Question: Generics and static types

[maljac]

I read the documentation and was not sure how I can extract a generic static type from a generic io-ts type:

// the io-ts type definition

const TConfigValue = <C extends t.Mixed>(codec: C) =>
  t.type({
    value: codec,
    override: t.boolean
  })

I was only able to generate specific static types:

const TConfigValueNumber = TConfigValue(t.number)
type ConfigValueNumber = t.TypeOf<typeof TConfigValueNumber>

How can I get a static generic type like this:

type ConfigValue<T> = {
  value: T,
  override: boolean
}

[mlegenhausen]

In this case you can not use t.TypeOf you need instead to define all interfaces by yourself.

export interface ConfigValue<T> {
  value: T
  override: boolean
}
export interface ConfigValueC<TC extends t.Mixed> extends t.Type<TConfigValue<t.TypeOf<T>>, TConfigValue<t.OutputOf<T>>, unknown> {}
export function ConfigValue<TC extends t.Mixed>(codec: TC): ConfigValueC<TC> {
  return t.type({
    value: codec,
    override: boolean
  })
}

[maljac]

Thank you Malte. This helped a lot. Because your answer is not 100% correct, for further reference the final solution I came up with:

type ConfigValue<T> = {
  value: T;
  override: boolean;
};

interface ConfigValueC<TC extends t.Mixed> extends t.Type<ConfigValue<t.TypeOf<TC>>, ConfigValue<t.OutputOf<TC>>, unknown> {}

function createConfigValueT<TC extends t.Mixed>(codec: TC): ConfigValueC<TC> {
  return t.type({
    value: codec,
    override: t.boolean,
  });
}

The issue can be closed.

[mlegenhausen]

You are welcome Malte 😉

[mrdulin]

@mlegenhausen I have the same issue.

define all interfaces by yourself

Does it make people define the properties twice? io-ts runtime generic type and TS generic type?

[edwardaskew]

It's a little hacky but this worked for me:

const TConfigValue = <C extends t.Mixed>(codec: C) =>
  t.type({
    value: codec,
    override: t.boolean
  });

type ConfigValue<T> = t.TypeOf<ReturnType<typeof TConfigValue<t.Type<T>>>>