Stream: beginners

Topic: Task event []*

view this post on Zulip Ghislain (Oct 12 2022 at 16:14):

In this example:

main =
    Task.attempt processTask \p ->
        when p is
            Ok s -> Stdout.line s
            Err ReadErr -> Stdout.line "Tried to read"
            Err WriteErr -> Stdout.line "Tried to write"

    |> Program.quick

processTask : Task.Task Str [ReadErr, WriteErr] []

Compiler output says:

This 2nd argument to attempt has an unexpected type:
...
The argument is an anonymous function of type:

    [Err [ReadErr, WriteErr], Ok Str] ->
    Task {} * [Write [Stdout]*]* ?

But attempt needs its 2nd argument to be:

    Result Str [ReadErr, WriteErr] -> Task {} * [] ?

Let's say processTask doesn't produce any event, why the compiler tells me to handle others by putting []*?

view this post on Zulip Ayaz Hafiz (Oct 12 2022 at 16:26):

This is due to a quirk of how type checking in Roc works. We're working on making it better! But for now, in this case, [] means it can be used in only a place where a [] is expected, and nothing more than a []. So you add []* to say "you can use this as a [], or any tag that has more variants than []"

view this post on Zulip Ghislain (Oct 12 2022 at 16:32):

Ok, thank you!

Last updated: Jul 26 2025 at 12:14 UTC