type narrowing not working as expected

[buoyant mason]

its impossible for arrayValues to be UnknownArrayDimension if size is ValueResult, so why does typescript think that arrayValues can still be UnknownArrayDimension on line 36?

https://www.typescriptlang.org/play?#code/PTAEEsHMDsHsCcCmoAuALcBnAUCgngA7IAKsm4K4s0oAvKAN4C+uhyAagIbwAqbdjFviKguAGwCuiAEqJMEsSgENUbAFygA5OKmz5izaCH8AotAkBbAMop4EgMYo9CpfRXDEGzWcs27j5wMjVhEAVWgAazgAd2gdGTkXZVUiL3Co2Fj4wJRDYxEASWgAN04xcAATHOSPLyLS8qrEoPzkdJjoAEF4eE48ABFwC0Rocmoa9S12zK6evsHh0apoPOw1j1Bu3rxq2mxQA9AAH0Z9w-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-PxKY3oo75tiyKagecAB6QnnCwTBAA

also, is there a more "elegant" way to define that? without having to duplicate the type just to change 2 properties

[lean delta]

@buoyant mason Here's a shortened URL of your playground link! You can remove the full link from your message.

mis#0279

Playground Link

Preview:ts // ignore this type Position = {} type VarType = {} type ValueResult = { type: 'ValueResult' } type EnumStructResult = { type: 'EnumStructResult' } type UnknownValueResult = { type: 'UnknownValueResult' } type InvalidResult = { type: 'InvalidResult' } type UnknownArrayDimension = { type: 'UnknownArrayDimension' } ...

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[quaint crescent]

typescript does have discriminated unions (narrowing based on literal properties)...

... but the narrowing doesn't go up a level

and so narrowing happens on the union in the first variant's size, but it doesn't continue outwards to the outer union (directly under type ArrayResult)

[buoyant mason]

mmm ok, so what should i do then, i dont think i should use runtime assertions

i could use Array.isArray i guess, but its adding unnecessary code to the runtime, maybe cast it with the as keyword?

[quaint crescent]

yeah i guess

[lean delta]

n_n#2622

Playground Link

Preview:ts ... type AnyResult = | ValueResult | EnumStructResult | UnknownValueResult | InvalidResult function sizeTypeIs<T extends AnyResult["type"]>( result: ArrayResult, type: T ): result is Extract<ArrayResult, {size: {type: T}}> { return result.size.type === type } ...

[quaint crescent]

another way is creating a helper function

it's a bit weird creating a helper function for this specific example

the benefit of it is when you need to check .size.type quite often

so you don't accidentally as as the wrong type

[quaint crescent]

but note that this specific function signature requires splitting ArrayResult further (see the playground for more info)

[dreamy ginkgo]

If you can change the underlying structure, "flattening" the .size.type into something like .sizeType would work

[lean delta]

Retsam19#2505

Playground Link

Preview:ts // ignore this type Position = {} type VarType = {} type ValueResult = { type: 'ValueResult' } type EnumStructResult = { type: 'EnumStructResult' } type UnknownValueResult = { type: 'UnknownValueResult' } type InvalidResult = { type: 'InvalidResult' } type UnknownArrayDimension = { type: 'UnknownArrayDimension' } ...

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[buoyant mason]

yeah i guess i could do that, tho im just checking the size type once or twice, no more

[quaint crescent]

yeah in that case as is probably your best option

[buoyant mason]

yea, the problem is that those objects, ValueResult etc, have more stuff inside, like the actual value, the expression/token position, etc

so i would have duplicated state, or whatever its called, and thats not good

[dreamy ginkgo]

@buoyant mason Well you'd flatten out those other fields, too.