Typescript: Design Meeting Notes, 6/14/2019

[DanielRosenwasser]

TypeScript Website Changes

• Starting to get a more investment into the playground!
• Clickable anchor links
  
  
  
  • Andrew commented that we should probably increase opacity for contrast.
  
  
• Removing the "fork me" ribbon
• Automatically serialize the playground to the URL bar on idle, lz compress the contents

Room for Improved Inference

• Feels like there is room for improvements in inference
  *
  ts let map = new Map([["", true], ["", 0]]);
  
  
  
  • In playground, gives you number is not assignable to boolean
    
    *
    
    ts type Foo = { success: boolean }; declare function bar<T>(f: () => T[]): T[]; let x: Foo[] = bar(() => Math.random() ? [{ success: true }] : [{ success: false }]);
  
  
  • Type '{ success: true; }[] | { success: false; }[]' is not assignable to type '{ success: true; }[]'.
  
  
• Problem a lot of the time comes down to "should we infer a union to T, or error that there's no common supertype"
  
  
  
  • Historically we didn't pick union types because we weren't sure about variance of usage.
  
  
  • Maybe we can do better based on variance as of strictFunctionTypes.
  
  
• What about function push<T>(arr: T[], elem: T)?
  
  
  
  • Nothing telling you you're wrong.
  
  
  • Bad.
  
  
  • Maybe we need some sort of variance annotation.
  
  
  • Not clear that that's the issue.
  
  

Strict Property Initialization

class Test {
    x: number = this.x;
}

• We decided to disallow x being referenced in its own initializer, but it maybe doesn't make as much sense in the presence of subtyping.
• Maybe adding a ! should allow you to disable the circularity error.
• Will this even work with changes to class property initialization?
  
  
  
  • Don't think so.
  
  
  • Existing engines don't seem to be compliant.
  
  
• Resolution: figure out what should happen per spec, update the Ember people, either version of postfix bang should work.

[sledorze]

Indeed, variance annotation would be very useful, specifically were it cannot be inferred (typically for declarations typings).