Jared Cone (Sep 29 2024 at 04:34): Jared Cone (Sep 29 2024 at 04:34):What's the difference between Task {} {}, Task {} [] and Task {} *? For the latter two, what would be the rust-equivalent type?
Luke Boswell (Sep 29 2024 at 04:46):Unit Type
Empty List
Anything -- in the return position there is nothing that can anything therefore this Task cannot fail.
Luke Boswell (Sep 29 2024 at 04:49):I think how the function can then used is where the difference is most significant. I'll try and make an example
Jared Cone (Sep 29 2024 at 04:52):Is there any difference between Unit vs Anything? For anything, would the Rust type still be RocResult<(), ()>?
Luke Boswell (Sep 29 2024 at 04:54):For example ... using [] this is concrete and won't unify with the [Exit _ _] type.
app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
main : Task {} [Exit I32 Str]
main =
foo!
Task.ok {}
foo : Task {} []
foo = Task.ok {}
── TYPE MISMATCH in main.roc ───────────────────────────────────────────────────
Something is off with the body of the main definition:
3│ main : Task {} [Exit I32 Str]
4│ main =
5│
6│ foo!
^^^^
This await call produces:
Task {} []
But the type annotation on main says it should be:
Task {} [Exit I32 Str]
To fix this we would need to return an open tag union like []_.
Luke Boswell (Sep 29 2024 at 04:54):So foo : Task {} []_ works
Luke Boswell (Sep 29 2024 at 04:56):Jared Cone said:
Is there any difference between Unit vs Anything? For anything, would the Rust type still be
RocResult<(), ()>?
You're specifically thinking about the interface between roc and the platform host here... This is currently generated as a RocResult and the * is represented in rust using ()
Luke Boswell (Sep 29 2024 at 04:57):Actually... you can't (shouldn't have) a * being passed to the host
Luke Boswell (Sep 29 2024 at 04:57):That will be a bug because it's size isn't fixed
Luke Boswell (Sep 29 2024 at 04:57):You want a concrete fixed sized type for things you pass across between roc and the host
Jared Cone (Sep 29 2024 at 05:01):Alrighty. Ignoring host interface then, what would be the functional difference in a function that returns a Task {} {} vs. a Task {} *? I was using Task {} {} to indicate a task that can't fail, but then I saw this in the tutorial:
Stdout.line has the type Task {} * because it doesn't produce any values when it finishes (hence the {}) and there aren't any errors that can happen when it runs (hence the *).
I suppose Task {} {} means a task can fail but there's nothing useful in the error, but how does Task {} * affect callers?
Luke Boswell (Sep 29 2024 at 05:07):It's about how the error type unifies with other things, this affects where and how you use the function.
Brendan Hansknecht (Sep 29 2024 at 05:25):Task {} {}: can succeed or fail but has no other information in either case.
Task {} []: empty tag for the failure case. This means it can not fail. Will unify with any other tag based errors (e.g. Task {} [SomeErr])
Task {} *: unconstrained type variable for the error case. This means it cannot fail. Can unify with anything.
Brendan Hansknecht (Sep 29 2024 at 05:27):Task {} {} and RocResult<(), ()> are equivalent. The other 2 are not actually safe to pass over the host boundary. Their exact implementation will change based on unification.
Jared Cone (Sep 29 2024 at 05:33):Would there be any reason to use Task {} [] instead of Task {} *? Like, any reason a designer would want their task to only unify with other tasks that have tag-based errors?
Luke Boswell (Sep 29 2024 at 05:37):Jared Cone said:
Would there be any reason to use
Task {} []instead ofTask {} *? Like, any reason a designer would want their task to only unify with other tasks that have tag-based errors?
I can't think of any reason why you would want to do that. I've used [] in the past because I didn't know the difference.
Brendan Hansknecht (Sep 29 2024 at 14:48):Only reason I can think of is to clarify that you expect a tag union for the error type. Like if all tasks use tag unions for errors and then a few have no error cases, I could see an argument for using [] over *. Especially given I think a number of people find an empty tag easier to understand than *.
Last updated: Jul 26 2025 at 12:14 UTC