Type narrowing of string literal unions #9314

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1. gcnew commented on Jun 22, 2016

   

   Marin Marinov (gcnew)

   on Jun 22, 2016

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   PS: Kudos for #9163 and all the work you've put in TS. It's a trully amazing language.
2. aluanhaddad commented on Jun 22, 2016

   

   Aluan Haddad (aluanhaddad)

   on Jun 22, 2016

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   Marin Marinov (@gcnew) interestingly, this works:

   type Easing = { val: 'ease-in' } | { val: 'ease-out' } | { val: 'ease-in-out' };
   
   function getEasingFunction(easing: Easing): (val: number) => number {
       switch (easing.val) {
           case 'ease-in': return t => t * t;
           case 'ease-out': return t => t * (2 - t);
           case 'ease-in-out': return t => t < 0.5 ? (2 * t * t) : -1 + (4 - 2 * t) * t;
       }
       // `easing` should be of type `never` here
       // however the actual type is `'ease-in' | 'ease-out' | 'ease-in-out'`
   }
3. gcnew commented on Jun 22, 2016

   

   Marin Marinov (gcnew)

   on Jun 22, 2016

   Last edited by gcnew

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   Aluan Haddad (@aluanhaddad) Yeah, another workaround that doesn't change Easing alias:

   type Easing = 'ease-in' | 'ease-out' | 'ease-in-out';
   
   function assertNever(x: never): never {
       throw `Not a never ${ x }`;
   }
   
   function getEasingFunction(easing: Easing): (val: number) => number {
       const x = { test: easing };
       switch (x.test) {
           case 'ease-in':     return t => t * t;
           case 'ease-out':    return t => t * (2-t);
           case 'ease-in-out': return t => t<0.5 ? (2*t*t) : -1+(4-2*t)*t;
   
           default:
               return assertNever(x);
       }
   }
4. ahejlsberg commented on Jun 22, 2016

   

   Anders Hejlsberg (ahejlsberg)

   on Jun 22, 2016

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   This is a duplicate of #7447.
5. Anders Hejlsberg (ahejlsberg)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Jun 22, 2016
6. Anders Hejlsberg (ahejlsberg)

   closed this as completedon Jun 22, 2016
7. microsoft

   locked and limited conversation to collaborators on Jun 19, 2018