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0 pointsk__4y ago0 comments

Is using "row polymorphism" the same as using a "structutal type system"?

I never heard about the former.

0 comments

Not really, and correct me if I'm wrong but afaik TS doesn't actually do row polymorphism so much as structural subtyping - although the difference between the two is pretty small and you can get pretty close to row polymorphism with structural subtyping + generics.

But even if these were the same thing and we want to be a bit pendantic since this is HN after all, structural type systems often support some kind of subtyping or row polymorphism, but it's not a strict requirement for a type system to be "structural". You could have a structural type system that doesn't allow

  { a: int, b: int }

to be used where

  { a: int }

is expected. How practical such a type system would be... I don't know. Flow type checker for JavaScript makes a distinction between "exact" types, i.e. object must have exactly the properties listed and no more, and "inexact" types where such subtyping is allowed.

seanwilson4y ago

> How practical such a type system would be... I don't know. Flow type checker for JavaScript makes a distinction between "exact" types, i.e. object must have exactly the properties listed and no more, and "inexact" types where such subtyping is allowed.

TypeScript doesn't have this check (https://github.com/Microsoft/TypeScript/issues/12936) and I've found it can be really error prone when you're wanting to return a copy of an object with some fields updated. Spot the bug in this example: https://www.typescriptlang.org/play?#code/C4TwDgpgBAKlC8UDeU...

mixedCase4y ago

FWIW, you can achieve row polymorphism in TypeScript, although it's not super intuitive.

  function rowPolymorphic<R extends { a: number }>(record: R): R & { a: string } {
    return {
      ...record,
      a: record.a.toString(),
    }
  }

  const rec = rowPolymorphic({
    a: 123,
    b: "string",
  })
  console.log(rec.b)

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