Tagged union common property types are not narrowed #10976

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1. Faris Mustafa (farism)

   changed the title [-]Tagged union common properties are not narrowed[/-] [+]Tagged union common property types are not narrowed[/+] on Sep 18, 2016
2. farism commented on Sep 18, 2016

   

   Faris Mustafa (farism)

   on Sep 18, 2016

   Last edited by farism

   Author

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   Actually.. this is user error. I believe the issue lies with doing the destructure in the reducer, i.e.

   function reducer({ kind, payload }: Action)

   Because I haven't switched on the kind at this point, flow control analysis cannot narrow down the type of payload. The following works fine

   function reducer(action: Action) {
     switch (action.kind) {
       case "move":
         return move(action.payload)
     }
   }

   I'm wondering if there is a way to make destructuring on tagged unions a bit more ergonomic? It seems like something that would be a common pattern, and having to duplicate destructures in each case could be cumbersome.
3. mhegazy commented on Sep 20, 2016

   

   Mohamed Hegazy (mhegazy)

   on Sep 20, 2016

   Contributor

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   once an object is destructed, the compiler can no longer make any assumptions about the relationships between the parts. Doing so requires data-flow analysis and alias tracking which is not trivial tasks.
4. Mohamed Hegazy (mhegazy)

   added

   Working as IntendedThe behavior described is the intended behavior; this is not a bugThe behavior described is the intended behavior; this is not a bug

   on Sep 20, 2016
5. Mohamed Hegazy (mhegazy)

   closed this as completedon Sep 20, 2016
6. felixfbecker commented on May 2, 2018

   

   Felix Becker (felixfbecker)

   on May 2, 2018

   Contributor

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   This would be really nice to have. I find myself often writing type guards inside filter() callbacks

   const elements = new Observable<HTMLElement | null>()
   elements.pipe(
     filter((element): element is NonNullable<typeof element> => !!element)
   )

   which is painful if you just want to narrow the type of one property/tuple element, because you need to repeat the type of the other members and cannot use destructuring:

   const mouseOvers = new Observable<MouseEvent>()
   
   mouseOvers.pipe(
     withLatestFrom(elements),
     filter((data): data is [typeof data[0], NonNullable<typeof data[1]>] => !!data[1])
   )

   vs

   mouseOvers.pipe(
     withLatestFrom(elements),
     filter(([, element]): element is NonNullable<typeof element> => !!element)
   )
7. microsoft

   locked and limited conversation to collaborators on Jul 31, 2018