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Mappable tuples with constraints are unwieldy due to lack of covariance #27351
Description
TypeScript Version: 3.1.0-dev.20180925
Search Terms: mappable tuples constraints covariance
I'm trying to make use of the new "mappable tuple and array types" coming in 3.1, and discovered that it's a bit awkward to work with arrays constrained to a particular type.
Code
interface WithFoo<T = any> {
foo: T;
}
type SelectFoos<A extends WithFoo[]> = { [K in keyof A]: A[K]['foo'] };Expected behavior:
Should compile.
Actual behavior:
Error: Type '"foo"' cannot be used to index type 'A[K]'..
Notes
It's possible to work around this using a conditional:
type SelectFoos<A extends WithFoo[]> = {
[K in keyof A]: A[K] extends WithFoo ? A[K]['foo'] : never
};but it seems like it should just work as is.
Activity
RyanCavanaugh commentedon Oct 1, 2018
Anders Hejlsberg (@ahejlsberg) bug? feature request?
ahejlsberg commentedon Oct 9, 2018
The issue here is that even though a homomorphic mapped type only maps the element type(s) of the array or tuple,
keyof Astill includes the non-numeric properties ofArray<T>(i.e."length","concat", etc.) and thereforeA[keyof A](the constraint ofA[K]) is a large and fairly useless union type that can't be indexed with"foo".We considered having
keyof Tfor an array or tuple type not include non-numeric properties, but it would be a significant breaking change. For example,A["length"]wouldn't work anymore.Given where we're at, your conditional type workaround seems reasonable.
jcalz commentedon Jun 3, 2019
When mapping over the keys
Kin a typeTconstrained to an array type, is it possible to narrowKinstead ofTorkeyof T? Assuming there were a "numericString" type, the suggestion is to treatlike
That would possibly give the expected behavior here without pretending that
keyof Tis limited to the numeric keys, soT['length']would still exist. Just a passing thought...