feat: add metadata support to Tagged type #723
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Friendly bump in case you forgot. If you're just busy, feel free to ignore this comment. |
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@sindresorhus thanks! I think I can tackle this this week :) |
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@sindresorhus Ok, I've updated this PR to rebase it on top of main and to add more docs that better explain the use cases for metadata and cover some of the details around assignability! |
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source/opaque.d.ts
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| @category Type | ||
| */ | ||
| export type Tagged<Type, Tag extends PropertyKey, TagMetadata = never> = Type & MultiTagContainer<Tag, TagMetadata>; |
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I ended up making a tag's default metadata never instead of void, as never has the more consistent/sensible (covariant) assignability semantics that we discussed in the other issue.
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| type MultiTagContainer<Token extends PropertyKey> = { | ||
| readonly [tag]: {[K in Token]: void}; | ||
| }; | ||
| type Tag<Token extends PropertyKey, TagMetadata> = TagContainer<{[K in Token]: TagMetadata}>; |
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In addition to adding the TagMetadata param, I decided to rename MultiTagContainer to Tag because it makes the IDE popups much clearer for tagged types. E.g. in these types:
type AbsolutePath = Tagged<string, 'AbsolutePath'>;
type NormalizedPath = Tagged<string, 'NormalizedPath'>;
type NormalizedAbsolutePath = AbsolutePath & NormalizedPath;Hovering over NormalizedAbsolutePath will now show
string & Tag<'AbsolutePath', never> & Tag<'NormalizedPath', never>instead of
string & MultiTagContainer<'AbsolutePath', never> & MultiTagContainer<'NormalizedPath', never>
| // UnwrapTagged/UnwrapOpaque should work on types with multiple tags. | ||
| const unwrapped3 = '' as UnwrapTagged<NormalizedAbsolutePath>; | ||
| const unwrapped4 = '' as UnwrapOpaque<NormalizedAbsolutePath>; | ||
| expectType<string>(unwrapped3); | ||
| expectType<string>(unwrapped4); |
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these are new tests that i added because i realized that i accidentally broke UnwrapTagged without them in an earlier version of this PR
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Thanks for writing this. Looks good 👍 |
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Thanks for merging! 💯🎉 |
Addition to support the use case described here #665 (comment)