• @soulsource
    43 months ago

    I wouldn’t call Monads a “language feature” of Rust.

    There are some types in the standard library that have Monad properties (Option, Result, Vec,…), but there is no Monad trait that would allow to be generic over them.

    There have been attempts to introduce such a trait, the most well known probably being the higher crate. However, even with that crate, it’s not easy to work with Monads.

    For instance, in the stable tool-chain, it’s currently (to my knowledge) not possible to express a Free Monad without using macros. If you care for the details, I’ve written a blog post about my Adventures with Free Monads and higher. With the Nightly toolchain it is possible to write a generic Free Monad, by the way, thanks to non-lifetime-binders.

    Other Monads, like State, Reader and Writer, are also challenging to implement in Rust. I did get them to work, however I failed to implement Applicative for them, so, while they are mathematically Monads, they do not have higher’s Monad trait. Here a possible future improvement of non-lifetime-binders could help. Again, a blog post: Writer and State Monad in Rust, based on higher.

    Oh, and last, but not least, do-notation in Rust is a bit inconvenient, because of lifetime rules and lambda captures. For instance, using non-copy types is messy. I’ve added explicit clone support to higher, but that’s a crutch, and overly verbose.

    • @haskman@programming.devOP
      33 months ago

      Yes, my thoughts exactly.

      This problem is not solved by monads, but by higher kinded types in general in languages like Haskell. They give you a uniform way to be generic over effects like async (Async<A>) vs sync (Identity<A>). Both of these can be treated as (F<A>) for all A. So a generic Into would look like the following, and no special syntax or semantics would be needed. The type system (if sound) would prevent you from misusing a trait like this.

      trait Into<F,T> {
         def into(self): F<T>;