Stream: beginners

Topic: combining multiple error rows

Gabriel Pickl (Dec 05 2022 at 13:45):

Let's say I have a function with the signature Result a aError, Result a bError -> Result a [DidNotMatch]???
How can I extend the possible errors by with both aError and bError? (This is a reduced example from a parser combinator thing I'm trying)

Richard Feldman (Dec 05 2022 at 14:52):

I haven't tried it, but I'd expect this to work:

Result a err, Result a err -> Result a [DidNotMatch]err

Richard Feldman (Dec 05 2022 at 14:53):

if you give it two Results with different error types, they should get unioned

Gabriel Pickl (Dec 05 2022 at 14:58):

aah, did not think of that, but that makes sense. thank you :)

Gabriel Pickl (Dec 05 2022 at 16:06):

Hmm, this does not quite work. When I write the following:

hardcoded : Str, (Str -> Parser b err) -> Parser b [ExpectedStr Str]err
hardcoded = \value, callback ->
    @Parser \input ->
        if Str.startsWith input value then
            when Str.splitFirst input value is
                Ok {after} ->
                    @Parser next = callback value
                    next after
                Err NotFound -> crash "Did not find item that was just found"
        else
            ParseFailed (ExpectedStr value)

I get an error message that [ParseFailed [ExpectedStr Str], Parsed b Str] and [ParseFailed err, Parsed b Str] cannot be unified. If I instead use the signature

hardcoded : Str, (Str -> Parser b [ExpectedStr Str]err) -> Parser b [ExpectedStr Str]err
hardcoded = \value, callback ->
    @Parser \input ->
        if Str.startsWith input value then
            when Str.splitFirst input value is
                Ok {after} ->
                    @Parser next = callback value
                    next after
                Err NotFound -> crash "Did not find item that was just found"
        else
            ParseFailed (ExpectedStr value)

this works, but seems wrong, since I am requesting that the callback might also return ExpectedStr Str and I'm doubling up the type in the type signature

Gabriel Pickl (Dec 05 2022 at 16:07):

(the signature of Parser is Parser a err := Str -> [Parsed a Str, ParseFailed err] right now)

Last updated: Jul 26 2025 at 12:14 UTC