why is type T = (x: T) => void valid whereas T = T isnt? #12525

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1. HerringtonDarkholme commented on Nov 28, 2016

   

   Herrington Darkholme (HerringtonDarkholme)

   on Nov 28, 2016

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   https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#310-type-aliases

   Note that object type literals, function type literals, and constructor type literals do not depend on types referenced within them and are therefore permitted to circularly reference themselves through type aliases.
2. DanielRosenwasser commented on Nov 28, 2016

   

   Daniel Rosenwasser (DanielRosenwasser)

   on Nov 28, 2016

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   The restriction that a type alias can't be referenced by itself at the top level has been the behavior since we implemented type aliases; however, you might recall that a while back, we started allowing type aliases to be referenced from within an object type.

   If we desugar T's declaration to use object type, we can that under the current rules, it's allowed.

   type T = {
       (value: T) => void
   };

   Overall this is actually pretty reasonable - even Haskell can't express something like this:

   type Wat = Wat

   <interactive>:1:1: error:
       Cycle in type synonym declarations:
         <interactive>:1:1-14: type Wat = Wat
   

   However, it has no problem provided that the type can be expanded, or "unrolled" one level at a time. In other words, as long as you have some sort of box-y thing containing the type:

   newtype Box a = Box a

   the type system will be okay.

   👍React with 👍1Reacted by Herrington Darkholme
3. Daniel Rosenwasser (DanielRosenwasser)

   added

   QuestionAn issue which isn't directly actionable in codeAn issue which isn't directly actionable in code

   on Nov 28, 2016
4. Daniel Rosenwasser (DanielRosenwasser)

   closed this as completedon Nov 28, 2016
5. zpdDG4gta8XKpMCd commented on Nov 28, 2016

   

   ZpdDG4gta (zpdDG4gta8XKpMCd)

   on Nov 28, 2016

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   is there any chance for recursive types in unions?

   type Expression = Binary <Expression> | Unary <Expression>
   
6. mhegazy commented on Nov 28, 2016

   

   Mohamed Hegazy (mhegazy)

   on Nov 28, 2016

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   is there any chance for recursive types in unions?

   tracked by #6230
7. microsoft

   locked and limited conversation to collaborators on Jun 19, 2018