Nested unions are "untagged"

[OliverJAsh]

I just ran into this bug which resulted in a runtime exception. 😢

import { unionize, ofType, UnionOf } from 'unionize';

const NestedAction = unionize({ Foo: {} }, { value: 'value' });
type NestedAction = UnionOf<typeof NestedAction>;

const Action = unionize({ Nested: ofType<NestedAction>() }, { value: 'value' });

declare const wrong: { value: {} };

// No error!!!
Action.Nested(wrong);

// Expected: (value: { tag: "Foo"; } & { value: {}; }) => …
// Actual: (value: Pick<{ tag: "Foo"; } & { value: {}; }, "value">) => …
// Why is `tag` omitted??
Action.Nested;

One known workaround is to force the nested union tag to be different:

-const NestedAction = unionize({ Foo: {} }, { value: 'value' });
+const NestedAction = unionize({ Foo: {} }, { tag: 'tag2', value: 'value' });

However that's a non-option when modelling Redux actions, since the tag must be type to conform to the usual Redux action type.

[OliverJAsh]

This change appears to fix it, but I'm probably breaking some other behaviour. Not sure what exactly because it seems there isn't a test for it.

 export declare type Creators<Record, TaggedRecord, TagProp extends string> = {
-  [T in keyof Record]: {} extends Required<UnTagged<Record[T], TagProp>>
+  [T in keyof Record]: {} extends Required<Record[T]>
     ? ((value?: {}) => TaggedRecord[keyof TaggedRecord])
-    : ((value: UnTagged<Record[T], TagProp>) => TaggedRecord[keyof TaggedRecord])
+    : ((value: Record[T]) => TaggedRecord[keyof TaggedRecord])

// error :-)
Action.Nested(wrong);
// no error :-)
Action.Nested(NestedAction.Foo({}));

Related:

• Don't flatten union-type values through UnTagged #39
• UnTagged was introduced in Allow including tag in value type #36. /cc @sledorze who introduced this and therefore might be able to think of a quick solution to this bug.

[OliverJAsh]

It seems UnTagged was added to allow the behaviour shown in this test:

https://github.com/sledorze/unionize/blob/9763034266f9b5aae61fcd93ecf8ea70efe7ad11/src/index.spec.ts#L94-L105

https://github.com/pelotom/unionize/pull/36/files#diff-6b623d6bcf5e7f06c466aa060ec9c4b6R94

However this test was removed recently for some reason: 87456d3

Furthermore, I just tried to revive the test and it seems to be throwing a type error.

So, where does this leave us with the issue I described originally?

Either:

• We still need UnTagged to allow for the behaviour shown in the test. If that's the case, we should 1) revive that test and 2) fix it because that test is now failing.
• We no longer need UnTagged, as we don't need the behaviour shown in the test. If that's the case, removing it will fix this issue.

@pelotom WDYT?

[pelotom]

I honestly cannot remember what UnTagged was for. Unless @sledorze wants to intercede and make a case for it, I'd say let's get rid of it.

[OliverJAsh]

#65