mapping over an array of tuple types does not give the correct type #11312

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1. kitsonk commented on Oct 3, 2016

   

   Kitson Kelly (kitsonk)

   on Oct 3, 2016

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   Tuples are never inferred. On the return, you have to be explicit:

   let arr: Array<[number, number]> = [[1, 2]]
   
   let mappedArr: Array<[number, number]> = arr.map(([x, y]) => {
       return <[number, number]> [x, y];
   });

   ... or ...

   let arr: Array<[number, number]> = [[1, 2]]
   
   let mappedArr: Array<[number, number]> = arr.map(([x, y]): [number, number] => {
       return [x, y];
   });

   But of course then you don't have to be explicit about the left hand side then:

   let arr: Array<[number, number]> = [[1, 2]]
   
   let mappedArr = arr.map(([x, y]): [number, number] => {
       return [x, y];
   });

   👍React with 👍2Reacted by altocumulus and Jason Rice
2. gcnew commented on Oct 3, 2016

   

   Marin Marinov (gcnew)

   on Oct 3, 2016

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   You can also manually augment map as proposed in #11252.
3. kitsonk commented on Oct 3, 2016

   

   Kitson Kelly (kitsonk)

   on Oct 3, 2016

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   Which means this is dupe of #6574, I had forgotten.
4. calebegg commented on Oct 28, 2016

   

   Caleb Eggensperger (calebegg)

   on Oct 28, 2016

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   I don't think this is a dupe; that issue is about mapping tuples to tuples (and also, it's fixed). This is about mapping arrays of tuples to arrays of tuples.

   The issue could be more succinctly stated as: "Tuple return types are not inferred". Here's a simple repro case:

   http://www.typescriptlang.org/play/#src=function%20foo(x%3A%20number%2C%20y%3A%20string)%20%7B%0D%0A%20%20%20%20return%20%5Bx%2C%20y%5D%3B%0D%0A%7D

   (note that the return type is (number | string)[] instead of [number, string])

   The fact that arrow functions therefore can't map tuples to tuples is just a consequence of that.
5. mhegazy commented on Oct 29, 2016

   

   Mohamed Hegazy (mhegazy)

   on Oct 29, 2016

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   This is another manifestation of #11152.

   map is declared as:

   map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[];

   so you see U is only inferred from the return type of the callback function. since tuple types are never inferred unless there is a contextual type (which in this case there is none), a regular array type is inferred for U, then the assignment check is done, and it fails.

   The piece that is missed here, is we know that the U[] will be assigned to mappedArr, and we should use this as a contextual type. #11152 tracks propagating using the return type of the contextual signature if one exists as another location for inference.
6. Mohamed Hegazy (mhegazy)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Oct 29, 2016
7. Mohamed Hegazy (mhegazy)

   closed this as completedon Oct 31, 2016
8. microsoft

   locked and limited conversation to collaborators on Jun 19, 2018