Haskell-style monads with Rust syntax.
Rust requires >>= to be self-modifying and not to return a value, so >>= becomes >> and return (keyword) becomes consume.
At the moment, Haskell's >> seems unnecessary in an eager language like Rust, but I could easily be convinced otherwise! Please let me know if you'd like it implemented.
This typechecks, compiles, and runs without a hitch:
rust
use rsmonad::prelude::*;
fn successor(x: u8) -> Maybe<u8> {
x.checked_add(1).map_or(Nothing, Just)
}
assert_eq!(
Just(3) >> successor,
Just(4),
);
assert_eq!(
Nothing >> successor,
Nothing,
);
assert_eq!(
Just(255) >> successor,
Nothing,
);
This as well, if you choose to enable the non-no-std bits:
rust
fn afraid_of_circles(x: u8) -> BlastDoor<()> {
if x == 0 { panic!("aaaaaa!"); }
Phew(())
}
assert_eq!(
Phew(42) >> afraid_of_circles,
Phew(())
);
assert_eq!(
Phew(0) >> afraid_of_circles,
Kaboom,
);
The logic of Haskell lists with the speed of Rust vectors:
rust
// from the wonderful Haskell docs: https://en.wikibooks.org/wiki/Haskell/Understanding_monads/List
fn bunny(s: &str) -> List<&str> {
List(vec![s, s, s])
}
assert_eq!(
List::consume("bunny") >> bunny,
List(vec!["bunny", "bunny", "bunny"]),
);
assert_eq!(
List::consume("bunny") >> bunny >> bunny,
List(vec!["bunny", "bunny", "bunny", "bunny", "bunny", "bunny", "bunny", "bunny", "bunny"]),
);
And even the notoriously tricky join-in-terms-of-bind with no type annotations necessary:
rust
let li = List::consume(List::consume(0_u8)); // List<List<u8>>
let joined = join(li); // --> List<u8>!
assert_eq!(joined, List::consume(0_u8));
Plus, we automatically derive QuickCheck::Arbitrary and property-test the monad laws, inextricable from defining a Monad in the first place.
Right now, you can use >> as sugar for bind only when you have a concrete instance of Monad like Maybe but not a general <M: Monad<A>>.
The latter still works but requires an explicit call to m.bind(f) (or, if you don't use the trait, Monad::<A>::bind(m, f)).
This should be fixed with the Rust's non-lifetime binder feature when it rolls out.