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HypeScript: Simplified TypeScript type system in TypeScript's own type system

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Re: HypeScript: Simplified TypeScript type system in TypeScript's own type system

#11

Post by staticassertion »

theK wrote: post #10

Am I the only one that feels uncomfortable with all that usage of strings in TS’s type system? Why not use pure literals instead of string literals? This is a genuine question, I’m trying to find out what the pros and cons where in the decision making process.

I think it's kind of cool. It reminds me a bit of C++ templates, which are "compile time duck typed". The benefit is that you get lose typing constructs that are evaluated strictly and at type check time, which is kinda nutty. It means that you can lean really heavily on the compiler.

Sort of like using Python to generate Typescript, as random example - my Python code doesn't have to typecheck, but its output does, so I can do absurd shit in Python and still feel good about the output.

One example is something like this: https://www.typescriptlang.org/docs/handbook/2/template-lite...

These are types that are built from string templates. Since strings are loose and can be manipulated in crazy ways like appending, we can now manipulate types in the same way. We can write types that are themselves parsers.

So idk if that's good or not, the downside in C++ is that TMP errors are fucking insane, but the upside is that I can have a function that says "pass me something and I'll call "iter" on it and I don't care what that thing is".

It also feelsy kinda more "typey". Types are just values. Types are just strings or numbers or whatever. They're things, with the constraint being that they must exist concretely (or be inferrable) when the type checker runs. No distinction between types and values seems like it's the ideal.

Re: HypeScript: Simplified TypeScript type system in TypeScript's own type system

#12

Post by p1necone »

theK wrote: post #10

Am I the only one that feels uncomfortable with all that usage of strings in TS’s type system? Why not use pure literals instead of string literals? This is a genuine question, I’m trying to find out what the pros and cons where in the decision making process.

In the context of type definitions, strings aren't really regular bare strings.

I.e. if I write

  type foo = {
     bar: "Vodka"
  }
I'm guaranteeing that the value of 'bar' on any object of that type must be "Vodka" - the type of bar is not string, it's literally "Vodka" because string values can be types.

This seems a little obscure and pointless, but you can put these string types in a union, and enforce that a value must be one of many values.

  function doTheThing(color: "RED" | "GREEN") {
      //do stuff
  }

  doTheThing("RED"); //ok
  doTheThing("GREEN"); //ok
  doTheThing("ORANGE"); //doesn't compile, because the type of the param *isn't* string, it's "RED" | "GREEN"
You can also define a type like

  type Mountain = { name: "EVEREST", height: 8848 } | { name: "K2", height: 8611 };
and then at compile time know that if mountain.name === "EVEREST" then height === 8848 because the types of name and height aren't string and number, they're "EVEREST" | "K2" and 8848 | 8611, and the compiler is smart enough to work out one based on the other.

==============================================

For extra context - a lot of this is for interoptability with Javascript code - you want to call some Javascript function with a stringly typed enum, and enforce only passing in valid values, but the Javascript code still just deals with strings.

A lot of Typescripts kind of insane flexibility is so you can introduce type safety to all sorts of dynamic Javascript code, without having to make sacrifices on the dynamic-ness of it.

Turns out this flexibility is actually kinda awesome to have in general even when you're not trying to refactor an existing JS codebase.

Re: HypeScript: Simplified TypeScript type system in TypeScript's own type system

#15

Post by RussianCow »

p1necone wrote: post #12
theK wrote: post #10

Am I the only one that feels uncomfortable with all that usage of strings in TS’s type system? Why not use pure literals instead of string literals? This is a genuine question, I’m trying to find out what the pros and cons where in the decision making process.

In the context of type definitions, strings aren't really regular bare strings. I.e. if I write type foo = { bar: "Vodka" } I'm guaranteeing that the value of 'bar' on any object of that type must be "Vodka" - the type of bar is not string, it's literally "Vodka" because string values can be types. This seems a little obscure and pointless, but you can put these string types in a union, and enforce that a value must…

To add to this, the reason it’s important for TypeScript to support string literals as values instead of using enums or something new is interop with existing JS. For example, think of an event handler where you pass the event name as the first argument; you can ensure at compile-time that an invalid event name isn’t passed.

Re: HypeScript: Simplified TypeScript type system in TypeScript's own type system

#17

Post by bobbylarrybobby »

p1necone wrote: post #12
theK wrote: post #10

Am I the only one that feels uncomfortable with all that usage of strings in TS’s type system? Why not use pure literals instead of string literals? This is a genuine question, I’m trying to find out what the pros and cons where in the decision making process.

In the context of type definitions, strings aren't really regular bare strings. I.e. if I write type foo = { bar: "Vodka" } I'm guaranteeing that the value of 'bar' on any object of that type must be "Vodka" - the type of bar is not string, it's literally "Vodka" because string values can be types. This seems a little obscure and pointless, but you can put these string types in a union, and enforce that a value must…

And don't forget about [template literal types](https://devblogs.microsoft.com/typescript/announcing-typescr...)!

```

type Color = "red" | "blue"; type Quantity = "one" | "two";

type SeussFish = `${Quantity | Color} fish`; // same as // type SeussFish = "one fish" | "two fish" // | "red fish" | "blue fish";

```

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