Aurélien Geron (Oct 19 2024 at 02:21): Aurélien Geron (Oct 19 2024 at 02:21):Hi folks,
Suppose you have a type alias for a tag union like this:
Colors : [Red, Green, Blue]
And now you want to define another type alias for a tag union containing more tags, like this:
MoreColors : [Red, Green, Blue, Yellow, Purple]
I'd like a way to define this new tag union using the first one.
I couldn't find that in the docs, and I tried several syntaxes that all failed, so I'm guessing it's not possible today. I'd like something like this (possibly with a different syntax):
MoreColors : Colors + [Yellow, Purple]
This would be particularly useful when several functions return mostly the same errors, with minor differences here and there. In fact, it might be useful to remove tags from a tag union (but that's probably not as frequently needed):
LessColors : Colors - [Red]
in theory, this should work:
MoreColors : [Yellow, Purple]Colors
Thanks Richard. However, the following program is giving me some errors:
app [main] {
pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br",
}
import pf.Stdout
Colors : [Red, Green, Blue]
MoreColors : [Yellow, Purple]Colors
numToTag : U64 -> MoreColors
numToTag = \num ->
when num is
0 -> Red
1 -> Green
2 -> Blue
3 -> Yellow
_ -> Purple
main =
numToTag 3 |> Inspect.toStr |> Stdout.line!
The errors are:
── TYPE MISMATCH in tag-union-union.roc ────────────────────────────────────────
Something is off with the body of the numToTag definition:
10│ numToTag : U64 -> MoreColors
11│ numToTag = \num ->
12│> when num is
13│> 0 -> Red
14│> 1 -> Green
15│> 2 -> Blue
16│> 3 -> Yellow
17│> _ -> Purple
This when expression produces:
[
Blue,
Green,
Purple,
Red,
Yellow,
]
But the type annotation on numToTag says it should be:
MoreColors
── INVALID_EXTENSION_TYPE in tag-union-union.roc ───────────────────────────────
This tag union extension type is invalid:
8│ MoreColors : [Yellow, Purple]Colors
^^^^^^
Note: A tag union extension variable can only contain a type variable
or another tag union.
── UNDERSCORE NOT ALLOWED HERE in tag-union-union.roc ──────────────────────────
The definition of MoreColors includes an inferred (_) type:
1│ app [main] {
Type alias definitions may not use inferred types (_).
────────────────────────────────────────────────────────────────────────────────
3 errors and 0 warnings found in 30 ms
.
You can run the program anyway with roc run tag-union-union.roc
yeah I thought I remembered there being a longstanding bug in the implementation there :sweat_smile:
Richard Feldman (Oct 19 2024 at 03:14):I think there may be an issue for it
Aurélien Geron (Oct 19 2024 at 03:14):Haha, no worries, it's good to know that it's at least supposed to work, there's no emergency
Sam Mohr (Oct 19 2024 at 04:52):You know what might help with this? That .. syntax suite:
MoreColors : [Yellow, Purple, ..(Colors typeVar)]
We could even allow multiple extensions, that would help keep flatter unions if desired.
Aurélien Geron (Oct 21 2024 at 00:28):Nice! What's the typeVar for here? Would Yellow, Purple, ..Colors] work as well?
Sam Mohr (Oct 21 2024 at 03:11):I'd say yes
Last updated: Jun 16 2026 at 16:19 UTC