Allow abstract class for local declarations and expressions #9110

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1. aluanhaddad commented on Jun 13, 2016

   

   Aluan Haddad (aluanhaddad)

   on Jun 13, 2016

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   Yes it should but it doesn't make a lot of sense to do.
2. yortus commented on Jun 13, 2016

   

   Troy Gerwien (yortus)

   on Jun 13, 2016

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   It might make sense according to the same rationale that TypeScript supports other local types. It would be helpful to see a motivating example showing the value of a local abstract class.

   Note this was sort-of anticipated in the proposal for abstract classes, under the heading "To Be Discussed".
3. ulrichb commented on Jun 13, 2016

   

   ulrichb

   on Jun 13, 2016

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   I had no real use case, I just wanted to scope the abstract class and its derived types to avoid having global symbols. The workaround was to use a surrounding namespace. It was an artificial sample for a TypeScript talk.

   Nevertheless, as local types (for non-abstract classes, interfaces, and enums) are supported, IMO missing support for abstract classes means that this feature is incomplete. Also because the compiler output seems to be correct (without --noEmitOnError) except the mentioned error message.

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4. Mohamed Hegazy (mhegazy)

   changed the title [-]"Modifiers cannot appear here" for abstract class in function[/-] [+]Allow abstract class for local declarations and expressions[/+] on Jun 13, 2016
5. Mohamed Hegazy (mhegazy)

   added

   SuggestionAn idea for TypeScriptAn idea for TypeScript

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   Good First IssueWell scoped, documented and has the green lightWell scoped, documented and has the green light

   on Jun 13, 2016
6. Mohamed Hegazy (mhegazy)

   assigned

   ghost

   on Jun 13, 2016
7. aluanhaddad commented on Jun 13, 2016

   

   Aluan Haddad (aluanhaddad)

   on Jun 13, 2016

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   I agree this should be supported I was just curious what the actual use case was since the base class cannot escape the function anyway, it's only implementations are going to reside within the same scope which should be fairly narrow. However if you're using the function to avoid polluting the global namespace but want to use an abstract class in your design that seems very reasonable. I've been spending too much time writing external modules...

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8. ghost closed this as completedin #9994on Jul 28, 2016
9. rauschma commented on Aug 10, 2016

   

   Axel Rauschmayer (rauschma)

   on Aug 10, 2016

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   This is a use case for abstract class expressions: implementing mixins via subclass factories.

   const MyMixin = (S : { new () : Object }) => abstract class extends S {
       abstract foo() {}
       bar() {}
   };
   
   // Must implement foo()
   class MyClass extends MyMixin(Object) {
   }

   (I don’t feel 100% confident about the type of S; there must be a better way to type it.)
11. Mohamed Hegazy (mhegazy)

    added this to the TypeScript 2.0.1This milestone has been deleted milestone on Aug 17, 2016
12. Mohamed Hegazy (mhegazy)

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    FixedA PR has been merged for this issueA PR has been merged for this issue

    on Aug 17, 2016
13. microsoft

    locked and limited conversation to collaborators on Jun 19, 2018