Stream: beginners

Topic: ✔ should tag "homonyms"(?) be allowed?

jan kili (Mar 26 2025 at 19:38):

I don't know the proper term(s) for this situation, but it seems that two tags with the same name but different payload types can't interact with each other. Is this a feature or a bug?

expect Foo == Foo           # true
expect Foo(Bar) != Foo(Baz) # true, because payloads can unify to [Bar, Baz]
expect Foo(Bar) != Foo      # fails type checking, so does Foo("Bar") etc

── TYPE MISMATCH in repro.roc ──────────────────────────────────────────────────

This 2nd argument to != has an unexpected type:

10│  expect Foo(Bar) != Foo
                        ^^^

This Foo tag has the type:

    [Foo]

But != needs its 2nd argument to be:

    [Foo [Bar]]a

────────────────────────────────────────────────────────────────────────────────

1 error and 0 warnings found in 31 ms

jan kili (Mar 26 2025 at 19:44):

The simplest workaround seems to be to wrap any incompatible payload types in arbitrary tags, like

Foo(A(123))
Foo(B("4"))
Foo(Empty)

Brendan Hansknecht (Mar 26 2025 at 22:40):

Technically we could allow it, but seems more error prone and confusing if the same union could have Foo and Foo a in it.

Brendan Hansknecht (Mar 26 2025 at 22:41):

That said, it is important to allow Foo and Foo a to exist in separate tag unions. Cause that can easily happen if two libraries just happen to use the same tag name.

Brendan Hansknecht (Mar 26 2025 at 22:42):

I'm really not sure about a use case, but I do get that from a theoretical perspective it could be allowed.

Richard Feldman (Mar 26 2025 at 22:47):

this is the correct behavior

Richard Feldman (Mar 26 2025 at 22:48):

I thought through this scenario years ago and concluded it would break things if we did it the other way, although I don't remember the specific reason anymore and would have to reconstruct the reasoning :sweat_smile:

jan kili (Mar 27 2025 at 00:21):

Brendan Hansknecht said:

That said, it is important to allow Foo and Foo a to exist in separate tag unions.

:check:
Yup, this works - they just can't interact.

jan kili (Mar 27 2025 at 00:22):

Richard Feldman said:

this is the correct behavior

Okie dokie!

Notification Bot (Mar 27 2025 at 00:22):

JanCVanB has marked this topic as resolved.

Last updated: Jul 26 2025 at 12:14 UTC