Support referencing one constant in another constant

[nickidlugash]

Can we support referencing one constant in another constant?

E.g.
"place-halo": "#faf3ed  ",
"country-halo": "place-halo"

Or

`"land": ""#ebe5e0",
`"country-halo": { "lighten": ["land", 10] }`

(The syntax in the second example is not valid – I just made it up to demonstrate that the ability to reference other constants could be useful once we have color functions: #38)

[tmcw]

Hm, I'm not sure if this is a good can of worms to open:

"place-halo": "country-halo",
"country-halo": "place-halo"

[nickidlugash]

Hm, I'm not sure if this is a good can of worms to open

@tmcw Can you elaborate? :)

[tmcw]

In the example that I posted: if you support variables referencing each other, you support referencing each other in infinite loops: place-halo goes to country-halo, and vice versa, forever. We can detect these kinds of loops, but they make the implementation a little more complex.

Variable references will also make debugging a bit trickier - instead of just saying "constant reference here was invalid", we'd have to look up possibly multiple levels of reference to give a more specific error.

[nickidlugash]

@tmcw I see. I had assumed in that example that it would error right away because the first constant is trying to reference a constant that has not yet been defined, but I didn't realize that the order does not matter.

If we don't think that referencing constants within constants is a good idea, perhaps we should just push this type of functionality to the future GUI, and not worry about it for now?

[tmcw]

If we don't think that referencing constants within constants is a good idea, perhaps we should just push this type of functionality to the future GUI, and not worry about it for now?

👍 This is something we can keep in our pocket for now and implement in a GUI after we have more knowledge of how we visually design maps anyway.

[jfirebaugh]

@nickidlugash Is this still needed given current UI technology?

[nickidlugash]

@jfirebaugh the current UI is not yet there to support something like this (this is basically the idea behind constant transforms, but it is not implemented yet). I think we're pretty set on this eventually being something the UI will handle though, so we can close here.

[nickidlugash]

I think we're pretty set on this eventually being something the UI will handle though, so we can close here.

Um...maybe not? #38 (comment)

[jfirebaugh]

Haha... oops, I guess there's a downside to merging the history. This bug was closed by a commit from the lint repository that fixed cutting-room-floor/mapbox-gl-style-lint#53.

[jfirebaugh]

RIP constants.