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"How types make hard problems easy" (or at least reduce cognitive load over time) (mayhul.com)

submitted 2 weeks ago by [email protected] to c/[email protected]

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[–] traches 2 points 2 weeks ago* (last edited 2 weeks ago) (5 children)

I’ve tried to use that NonEmptyArray type in the past and it was a real pain in the ass getting the type checker to believe that no, for realsies this array is not empty I just checked the length two lines ago. Is there some trick I don’t know or has it gotten smarter about that in recent updates?

[–] [email protected] 4 points 2 weeks ago (4 children)

Have you actually implemented a custom type guard or just asserted size?

[–] traches 1 points 2 weeks ago (1 children)

This is what I'm talking about:

Code for copy-pasting:

type NonEmptyArray<T> = [T, ...T[]];

function neverEmpty<T>(array: T[]): NonEmptyArray<T> | null {  
    if (array.length === 0) return null

    return array
}

[–] [email protected] 5 points 2 weeks ago (1 children)

type NonEmptyArray<T> = [T, ...T[]];

function isNonEmptyArray<T>(arr: T[]): arr is NonEmptyArray<T> {
    return arr.length > 0;
}

function neverEmpty<T>(array: T[]): NonEmptyArray<T> | null {  
    if (!isNonEmptyArray(array)) return null

    return array
}

[–] traches 4 points 2 weeks ago (1 children)

Hey cool, I learned something. Thanks!

[–] [email protected] 2 points 2 weeks ago

more context: https://stackoverflow.com/questions/56006111/is-it-possible-to-define-a-non-empty-array-type-in-typescript

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