How to match a custom type/interface?

[KlausJokisuo]

Heya,
I was wondering how I could check if the match is the correct type.

I know that I can check if the match is null, but I would like to check the correct type too.

type User = {
  age: number;
  name: string;
  uid: string;
};

type NullOrUser = User | null;

const nullOrUser: NullOrUser = {
  age: 10,
  name: 'Fuu',
  uid: 'Faa',
};

match(nullOrUser)
  .with(P.checkTypeIsUser, () => console.log('It´s User')) // <--- This line here
  .with(P.nullish, () => console.log('It´s null'))
  .exhaustive();

Thanks 👍

[gvergnaud]

Hey!

The recommended way to achieve this is to create a user pattern and then infer the type from the pattern:

const userPattern = {
    age: P.number,
    name: P.string,
    uid: P.string,
};

type User = P.infer<typeof userPattern>;

type NullOrUser = User | null;

const nullOrUser: NullOrUser = {
    age: 10,
    name: 'Fuu',
    uid: 'Faa',
};

match(nullOrUser)
    .with(userPattern, () => console.log('It´s User'))
    .with(P.nullish, () => console.log('It´s null'))
    .exhaustive();

[KlausJokisuo]

Thanks!

[kturmel]

How can you then P.select in your custom pattern?

Context

type None = {
  _tag: 'None'
}

type Some<T> = {
  _tag: 'Some'
  value: T
}

type Option<T> = Some<T> | None

Example

const optionPattern = {
  _tag: 'Some',
  value: P.any
}

const some = <T>(v: T): Option<T> => ({ _tag: 'Some', value: v })

match(some(2))
  .with(optionPattern, (v) => ...) // v is { _tag: 'Some', value: 2 } or shortly Some<2>
  .otherwise(() => ...)

How can I select the value from the option then ?

match(some(2))
  .with({ _tag: 'Some', value: P.select() }, (v) => ...) // v is 2
  .otherwise(() => ...)

Like this but with custom pattern? Do I have to repeat myself the _tag: 'Some' everytime?

yes, this is some kind of topic necro

[gvergnaud]

Your solution seems to be working if the tag key is defined as a constant:

type Option<T> = Some<T> | None

const optionPattern = {
  _tag: 'Some' as const, // 👈 added `as const`
  value: P.select()
}

const some = <T>(v: T): Option<T> => ({ _tag: 'Some', value: v })

match(some(2))
  .with(optionPattern, (v) => { v + 1 }) // v: number
  .otherwise(() =>  {})

Playground

[kturmel]

Oh this is banger!

Thanks for your solution.