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Claudia Meadows (dead-claudia)

opened on Sep 6, 2017

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Edit: keyof works as I thought; mapped types are just broken in interfaces. I also clarified a few things.

Allowing negated types and F-bounded mapped type variables would solve several issues:

• Subtraction types: Subtraction types #4183 - The proposed A - B could be encoded as A & !B as per this proposal.
• Mapped conditional types: Mapped conditional types #12424 - For key-based conditions, this + F-bounded mapped types would allow that to be solved without explicit support.
• Promises - Having negated types makes it possible to prevent thenables from being used as generic Promise parameters.

Here's my proposal:

1. Allow negated types, using !T to represent the set of all types that do not conform to T (i.e. not assignable to T). This is pretty simple to digest.

2. Expand mapped types to allow the key variable to be F-bounded, like in {[P in Keys<T, P>]: ...}.

The first is sufficient to cover subtraction types and Promise type issues, and the other two make mapped conditional types possible.

Reacted by Joe Calzaretta, Alex, Shai Reznik, Andrey Godyaev, Stephen Laughton, Alexis Fontaine and Troy Gerwien

Reacted by Aluan Haddad, ezequiel-umu, Remo H. Jansen, Sean M. Vieira and Stephen Laughton

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1. aluanhaddad commented on Sep 6, 2017

   

   Aluan Haddad (aluanhaddad)

   on Sep 6, 2017

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   Big 👍 on the proposed syntax. I think the negation operator is indeed quite intuitive in the context of mapped types/keyof and is a great idea. My only concern is that I'm not sure what it means outside of an intersection context. "Does not conform to T" could be construed as meaning not assignable to T which is very different from meaning not having any of the keys of T. This may just need further clarification.

   Minor nitpick, but I believe your second item is already valid today.
2. dead-claudia commented on Sep 7, 2017

   

   Claudia Meadows (dead-claudia)

   on Sep 7, 2017

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   Aluan Haddad (@aluanhaddad)

   My only concern is that I'm not sure what it means outside of an intersection context.

   This comment should explain type negation outside a union context (partial Promise fix).

   "Does not conform to T" could be construed as meaning not assignable to T which is very different from meaning not having any of the keys of T.

   "not assignable to T" is actually the intended meaning for negated types. This happens to be equivalent to "not having any of the keys of T" in the context of object types, but it carries more expected semantics for primitives.

   Minor nitpick, but I believe your second item is already valid today.

   Yeah...I figured this out; I just ran into this odd bug.
3. Claudia Meadows (dead-claudia)

   changed the title [-]Allow negated types + expand `keyof` to accept any type[/-] [+]Allow negated types + F-bounded mapped types[/+] on Sep 7, 2017
4. SalathielGenese commented on Mar 9, 2018

   

   Salathiel Genese (SalathielGenese)

   on Mar 9, 2018

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   I upvote A & !B especially the !B part... Over Exclude from #21847
5. mhegazy commented on Mar 9, 2018

   

   Mohamed Hegazy (mhegazy)

   on Mar 9, 2018

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   Looks like the same feature as #4196, just different proposal. i suggests adding the proposal to #4196 and keeping the discussion to one issue.
6. Mohamed Hegazy (mhegazy)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Mar 9, 2018
7. typescript-bot commented on Mar 23, 2018

   

   TypeScript Bot (typescript-bot)

   on Mar 23, 2018

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   Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.
8. TypeScript Bot (typescript-bot)

   closed this as completedon Mar 23, 2018
9. microsoft

   locked and limited conversation to collaborators on Jul 25, 2018

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