Bad type inference from return type #16099

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1. ikatyang commented on May 26, 2017

   

   Ika (ikatyang)

   on May 26, 2017

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   You have to provide some information for TS to infer types, otherwise it wouldn't know what name is.

   for example

   // <ItemType extends string>
   
   declare function flatMap<ItemType extends string, ProjectedItemType>(projectorFn: (item: ItemType) => ProjectedItemType[]): (aggregate: ProjectedItemType[], item: ItemType) => ProjectedItemType[];
   
   ['alice', 'bob'].reduce(flatMap(name => name.split('')), []);

   or

   // (name: string) => name.split('')
   
   declare function flatMap<ItemType, ProjectedItemType>(projectorFn: (item: ItemType) => ProjectedItemType[]): (aggregate: ProjectedItemType[], item: ItemType) => ProjectedItemType[];
   
   ['alice', 'bob'].reduce(flatMap((name: string) => name.split('')), []);
2. dawidgarus commented on May 26, 2017

   

   dawidgarus

   on May 26, 2017

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   Ika (@ikatyang) I know, but it would be nice if compiler could work out type of name from return type.
   reduce expects (something[], string): something[] function and flatMap returns (ProjectedItemType[], ItemType): ProjectedItemType[]. When compiler doesn't know what is ItemType as assumes that it's {}, it could use that knowledge to infer that ItemType is string.
3. ahejlsberg commented on May 26, 2017

   

   Anders Hejlsberg (ahejlsberg)

   on May 26, 2017

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   Duplicate of #15680. Fixed by #16072.

   👍React with 👍1Reacted by Aluan Haddad
4. Anders Hejlsberg (ahejlsberg)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on May 26, 2017
5. mhegazy commented on Jun 9, 2017

   

   Mohamed Hegazy (mhegazy)

   on Jun 9, 2017

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   Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.
6. Mohamed Hegazy (mhegazy)

   closed this as completedon Jun 9, 2017
7. microsoft

   locked and limited conversation to collaborators on Jun 14, 2018