Stream: beginners

Topic: ✔ Reading file and handling errors

Seth Workman (Dec 07 2023 at 21:42):

I'm confused about how to handle errors here as I'm getting a compiler error

run =
   part1 <-
      Path.fromStr "day1_input.txt"
         |> File.readUtf8
         |> Task.await
   Stdout.line "\(part1)"

handleErr = \err ->
    when err is
        FileReadErr -> Stderr.line "Error"
        FileReadUtf8Err -> Stderr.line "Error"
main =
   run |> Task.onErr handleErr

19│     |> Task.onErr handleErr
                      ^^^^^^^^^

This handleErr value is a:

    [
        FileReadErr,
        FileReadUtf8Err,
    ] -> Task {} *

But |> needs its 2nd argument to be:

    [
        FileReadErr Path.Path InternalFile.ReadErr,
        FileReadUtf8Err Path.Path [BadUtf8 Utf8ByteProblem Nat]*,
    ] -> Task {} *

Seth Workman (Dec 07 2023 at 21:43):

It looks like something like this would work

        FileReadErr _ _ -> Stderr.line "Error"
        FileReadUtf8Err _ _ -> Stderr.line "Error"

Seth Workman (Dec 07 2023 at 21:44):

Is there a better way to handle these errors?

Luke Boswell (Dec 07 2023 at 21:45):

Your very close, just when you pattern match on the error yags in handleErr you are not including the tag payloads.

Luke Boswell (Dec 07 2023 at 21:47):

If you havent seen it the https://www.roc-lang.org/examples/Tasks/README.html example might help, but I'm guessing from your code that you have as your using that same pattern.

Luke Boswell (Dec 07 2023 at 21:55):

I highly recommend adding a type annotation in situations like this, it can help debug the issue and by pinning the types at that point the compiler can help check your understanding. You can always blow it away once your done with it and have working code.

Seth Workman (Dec 07 2023 at 21:56):

I think I'm starting to get it a bit more now, thanks! I'll read that over again and see what else I can pick up

Notification Bot (Dec 07 2023 at 21:56):

Seth Workman has marked this topic as resolved.

Ryan Bates (Dec 07 2023 at 21:59):

@Seth Workman you can also import local files directly which doesn't require error handling. The compiler handles it.

app ...
    imports [
        "day1_input.txt" as input : Str,
        ...
    ]

Last updated: Jul 26 2025 at 12:14 UTC