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Comment by tshaddox | Hacker News Reader

For TypeScript I'm not really sure what you would expect them to change just to avoid being Turing complete. Its Turing completeness is a straightforward consequence of its basic building blocks: iteration, conditional types, and recursive type definitions. Of course TypeScript could go out of its way to avoid Turing completeness by just specifying a maximum recursion depth or something similar, but why?

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> specifying a maximum recursion depth or something similar

Doesn't TS do exactly this? It doesn't seem like setting a cap on recursion depth would disqualify something from being Turing complete. Or am I misunderstanding your comment?

When recursion is the only method of iteration, of course it would.

Is the recursion limit an implementation detail of the compiler though, or something statically built into the language? If the former there's nothing to prevent the language itself from being Turing complete.

(Similar to how every Turing complete programming language today runs on something technically weaker than a Turing machine due to finite memory.)

There’s no spec so the compiler is the language.

What is the other method of iteration? Also, TypeScript does have a recursion limit[0].

[0] https://herringtondarkholme.github.io/2023/04/30/typescript-...

Do you mean within a state machine or in practice?

Iteration (loops) are typically represented as jump or branch instructions while recursion requires the creation of a new stack frame on the call stack.

No, in TypeScript's type system. As far as I know, recursion is the only method of iteration but the comment I was replying to seemed to imply otherwise.

The comment does not imply otherwise

I thought you did because of the "would". So you're saying that TypeScript is not Turing complete?

Nothing is Turing complete. But it is the stronger form of incomplete where even on a machine with truly unlimited memory it’d still be unable to model some (almost all, formally) executions.

>Turing completeness could be disabled, if it is checked that a type cannot use itself in its definition (or in a definition of an referenced type) in any way, not just directly as it is tested currently. This would make recursion impossible.

That sounds like a reasonable resolution.

https://github.com/microsoft/TypeScript/issues/14833

You can't do that because recursion is a feature, not a bug. It lets you do really cool things, such as:

    type MultiDimArray<T> = T[] | MultiDimArray<T>[];

    type LinkedList<T> = {
        value: T,
        next: LinkedList<T> | null,
    };

    type Json = string | number | null | Json[] | { [k: string]: Json };

Possibly a dumb question but why would Turing incompleteness be desirable in a language whose entire purpose is generating code in a Turing complete language?

I'm not an expert on type theory, but I would guess "Turing complete types" make type checking an undecidable problem, similar to how dependent types make type inference undecidable, since dependent types let you more or less encode arbitrary things into a type (e.g. p is a value of type Prime). That at the very least gives you an infinite search space, and the possibility to encode e.g. a halting property into types.

The problem being discussed is not that typescript the language that compiles to JavaScript is Turing complete, but rather that typescript types the compile time code necessary to validate that typescript are Turing complete. The consequence is compilation speed foot guns, readability and complexity. Not the end of the world, but the sort of thing some people reasonably consider a downside

Not allowing types to be nested within themselves would be a massive downside. You'd lose the ability to model anything tree-like in a reasonable manner. There are some obvious workarounds, but they're so inconvenient that nobody using a scripting language would ever want to be limited to using them.

IMO, TypeScript fails as a developer tool. Developer tools should guide the developer towards making good decisions. TS guides developers towards making poor decisions by facilitating code with complex interfaces.

JavaScript taxes interface complexity through increased cognitive load. Typescript frees developers from that tax and harms architecture in the process.

Funny how business gurus keep saying over and over that incentives are everything and yet when it comes to choosing developer tools for their businesses, they choose tools which promote terrible incentives, instead they choose "that which sounds good"... It's like communism. It sounds good in theory but it fails spectacularly at the incentive layer... It fails at the most important layer.

Just write your JS first and then type it afterward. Best of both.

It's the exact opposite; TypeScript discourages complex interfaces by making them explicit. It's far too easy in a dynamic language for devs to introduce absurdly complex interfaces without understanding the cognitive load it's imposing on other users. In TypeScript, they're forced to type it...which immediately reveals that complexity - if they're able to do it all!

This isn't theoretical; the only reason TypeScript allows for such complex interfaces is precisely because JavaScript devs have created those interfaces, and TypeScript makes a best-effort to make as much JS code type-able as possible.

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