Styling multiple related layers using variables

[pnorman]

I have a large number of layers which mix very similar properties in different ways. e.g.

layer(:road_base_casing, :source=>:spirit, :source_layer=>:roads) {
    filter foo
    line_width A
    line_color M
    line_cap round
}

layer(:road_tunnel_casing, :source=>:spirit, :source_layer=>:roads) {
    filter foo & X
    line_width A
    line_color N
}

layer(:road_base_fill, :source=>:spirit, :source_layer=>:roads) {
    filter foo
    line_width B
    line_color N
}

layer(:road_casing, :source=>:spirit, :source_layer=>:roads) {
    filter foo & !X & !Y
    line_width A
    line_color M
}

In reality, all those variables are fairly complicated expressions using interpolates, matches, etc

I don't want to attempt something this complex with cascading but instead want to use variables. I have an example of how to use variables for numbers, colors, and other simple values.

I don't know how to assign part of an expression or conditional to a variable to reuse it in multiple definitions. I tried

ROAD_FILTER = (highway.in('motorway', 'trunk', 'primary', 'secondary', 'tertiary',
    'unclassified', 'residential', 'living_street') |
    ((highway == 'service') & (((minor == nil) & (zoom() >= 14)) | (zoom() >= 15))))

but it doesn't know what to do with the highway.in and gives

(eval):1:in `block in <top (required)>': undefined method `in' for nil:NilClass (NoMethodError

[systemed]

Ruby appears to be happy to hoist a definition outside the layer in which it's defined. So, for example, I have:

	layer(:roads_casing_11, :zoom=>11..12, :source_layer=>:roads) {
		Is_Minor = _!(type.in(MAIN_ROADS))
		Is_Rural_Minor = (urban< 1) & (_!(type.in(MAIN_ROADS)))
		Is_Urban_Minor = (urban>=1) & (_!(type.in(MAIN_ROADS)))
		filter (urban<2) | (type.in(MAIN_ROADS))

		line_cap :round
		line_color case_when(Is_Urban_Minor, UNCASED_MED_GREY.to_hex_color, CASING_LIGHTER.to_hex_color)

		line_width interpolate([:linear], zoom(),
			10, case_when(
				(urban >= 2) & (type.in(MAIN_ROADS)), urban_c10,
				Is_Rural_Minor, minor_c10,
				Is_Urban_Minor, urbmn_c10,
				rural_c10),
			15, case_when(
				(urban >= 2) & (type.in(MAIN_ROADS)), urban_c15,
				Is_Rural_Minor, minor_c15,
				Is_Urban_Minor, urbmn_c15,
				rural_c15),
			18, case_when(
				(urban >= 2) & (type.in(MAIN_ROADS)), urban_c18,
				rural_c18)
		)
	}

	layer(:roads_casing, :zoom=>12..22, :source_layer=>:roads) {
		filter _!((tunnel==true) | (unpaved==true))
		line_cap :round
		line_color case_when(Is_Urban_Minor, UNCASED_MED_GREY.to_hex_color, CASING_GREY.to_hex_color)
		line_width interpolate([:linear], zoom(),
			10, case_when( Is_Urban_Minor, urbmn_c10, Is_Minor, minor_c10, urban>=2, urban_c10, rural_c10 ),
			15, case_when( Is_Urban_Minor, urbmn_c15, Is_Minor, minor_c15, urban>=2, urban_c15, rural_c15 ),
			18, case_when( urban>=2, urban_c18, rural_c18 )
		)
	}

where Is_Urban_Minor etc. can be reused in subsequent definitions.

[pnorman]

I was going to use this but rubocop didn't like constants in blocks. I can easily disable that cop¹ but it reminded me why I wasn't a fan of this solution.

In the case I was working on I had several road casing layers, then a mix of fill and casing layers. If I define a variable for CasingColor and FillColor the first time I use them then the definitions are at two completely different parts of the file. If I rearrange the layer order I have to make sure that the first casing or fill layer has the definition in it. If I define them both in the first layer I have a fill definition in a casing layer.

baz = Glug::Condition.new.from_list(:match, ['static', 'motorway', '#b40017', :red]) gives me a variable which works, but is ugly. I think the solution is to allow Condition to take a block. I'm sufficiently deep in ruby DSL stuff I'm going to try to find some help.

Footnotes

1. And will have to since outside the layer block is still within the stylesheet block. ↩