Matthieu Pizenberg (May 09 2024 at 13:11): Matthieu Pizenberg (May 09 2024 at 13:11):I have the following code:
shiftStr = Stdin.line!
shift <- Str.toI8 shiftStr |> Result.map
dbg shift
and the compiler doesn’t like it, giving me a long error about a type mismatch between something which is a Task ... and something which is a Result .... Maybe there is a section in the tutorial I’ve skipped that give an example similar to this I’m trying to do?
Brendan Hansknecht (May 09 2024 at 13:17):Your code is nested lambdas that return both tasks and results.
Brendan Hansknecht (May 09 2024 at 13:17):Result.map returns a result
Matthieu Pizenberg (May 09 2024 at 13:17):Do you have an example of how to read an integer from stdin?
Brendan Hansknecht (May 09 2024 at 13:18):Task.await (called by !) returns a task/requires a task
Brendan Hansknecht (May 09 2024 at 13:18):So they aren't capatible
Brendan Hansknecht (May 09 2024 at 13:21):https://www.roc-lang.org/packages/basic-cli/Task#fromResult should help
Brendan Hansknecht (May 09 2024 at 13:21):shift = Str.toI8 shiftStr |> Task.fromResult!
Brendan Hansknecht (May 09 2024 at 13:22):That will let you just accumulate the errors
Brendan Hansknecht (May 09 2024 at 13:22):Could also just use a raw when ... is on the result.
Matthieu Pizenberg (May 09 2024 at 13:25):Thanks ok. I was trying the exercise of having all in one indentation. This seems to work
shiftStr = Stdin.line!
shift <- Str.toI8 shiftStr |> Task.fromResult |> Task.await
Mainly to just not care about the error path
Matthieu Pizenberg (May 09 2024 at 13:27):This version works too
shiftStr = Stdin.line!
shift = (Str.toI8 shiftStr |> Task.fromResult)!
The parens are required in that last example? If so, pretty sure that is a bug in the ! desugaring
Matthieu Pizenberg (May 09 2024 at 16:51):yes the parens are required
Richard Feldman (May 09 2024 at 17:21):yeah should be able to do |> Task.fromResult! instead, sounds like a bug :thumbs_up:
Anton (May 10 2024 at 09:06):I would advise against mixing ! and <- (backpassing) in the same code unless you're very experienced with both. We may get rid of backpassing in favor of !.
Karakatiza (May 10 2024 at 11:37):Does anyone know why specifically the initial version doesn't compile? I thought syntactically it is valid?
Matthieu, are there any other statements after dbg in your original code? If not, maybe the body of the function is incomplete - you declare a backpassed argument, but then you might have no statements which I think is invalid syntax, you would atleast need to return something, e.g. that argument
Brendan Hansknecht (May 10 2024 at 12:40):Result.map returns a Result
Brendan Hansknecht (May 10 2024 at 12:40):Task.await requires an inner function that returns a Task
Brendan Hansknecht (May 10 2024 at 12:40):So it is a type mismatch
Ian McLerran (May 10 2024 at 14:51):Speaking of mixing back passing and bang syntax, I discovered a bug where using the bang operator in a when block causes a compiler panic, but back passing does not. Haven’t got a min repo yet though, so haven’t determined for sure that it’s related to the when.
Matthieu Pizenberg (May 10 2024 at 15:16):Karakatiza said:
Does anyone know why specifically the initial version doesn't compile? I thought syntactically it is valid?
Matthieu, are there any other statements after dbg in your original code? If not, maybe the body of the function is incomplete - you declare a backpassed argument, but then you might have no statements which I think is invalid syntax, you would atleast need to return something, e.g. that argument
yep, as Brendan says, it was a type mismatch between a Result and a Task
Last updated: Jul 26 2025 at 12:14 UTC