Inconsistent String Literal Type Propagation #15271

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1. ahejlsberg commented on Apr 19, 2017

   

   Anders Hejlsberg (ahejlsberg)

   on Apr 19, 2017

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   This is working as intended. In the call strEnum(["MY_TYPE"]), the "MY_TYPE" literal has a widening literal type (see #11126) because it was never given an explicit type through a type annotation or a type assertion. You get the desired result if you explicitly assert a type:

   const myTypeEnum = strEnum(["MY_TYPE" as "MY_TYPE"])

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2. Anders Hejlsberg (ahejlsberg)

   added

   Working as IntendedThe behavior described is the intended behavior; this is not a bugThe behavior described is the intended behavior; this is not a bug

   on Apr 19, 2017
3. bilalq commented on Apr 20, 2017

   

   Bilal Quadri (bilalq)

   on Apr 20, 2017

   Last edited by bilalq

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   I'm left a bit surprised by the type widening here. In a project using react-redux, I'm having to declare types that I would expect the compiler to be able to infer.

   In all of the following examples, I would expect the code to compile. However, there are many where it does not:

   // Example 1
   // DOES NOT compile
   interface MyAction {
     type: 'MY_ACTION'
   }
   const action = {
     type: 'MY_ACTION'
   }
   const reassignment: MyAction = action;
   
   
   // Example 2
   // DOES NOT compile
   const MY_ACTION = 'MY_ACTION';
   interface MyAction {
     type: typeof MY_ACTION
   }
   const action = {
     type: MY_ACTION
   }
   const reassignment: MyAction = action;
   
   
   // Example 3
   // DOES compile
   const MY_ACTION: 'MY_ACTION' = 'MY_ACTION';
   interface MyAction {
     type: typeof MY_ACTION
   }
   const action = {
     type: MY_ACTION
   }
   const reassignment: MyAction = action;
   
   
   // Example 4
   // DOES compile
   const MY_ACTION = 'MY_ACTION';
   interface MyAction {
     type: typeof MY_ACTION
   }
   const action: MyAction = {
     type: MY_ACTION
   }
   const reassignment: MyAction = action;

   Are all these situations truly as intended? Is this because the object is not guaranteed to be immutable? If that's the case, the widening difference between example 2 and 3 is throwing me off.
4. RyanCavanaugh commented on Apr 20, 2017

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Apr 20, 2017

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   Are all these situations truly as intended?

   Consider a slight variant:

   const DEFAULT_COLOR = 'RED';
   const style = {
     background: DEFAULT_COLOR;
   };
   if (user.useNeatBackground) {
     // Error? Cannot convert "BLUE" to "RED" ?
     style.background = 'BLUE';
   }

   Without explicit intent, it's very tricky to infer literal types. You can quickly find yourself issuing errors on code that is extremely unsuspicious.
5. mattnedrich commented on Apr 20, 2017

   

   Matt Nedrich (mattnedrich)

   on Apr 20, 2017

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   Thanks for the clarification! I'm going to close.
6. Matt Nedrich (mattnedrich)

   closed this as completedon Apr 20, 2017
7. bilalq commented on Apr 20, 2017

   

   Bilal Quadri (bilalq)

   on Apr 20, 2017

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   Hmm... that case certainly makes sense. I don't know if it's prohibitively expensive to do this, but could the compiler avoid widening the type within a certain scope if it can guarantee that an object is never mutated? Though even in that case

   Another possible alternative here that might be nice would be to introduce a keyword in the type layer called literal.

   const SUBMIT_COMMENT_SUCCEEDED: literal = 'SUBMIT_COMMENT_SUCCEEDED';

   It expresses intent clearly, and avoids the triple repetition that we currently have to do:

   const SUBMIT_COMMENT_SUCCEEDED: 'SUBMIT_COMMENT_SUCCEEDED' = 'SUBMIT_COMMENT_SUCCEEDED';

   Would this be a proposal that might be considered? Or are there downsides to this?

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8. microsoft

   locked and limited conversation to collaborators on Jun 14, 2018