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The Rust async story would be much nicer if they'd put in the hard work up front to support higher-kinded types, as then it could have a monadic async API like

The Rust async story would be much nicer if they'd put in the hard work up front to support higher-kinded types, as then it could have a monadic async API like OCaml or Haskell. I don't know anyone who's used async in both Rust and Haskell who prefers the Rust approach. It'd also fix oddities like why it's possible to write a function like the following in C++ but not Rust: template<template<typename> class MyContainer> int getFirstInt(const MyContainer<int>& myContainer) { return myContainer[0]; } (In Rust it's not possible to write something like MyContainer<int>; only the int is allowed to be generic).

chrismorgan 3y ago
Are you aware of generic associated types, which landed in 1.65 (late last year)? To a considerable extent, they’re Rust’s answer to higher-kinded types. Less expressive in some ways, but fitting Rust way better, as basically a relaxation of a former restriction, rather than a new feature. If I’m reading your snippet right, GATs let you express exactly that, though minus `template`’s duck-typiness so you’d have to spell out what contract the container must adhere to.
- logicchains 3y ago
  >less expressive in some ways, but fitting Rust way better, as basically a relaxation of a former restriction, rather than a new feature They're not expressive enough to implement a monadic async library, as far as I'm aware, so they don't fulfill the role of proper HKTs in this context. >minus `template`’s duck-typiness so you’d have to spell out what contract the container must adhere to. Do you think it's consistent that the T in MyContainer<T> can be "duck typed" but not the MyContainer?
  - estebank 3y ago
    In Rust neither is duck typed. For T you must specify the traits it must have. This makes compilation fail at definition, not at the point of incorrect usage. For an example of GATs, you can see the following: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=0691091fe290a60f9f13066f7299c000 https://play.rust-lang.org/?version=stable&mode=debug&editio...
  - zozbot234 3y ago
    > They're not expressive enough to implement a monadic async library, as far as I'm aware AIUI you need GC in order to do that. There's a reason why Rust's async/await support needs compiler magic. (I'd like to see standard interfaces for "pluggable" garbage-collected arenas in Rust, but this will need to wait until after Allocators/Storages get fully stabilized.)

chrismorgan 3y ago

Are you aware of generic associated types, which landed in 1.65 (late last year)? To a considerable extent, they’re Rust’s answer to higher-kinded types. Less expressive in some ways, but fitting Rust way better, as basically a relaxation of a former restriction, rather than a new feature. If I’m reading your snippet right, GATs let you express exactly that, though minus `template`’s duck-typiness so you’d have to spell out what contract the container must adhere to.

logicchains 3y ago
>less expressive in some ways, but fitting Rust way better, as basically a relaxation of a former restriction, rather than a new feature They're not expressive enough to implement a monadic async library, as far as I'm aware, so they don't fulfill the role of proper HKTs in this context. >minus `template`’s duck-typiness so you’d have to spell out what contract the container must adhere to. Do you think it's consistent that the T in MyContainer<T> can be "duck typed" but not the MyContainer?
- estebank 3y ago
  In Rust neither is duck typed. For T you must specify the traits it must have. This makes compilation fail at definition, not at the point of incorrect usage. For an example of GATs, you can see the following: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=0691091fe290a60f9f13066f7299c000 https://play.rust-lang.org/?version=stable&mode=debug&editio...
- zozbot234 3y ago
  > They're not expressive enough to implement a monadic async library, as far as I'm aware AIUI you need GC in order to do that. There's a reason why Rust's async/await support needs compiler magic. (I'd like to see standard interfaces for "pluggable" garbage-collected arenas in Rust, but this will need to wait until after Allocators/Storages get fully stabilized.)