Generics as Type Classes

Description

Rust's type system is designed more like functional languages (like Haskell) rather than imperative languages (like Java and C++). As a result, Rust can turn many kinds of programming problems into "static typing" problems. This is one of the biggest wins of choosing a functional language, and is critical to many of Rust's compile time guarantees.

A key part of this idea is the way generic types work. In C++ and Java, for example, generic types are a meta-programming construct for the compiler. vector<int> and vector<char> in C++ are just two different copies of the same boilerplate code for a vector type (known as a template) with two different types filled in.

In Rust, a generic type parameter creates what is known in functional languages as a "type class constraint", and each different parameter filled in by an end user actually changes the type. In other words, Vec<isize> and Vec<char> are two different types, which are recognized as distinct by all parts of the type system.

This is called monomorphization, where different types are created from polymorphic code. This special behavior requires impl blocks to specify generic parameters. Different values for the generic type cause different types, and different types can have different impl blocks.

In object-oriented languages, classes can inherit behavior from their parents. However, this allows the attachment of not only additional behavior to particular members of a type class, but extra behavior as well.

The nearest equivalent is the runtime polymorphism in Javascript and Python, where new members can be added to objects willy-nilly by any constructor. However, unlike those languages, all of Rust's additional methods can be type checked when they are used, because their generics are statically defined. That makes them more usable while remaining safe.

Example

Suppose you are designing a storage server for a series of lab machines. Because of the software involved, there are two different protocols you need to support: BOOTP (for PXE network boot), and NFS (for remote mount storage).

Your goal is to have one program, written in Rust, which can handle both of them. It will have protocol handlers and listen for both kinds of requests. The main application logic will then allow a lab administrator to configure storage and security controls for the actual files.

The requests from machines in the lab for files contain the same basic information, no matter what protocol they came from: an authentication method, and a file name to retrieve. A straightforward implementation would look something like this:


enum AuthInfo {
    Nfs(crate::nfs::AuthInfo),
    Bootp(crate::bootp::AuthInfo),
}

struct FileDownloadRequest {
    file_name: PathBuf,
    authentication: AuthInfo,
}

This design might work well enough. But now suppose you needed to support adding metadata that was protocol specific. For example, with NFS, you wanted to determine what their mount point was in order to enforce additional security rules.

The way the current struct is designed leaves the protocol decision until runtime. That means any method that applies to one protocol and not the other requires the programmer to do a runtime check.

Here is how getting an NFS mount point would look:

struct FileDownloadRequest {
    file_name: PathBuf,
    authentication: AuthInfo,
    mount_point: Option<PathBuf>,
}

impl FileDownloadRequest {
    // ... other methods ...

    /// Gets an NFS mount point if this is an NFS request. Otherwise,
    /// return None.
    pub fn mount_point(&self) -> Option<&Path> {
        self.mount_point.as_ref()
    }
}

Every caller of mount_point() must check for None and write code to handle it. This is true even if they know only NFS requests are ever used in a given code path!

It would be far more optimal to cause a compile-time error if the different request types were confused. After all, the entire path of the user's code, including what functions from the library they use, will know whether a request is an NFS request or a BOOTP request.

In Rust, this is actually possible! The solution is to add a generic type in order to split the API.

Here is what that looks like:

use std::path::{Path, PathBuf};

mod nfs {
    #[derive(Clone)]
    pub(crate) struct AuthInfo(String); // NFS session management omitted
}

mod bootp {
    pub(crate) struct AuthInfo(); // no authentication in bootp
}

// private module, lest outside users invent their own protocol kinds!
mod proto_trait {
    use std::path::{Path, PathBuf};
    use super::{bootp, nfs};

    pub(crate) trait ProtoKind {
        type AuthInfo;
        fn auth_info(&self) -> Self::AuthInfo;
    }

    pub struct Nfs {
        auth: nfs::AuthInfo,
        mount_point: PathBuf,
    }

    impl Nfs {
        pub(crate) fn mount_point(&self) -> &Path {
            &self.mount_point
        }
    }

    impl ProtoKind for Nfs {
        type AuthInfo = nfs::AuthInfo;
        fn auth_info(&self) -> Self::AuthInfo {
            self.auth.clone()
        }
    }

    pub struct Bootp(); // no additional metadata

    impl ProtoKind for Bootp {
        type AuthInfo = bootp::AuthInfo;
        fn auth_info(&self) -> Self::AuthInfo {
            bootp::AuthInfo()
        }
    }
}

use proto_trait::ProtoKind; // keep internal to prevent impls
pub use proto_trait::{Nfs, Bootp}; // re-export so callers can see them

struct FileDownloadRequest<P: ProtoKind> {
    file_name: PathBuf,
    protocol: P,
}

// all common API parts go into a generic impl block
impl<P: ProtoKind> FileDownloadRequest<P> {
    fn file_path(&self) -> &Path {
        &self.file_name
    }

    fn auth_info(&self) -> P::AuthInfo {
        self.protocol.auth_info()
    }
}

// all protocol-specific impls go into their own block
impl FileDownloadRequest<Nfs> {
    fn mount_point(&self) -> &Path {
        self.protocol.mount_point()
    }
}

fn main() {
    // your code here
}

With this approach, if the user were to make a mistake and use the wrong type;

fn main() {
    let mut socket = crate::bootp::listen()?;
    while let Some(request) = socket.next_request()? {
        match request.mount_point().as_ref()
            "/secure" => socket.send("Access denied"),
            _ => {} // continue on...
        }
        // Rest of the code here
    }
}

They would get a syntax error. The type FileDownloadRequest<Bootp> does not implement mount_point(), only the type FileDownloadRequest<Nfs> does. And that is created by the NFS module, not the BOOTP module of course!

Advantages

First, it allows fields that are common to multiple states to be de-duplicated. By making the non-shared fields generic, they are implemented once.

Second, it makes the impl blocks easier to read, because they are broken down by state. Methods common to all states are typed once in one block, and methods unique to one state are in a separate block.

Both of these mean there are fewer lines of code, and they are better organized.

Disadvantages

This currently increases the size of the binary, due to the way monomorphization is implemented in the compiler. Hopefully the implementation will be able to improve in the future.

Alternatives

If a type seems to need a "split API" due to construction or partial initialization, consider the Builder Pattern instead.
If the API between types does not change -- only the behavior does -- then the Strategy Pattern is better used instead.

Rust Design Patterns