this post was submitted on 05 Jan 2025
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Programming

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"How types make hard problems easy" (or at least reduce cognitive load over time) (mayhul.com)
submitted 3 days ago by [email protected] to c/programming
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[–] [email protected] 2 points 3 days ago* (last edited 3 days ago) (1 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 3 days ago (1 children)

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

[–] [email protected] 1 points 2 days 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 days 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
}