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Mappable tuples with constraints are unwieldy due to lack of covariance #27351
Description
Activity
Anders Hejlsberg (@ahejlsberg) bug? feature request?
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.
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 treattype MapArray<T extends {x: string}[]> = {[K in keyof T]: T[K]['x']}
like
type MapArray<T extends {x: string}[]> = {[K in keyof T]: T[K & numericString]['x']
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...
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
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:
but it seems like it should just work as is.
Playground Link