use cases

[boneskull]

Hi,

Heard about ArkType off of Reddit and thought I'd check it out. I started playing with it, but am struggling to understand how it has a "1:1" relationship to TS. Does this mean that (theoretically) anything expressible in TS is expressible in ArkType?

So for example, I am building a small library. My library allows a consumer to provide it a function in a set of options. The function must be of a certain shape, but (and maybe this is just a documentation issue, since it's early) I couldn't figure out how to express a) the parameters and return value of the function, and b) how to express a "function" at all. Regarding b), the type of the options (via typeof myType.infer) has unknown where the function should be:

const myOpts = type({
  callback: 'Function' // this is as far as I got
});
type MyOpts = typeof myOpts.infer // {callback: unknown}

Is this a use case that ArkType intends to cover? Or is it focused on validating user (e.g., web form) input?

[ssalbdivad]

Actually this is a subtle bug with the way specifically Function is inferred, as in general we use functional types to represent morphs within our type system.

If you mouse over the type itself, you'll see this:

const myOpts: Type<{
    callback: (...args: any[]) => unknown;
}>

But it's incorrectly reduced to its return type, because we make the mistake of assuming it's a morph (i.e. a validator that accepts a given input type then transforms it).

That said, generally speaking a validation library is not useful when validating functional values because they have no associated type at runtime. The only data you can determine about a function by actually inspecting the object at runtime is how many arguments it takes (will be its length property), so there's very little to be gained by validating functions at runtime. Since ArkType's goal is to implement TS syntax that can be meaningfully checked at runtime, we have no plans to build functional type parsing.

That said, you can easily compose ArkType validators to validate a function's input then run arbitrary logic (that's essentially what a morph is). That can be achieved like this:

const getLength = morph("string", (s) => s.length)

// or
const getLength2 = type(["string", "|>", (s) => s.length])

// data is typed as number | undefined
const { data } = getLength("foo")

I've logged a bug here #721 that I will address in the upcoming beta release to fix the general type of the Function keyword!

[boneskull]

Thanks for the prompt reply! Good to know. Yes, as I was playing with it, I realized there wasn't any way to know what a function returns given the lack of meaningful introspection in JS.

I kept wanting a something that converts an arbitrary type to a validator