MystPi (Oct 04 2024 at 16:51): MystPi (Oct 04 2024 at 16:51):As far as I can tell, you can't have a custom function to use with ! or ?, right? Any reason for this? I feel this feature would be so much more powerful if you could have a custom bind function. I wanted to create a monad-based parser package, and having a custom version of ! or ? would make consuming the API much better. Coming from Gleam and being used to its very powerful use feature (backpassing), I was kind of unpleasantly surprised that this wasn't already possible.
Anton (Oct 04 2024 at 17:48):We used to have backpassing, but we got rid of it because it was a source of confusion and it increased the Roc learning curve. We've been getting along great so far with just ! and ? :)
MystPi (Oct 05 2024 at 19:12):Personally, even though I was new to FP when learning Gleam, I was never confused about backpassing. It seemed quite logical since the desugaring rules are always the same.
MystPi (Oct 05 2024 at 19:15):Anway—back to my original q—will there be a feature enabling custom version of ! and/or ? in Roc, something akin to
with Parser.andThen do
Parser.token! ...
ident = parse_ident!
Parser.token! ...
Parser.return (SomeAst ident)
I actually kind of like that syntax that I just made up, haha
Brendan Hansknecht (Oct 05 2024 at 21:08):There are no such plans currently. Only task and result
Jared Cone (Oct 05 2024 at 21:08):Couldn't you just use tasks for that as-is?
Luke Boswell (Oct 05 2024 at 22:00):@MystPi have you seen the purity inference proposals?
Brendan Hansknecht (Oct 05 2024 at 22:20):I don't think these are really related to be clear.
What @MystPi wants is being able to chain no Task/Result types. For example generators, parsers, and the like. This is ident <- Parser.ident |> Parser.andThen in the backpassing world. So any Task or Result solution may not apply. That said, Task may apply if the parser is running Tasks. Most likely the parser is just over a Str, so no Task is wanted at all
Brendan Hansknecht (Oct 05 2024 at 22:21):This is a common monadic pattern
Brendan Hansknecht (Oct 05 2024 at 22:21):The closest we have today is record builders
Brendan Hansknecht (Oct 05 2024 at 22:21):Like what is done for weaver
Brendan Hansknecht (Oct 05 2024 at 22:21):Otherwise, this all has to be done manually
Brendan Hansknecht (Oct 05 2024 at 22:22):This also can be done pretty well with shadowing (or mutable variables), but is more verbose than implicit state thread like is done with monads.
Brendan Hansknecht (Oct 05 2024 at 22:23):If we get mutable varibles _ suffix, the parser could be:
state_ = ...
state_ = Parser.token state_ ...
state_, ident = parse_ident state_
state_ = Parser.token state_ ...
state_ = Parser.return state_ (SomeAst ident)
A lot more verbose and less nice than having arbitrary monadic chaining with ! or backpassing.
I think if we get enough demand for it, we may one day open up ! like is suggested above (or some other similar solution), but currently there has been minimal demand.
MystPi (Oct 06 2024 at 15:32):Got it—I see that Roc's syntax is still very much in the development stage, which I can def understand
MystPi (Oct 06 2024 at 15:33):Luke Boswell said:
MystPi have you seen the purity inference proposals?
No I haven't, though after reading (skimming) it I can't say I really get what it's about, haha. Not sure how it applies to my question either
Brendan Hansknecht (Oct 06 2024 at 16:04):I see that Roc's syntax is still very much in the development stage
Yeah, it has changed a lot over the last year. Generally speaking, we start with the more minimal version of a feature (eg ! just for task) before we expand to something more powerful (eg generic !)
Last updated: Jul 26 2025 at 12:14 UTC