How do you define an array of generics in TypeScript?

Question

Let's say I have a generic interface like the following:

interface Transform<ArgType> {
    transformer: (input: string, arg: ArgType) => string;
    arg: ArgType;
}

And then I want to apply an array of these Transform to a string. How do I define this array of Transform such that it validates that <ArgType> is equivalent in both Transform.transformer and Transform.arg? I'd like to write something like this:

function append(input: string, arg: string): string {
    return input.concat(arg);
}

function repeat(input: string, arg: number): string {
    return input.repeat(arg);
}

const transforms = [
    {
        transformer: append,
        arg: " END"
    },
    {
        transformer: repeat,
        arg: 4
    },
];

function applyTransforms(input: string, transforms: \*what type goes here?*\): string {
    for (const transform of transforms) {
        input = transform.transformer(input, transform.arg);
    }

    return input;
}

In this example, what type do I define const transforms as in order for the type system to validate that each item in the array satisfies the generic Transform<ArgType> interface?

Answer 1

(Using TS 3.0 in the following)

If TypeScript directly supported existential types, I'd tell you to use them. An existential type means something like "all I know is that the type exists, but I don't know or care what it is." Then your transforms parameter have a type like Array< exists A. Transform<A> >, meaning "an array of things that are Transform<A> for some A". There is a suggestion to allow these types in the language, but few languages support this so who knows.

You could "give up" and just use Array<Transform<any>>, which will work but fail to catch inconsistent cases like this:

applyTransforms("hey", [{transformer: repeat, arg: "oops"}]); // no error

But as you said you're looking to enforce consistency, even in the absence of existential types. Luckily, there are workarounds, with varying levels of complexity. Here's one:

---

Let's declare a type function which takes a T, and if it a Transform<A> for some A, it returns unknown (the new top type which matches every value... so unknown & T is equal to T for all T), otherwise it returns never (the bottom type which matches no value... so never & T is equal to never for all T):

type VerifyTransform<T> = unknown extends
  (T extends { transformer: (input: string, arg: infer A) => string } ?
    T extends { arg: A } ? never : unknown : unknown
  ) ? never : unknown

It uses conditional types to calculate that. The idea is that it looks at transformer to figure out A, and then makes sure that arg is compatible with that A.

Now we can type applyTransforms as a generic function which only accepts a transforms parameter which matches an array whose elements of type T match VerifyTransform<T>:

function applyTransforms<T extends Transform<any>>(
  input: string, 
  transforms: Array<T> & VerifyTransform<T>
): string {
  for (const transform of transforms) {
    input = transform.transformer(input, transform.arg);
  }
  return input;
}

Here we see it working:

applyTransforms("hey", transforms); // okay

If you pass in something inconsistent, you get an error:

applyTransforms("hey", [{transformer: repeat, arg: "oops"}]); // error

The error isn't particularly illuminating: "[ts] Argument of type '{ transformer: (input: string, arg: number) => string; arg: string; }[]' is not assignable to parameter of type 'never'." but at least it's an error.

---

Or, you could realize that if all you're doing is passing arg to transformer, you can make your existential-like SomeTransform type like this:

interface SomeTransform {
  transformerWithArg: (input: string) => string;
}

and make a SomeTransform from any Transform<A> you want:

const makeSome = <A>(transform: Transform<A>): SomeTransform => ({
  transformerWithArg: (input: string) => transform.transformer(input, transform.arg)
});

And then accept an array of SomeTransform instead:

function applySomeTransforms(input: string, transforms: SomeTransform[]): string {
  for (const someTransform of transforms) {
    input = someTransform.transformerWithArg(input);
  }

  return input;
}

See if it works:

const someTransforms = [
  makeSome({
    transformer: append,
    arg: " END"
  }),
  makeSome({
    transformer: repeat,
    arg: 4
  }),
];

applySomeTransforms("h", someTransforms);

And if you try to do it inconsistently:

makeSome({transformer: repeat, arg: "oops"}); // error

you get an error which is more reasonable: "Types of parameters 'arg' and 'arg' are incompatible. Type 'string' is not assignable to type 'number'."

---

Okay, hope that helps. Good luck.