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by satvikpendem 1218 days ago
Why is it disappointing? TypeScript's type system, via pseudo-dependent-types, is much more powerful than Elm's, Rust's, Haskell's etc. The only problem is that it's not sound, but JS itself is not sound, so TS can never be.
1 comments
creata 1218 days ago
> JS itself is not sound, so TS can never be.

What?

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danabrams 1218 days ago
The type system is not sound… that is there are things which the type checker cannot prove.

Not that the languages themselves are unsound.

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creata 1218 days ago
1. That's a horrible description of unsoundness; you're making it sound like incompleteness!

2. What I meant to ask was: what does satvikpendem mean by "JS is unsound"? It's a dynamically typed language, so they can't be talking about soundness in the type system...

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orta 1218 days ago
I did a bit of a writeup on the gaps in soundness for TypeScript in the docs, https://www.typescriptlang.org/play?strictFunctionTypes=fals...
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satvikpendem 1218 days ago
I should clarify, the JS type system is unsound, and because JS is valid TS, TS is by extension also unsound. Not sure why you'd think a dynamically typed language cannot be unsound, it still has types, they're just not static.
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creata 1218 days ago
Define soundness. Here's something close to the definitions that I've seen:

> The central result we wish to have for a given type-system is called soundness. It says this. Suppose we are given an expression (or program) e. We type-check it and conclude that its type is t. When we run e, let us say we obtain the value v. Then v will also have type t. - https://papl.cs.brown.edu/2014/safety-soundness.html

This only makes sense in the context of static types afaict, because you do not "typecheck an expression" in a dynamically typed language.

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satvikpendem 1217 days ago
Types can be checked at runtime too, not just compile time. A language is sound if all types, even at runtime, are what they say they are. Zod for TypeScript is one example of checking this at runtime, although it's not fully safe either.
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satvikpendem 1218 days ago
Soundness is not decidability. The reason TS is unsound is not directly because it's undecidable, as any Turing complete system is undecidable, and the TS type system is Turing complete.
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