    pub content: Vec<BookItem>,
    renderer: R,
}

impl<R> MDBook<R> where R: Renderer {

    pub fn new(path: &PathBuf) -> Self {

        MDBook {
            title: String::from(""),
            author: String::from(""),
            content: vec![],
            config: BookConfig::new()
                        .set_src(path.join("src"))
                        .set_dest(path.join("book")),
            renderer: HtmlHandlebars::new(), // <---- ERROR HERE
        }
    }
}

The Renderer trait is empty at the moment and the implementation for HtmlHandlebars is

pub struct HtmlHandlebars;

impl Renderer for HtmlHandlebars {

}

impl HtmlHandlebars {
    pub fn new() -> Self {
        HtmlHandlebars
    }
}

What am I doing wrong?

Answer 1

impl<R> MDBook<R> where R: Renderer {

    pub fn new(path: &PathBuf) -> Self {

These lines claim that for all types R that implement Renderer, there is a method new(path) that returns MDBook<R>. However, your implementation of the method always returns MDBook<HtmlHandlebars> regardless of what R is.

You could add a trait bound to R (or a method to Renderer) that allows constructing a value of type R in new. Alternatively, the method could accept the renderer as parameter, i.e. fn new(path: &Path, renderer: R) -> Self. Either way, you need a way to get your hands on a renderer (i.e., a value of type R) inside new.

If on the other hand you want to support something like this:

let book = MDBook::new(path);
if some_condition {
    book.set_renderer(SomeOtherThing::new());
}

then generics are the wrong tool for the job, since they make the choice of renderer part of the static type of book. You can remove the R type parameter completely, keep your trait and simply store a trait object (likely Box<Renderer>) in MDBook.