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Allow negated types + F-bounded mapped types #18280
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Big 👍 on the proposed syntax. I think the negation operator is indeed quite intuitive in the context of mapped types/
keyofand is a great idea. My only concern is that I'm not sure what it means outside of an intersection context. "Does not conform toT" could be construed as meaning not assignable toTwhich is very different from meaning not having any of the keys ofT. This may just need further clarification.Minor nitpick, but I believe your second item is already valid today.
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 toTwhich is very different from meaning not having any of the keys ofT."not assignable to
T" is actually the intended meaning for negated types. This happens to be equivalent to "not having any of the keys ofT" 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.
- changed the title
[-]Allow negated types + expand `keyof` to accept any type[/-][+]Allow negated types + F-bounded mapped types[/+]on Sep 7, 2017 I upvote
A & !Bespecially the!Bpart... OverExcludefrom #21847Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.
- locked and limited conversation to collaborators
on Jul 25, 2018
Edit:
keyofworks 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:
A - Bcould be encoded asA & !Bas per this proposal.Here's my proposal:
Allow negated types, using
!Tto represent the set of all types that do not conform toT(i.e. not assignable toT). This is pretty simple to digest.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.