jan kili (Jan 02 2025 at 15:26): jan kili (Jan 02 2025 at 15:26):(This might be specific to basic-cli@0.18.0.) I have a function that returns a custom Result, and during its construction I want to use Stdout.line! for info. However, this causes a type mismatch where the Result the block returns conflicts with the Result that line! returns, with no obvious way to deconflict them. What are some small/large changes I can make to this minimal reproduction to get it to compile?
app [main!] {
basic_cli: platform "https://github.com/roc-lang/basic-cli/releases/download/0.18.0/0APbwVN1_p1mJ96tXjaoiUCr8NBGamr8G8Ac_DrXR-o.tar.br",
}
import basic_cli.Stdout
main! = \_ ->
s = try get_string! {}
Stdout.line! s
get_string! : {} => Result Str _
get_string! = \_ ->
Str.fromUtf8 [100]
|> \result ->
when result is
Err e ->
when e is
BadUtf8 foo bar ->
try Stdout.line! "BadUtf8: $(Inspect.toStr foo) $(Inspect.toStr bar)"
Err "custom"
Ok s ->
Ok s
jan@framey:~/_code/JanCVanB/bug-repros/mixed-errs$ roc check
── TYPE MISMATCH in main.roc ───────────────────────────────────────────────────
This returns something that's incompatible with the return type of the
enclosing function:
16│ when result is
17│ Err e ->
18│ when e is
19│ BadUtf8 foo bar ->
20│> try Stdout.line! "BadUtf8: $(Inspect.toStr foo) $(Inspect.toStr bar)"
21│ Err "custom"
22│
23│ Ok s ->
24│ Ok s
This returns an Err of type:
[Err [StdoutErr [
AlreadyExists,
BrokenPipe,
Interrupted,
NotFound,
Other Str,
OutOfMemory,
PermissionDenied,
Unsupported,
]], …]
But I expected the function to have return type:
[Err Str, …]
────────────────────────────────────────────────────────────────────────────────
1 error and 0 warnings found in 28 ms.
jan@framey:~/_code/JanCVanB/bug-repros/mixed-errs$
You need to use a tag error
Brendan Hansknecht (Jan 02 2025 at 15:34):a Str error can not merge with the tag error being returned from Stdout.line!
Brendan Hansknecht (Jan 02 2025 at 15:34):That or you need to map the Stdout.line! error into a Str
jan kili (Jan 02 2025 at 15:35):Ah! Yes, Err (Custom "custom") resolves the type mismatch.
jan kili (Jan 02 2025 at 15:36):Perhaps I need a different minimal reproduction for this that I'm hitting in my real app:
This returns an `Err` of type:
[Err [StderrErr [
AlreadyExists,
BrokenPipe,
Interrupted,
NotFound,
Other Str,
OutOfMemory,
PermissionDenied,
Unsupported,
]], …]
But I expected the function to have return type:
[Err [
FileReadErr * *,
FileReadUtf8Err * *,
], …]d
because I assumed those two tag unions
( [StderrErr ...] and [FileReadErr ..., FileReadUtf8Err ...] )
would merge.
jan kili (Jan 02 2025 at 15:38):Real app function:
read_file! : PathToFile => Result Str _
read_file! = \file_path ->
file_path
|> Path.read_utf8!
|> \result ->
when result is
Err e ->
when e is
FileReadErr path io_err ->
try Stderr.line! "📰 ❗ Failed to read file $(Path.display path) because $(Inspect.toStr io_err)"
try Stderr.line! tip_for_calling_this_app
Err e
FileReadUtf8Err path io_err ->
try Stderr.line! "📰 ❗ Failed to read file $(Path.display path) because $(Inspect.toStr io_err)"
Err e
Ok file_contents ->
Ok file_contents
jan@framey:~/_code/JanCVanB/roc-typesetter$ roc check
── TYPE MISMATCH in main.roc ───────────────────────────────────────────────────
This returns something that's incompatible with the return type of the
enclosing function:
50│ when result is
51│ Err e ->
52│ when e is
53│ FileReadErr path io_err ->
54│> try Stderr.line! "📰 ❗ Failed to read file $(Path.display path) because $(Inspect.toStr io_err)"
55│ try Stderr.line! tip_for_calling_this_app
56│ Err e
57│
58│ FileReadUtf8Err path io_err ->
59│ try Stderr.line! "📰 ❗ Failed to read file $(Path.display path) because $(Inspect.toStr io_err)"
60│ Err e
61│
62│ Ok file_contents ->
63│ Ok file_contents
This returns an Err of type:
[Err [StderrErr [
AlreadyExists,
BrokenPipe,
Interrupted,
NotFound,
Other Str,
OutOfMemory,
PermissionDenied,
Unsupported,
]], …]
But I expected the function to have return type:
[Err [
FileReadErr Path.Path val,
FileReadUtf8Err Path.Path val,
], …] where val implements Inspect, val implements Inspect
────────────────────────────────────────────────────────────────────────────────
1 error and 0 warnings found in 32 ms.
jan@framey:~/_code/JanCVanB/roc-typesetter$
app [main!] {
basic_cli: platform "https://github.com/roc-lang/basic-cli/releases/download/0.18.0/0APbwVN1_p1mJ96tXjaoiUCr8NBGamr8G8Ac_DrXR-o.tar.br",
}
import basic_cli.Stdout
main! = \_ ->
s = try get_string! {}
Stdout.line! s
get_string! : {} => Result Str _
get_string! = \_ ->
Str.fromUtf8 [100]
|> \result ->
when result is
Err e ->
when e is
BadUtf8 foo bar ->
try Stdout.line! "BadUtf8: $(Inspect.toStr foo) $(Inspect.toStr bar)"
Err e
Ok s ->
Ok s
jan@framey:~/_code/JanCVanB/bug-repros/mixed-errs-2$ roc check
── TYPE MISMATCH in main.roc ───────────────────────────────────────────────────
This returns something that's incompatible with the return type of the
enclosing function:
16│ when result is
17│ Err e ->
18│ when e is
19│ BadUtf8 foo bar ->
20│> try Stdout.line! "BadUtf8: $(Inspect.toStr foo) $(Inspect.toStr bar)"
21│ Err e
22│
23│ Ok s ->
24│ Ok s
This returns an Err of type:
[Err [StdoutErr [
AlreadyExists,
BrokenPipe,
Interrupted,
NotFound,
Other Str,
OutOfMemory,
PermissionDenied,
Unsupported,
]], …]
But I expected the function to have return type:
[Err [BadUtf8 val val], …] where val implements Inspect, val implements Inspect
────────────────────────────────────────────────────────────────────────────────
1 error and 0 warnings found in 25 ms.
jan@framey:~/_code/JanCVanB/bug-repros/mixed-errs-2$
Yeah, this is a more nuanced issue
Brendan Hansknecht (Jan 02 2025 at 15:46):Essentially to be used in the when e is block, e must be either FileReadErr path io_err or FileReadUtf8Err path io_err
Brendan Hansknecht (Jan 02 2025 at 15:47):This is a closed tag due to the when
Brendan Hansknecht (Jan 02 2025 at 15:47):e is then returned
Brendan Hansknecht (Jan 02 2025 at 15:47):Suddenly, that closed tag is being merged with [StderrErr ...] due to the try StdErr.line
jan kili (Jan 02 2025 at 15:48):Ayyy, nice, so adding _ -> Err e seems to fix it.
Brendan Hansknecht (Jan 02 2025 at 15:49):I think you could add a _ -> crash "unreachable" to your when e is to fix it.
That or instead of returning e, you need to return a new error.
FileReadErr path io_err ->
try Stderr.line! "📰 ❗ Failed to read file $(Path.display path) because $(Inspect.toStr io_err)"
try Stderr.line! tip_for_calling_this_app
- Err e
+ Err (FileReadErr path io_err)
Ooh that's nicer - especially since in real app functions this pattern is usually combined with error customization
Brendan Hansknecht (Jan 02 2025 at 15:50):This is an interesting sharp edge try/? when combined with when err is
jan kili (Jan 02 2025 at 15:51):Yeah, because I'm in a function that returns a Result, when I see the typechecker message say "this function returns" I don't think "oh yeah when is a function that returns a sometimes-closed union"
jan kili (Jan 02 2025 at 15:52):I read those typechecker messages as referring to the enclosing function's Result Err, but they're not.
Notification Bot (Jan 02 2025 at 15:53):JanCVanB has marked this topic as resolved.
jan kili (Jan 02 2025 at 15:54):I'm happy to continue discussing how to un-sharpen that edge here or elsewhere.
jan kili (Jan 02 2025 at 15:55):Side note - where in the typechecker messages does it indicate a closed tag union? I read all those ...]-ending types as being open.
Brendan Hansknecht (Jan 02 2025 at 15:58):I think this is the only open union cause it is followed by d:
[Err [
FileReadErr * *,
FileReadUtf8Err * *,
], …]d
That said, any union returned by a function is automatically made open (solves most headaches from open vs closed unions)
Brendan Hansknecht (Jan 02 2025 at 15:59):Anyway, sounds like we should at least improve a few error messages here.
jan kili (Jan 02 2025 at 16:00):What does ...] indicate?
Brendan Hansknecht (Jan 02 2025 at 16:00):That it left out information cause it was unimportant for the diff
Brendan Hansknecht (Jan 02 2025 at 16:00):In this case, it elided the Ok cases
jan kili (Jan 02 2025 at 16:01):Oh, gotcha. Yeah that's outside the Err tag union.
Brendan Hansknecht (Jan 02 2025 at 16:01):Eventually we plan to use ... for open unions (and combining unions with ...OtherUnions), but we don't do that yet.
jan kili (Jan 02 2025 at 16:02):Wow, so none of the above Err tags are open, then.
jan kili (Jan 02 2025 at 16:03):That makes sense. I think I was biased toward confusion here because I remember seeing that ...] is the upcoming openness syntax.
jan kili (Jan 02 2025 at 16:04):I should've counted my matching [ ]s more closely!
Brendan Hansknecht (Jan 02 2025 at 16:35):Wow, so none of the above Err tags are open, then
Except that all tags returned from functions are automatically made open. So it is kinda confusing.
Last updated: Jul 26 2025 at 12:14 UTC