Typing generic functions (like a transformer) correctly

[bjesuiter]

Given the following function type:
ChunkTransformFunc<P, R> = (chunk: P) => R

I want to be able to express:

1. The Param should be of type P
2. The Return Type should be of type R (normally unrelated to P)

ChatGPT Gives me something like this:

const ChunkTransformFuncSchema = z
  .function(z.zodTuple([z.unknown() as any]), z.unknown())
  .args(z.zodTuple([z.unknown() as any] as const))
  .transform((func) => {
    const [inputSchema, outputSchema] = z.getParsedType(func);
    return z.function(inputSchema[0], outputSchema);
  });

But this has some problems:

1. z.zodTuple does not exist => rewriting to z.tuple()
2. Then this line does not work anymore, due to readonly something somewhere: .args(z.tuple([z.unknown() as any] as const));

I would like to have something like a z.generic() function which allows a generic to be defined as input and to be retrieved as an output type for example.
Maybe like this:

z
.function(z.generic('P'), z.generic('R'))
.args(z.generic('P))
.returns(z.generic('R'))

[colinhacks]

This is known as a "higher-kinded type" and it isn't representable in TypeScript: microsoft/TypeScript#1213

[klesun]

Just my 5 cents for future readers, HKT is actually kind of possible in typescript through this, albeit a bit verbose: microsoft/TypeScript#40928 (you need to define an interface for the callback each time)

export interface HKT {
  param: unknown;
  result: unknown;
}
export type Apply<f extends HKT, x>
  = (f & { param: x })['result'];

interface BoxHKT extends HKT {
  result: [this['param']];
}

type Test3 = Apply<BoxHKT, number> // be deduced to `[number]`