Allow attachments to return non-void

[janosh]

Describe the problem

maybe this should be a discussion. if so, feel free to convert.

this Svelte attachment picks black or white text color to maximize contrast with node's background color:

export function contrast_color(
  text_color_threshold: number = 0.7,
  choices: [string, string] = [`black`, `white`], // one dark and one light color (in that order)
) {
  return (node: HTMLElement | null) => {
    const light_bg = luminance(get_bg_color(node)) > text_color_threshold
    const text_color = light_bg ? choices[0] : choices[1] // white text for dark backgrounds, black for light
    if (node) node.style.color = text_color
    return text_color
  }
}

in some cases, i'd like to call it outside a Svelte component and have it return the text color it would apply to the node:

const text_color = contrast_color(0.6)(null)

unfortunately, returning the color string from the attachment raises a type error when applied to a Svelte component:

Type string is not assignable to type void | (() => void)

Describe the proposed solution

maybe the allowed attachment return type could be broadened to enable this?

Importance

nice to have

[Conduitry]

What does get_bg_color(null) do? How does it know what node to get the background color of?

Regardless, this doesn't sound like a good idea to me, and this doesn't sound like a pattern that we should encourage. I would definitely suggest just using two different functions, and have the attachment one call the other one to find out what color to use.

[janosh]

I would definitely suggest just using two different functions

yes, that's what i ended up doing