Define functional setters using Setfield

Project.toml

@@ -15,6 +15,7 @@ MacroTools = "1914dd2f-81c6-5fcd-8719-6d5c9610ff09"
 NodeJS = "2bd173c7-0d6d-553b-b6af-13a54713934c"
 REPL = "3fa0cd96-eef1-5676-8a61-b3b8758bbffb"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
+Setfield = "efcf1570-3423-57d1-acb7-fd33fddbac46"
 TableTraits = "3783bdb8-4a98-5b6b-af9a-565f29a5fe9c"
 URIParser = "30578b45-9adc-5946-b283-645ec420af67"
 
@@ -27,6 +28,7 @@ IteratorInterfaceExtensions = "^0.1.1, ^1"
 JSON = "≥ 0.19.0"
 MacroTools = "≥ 0.4.4"
 NodeJS = "≥ 0.5.1"
+Setfield = "≥ 0.4.0"
 TableTraits = "^0.3.1, ^0.4, ^1"
 URIParser = "≥ 0.4.0"
 julia = "1"

src/VegaLite.jl

@@ -11,6 +11,7 @@ using FilePaths
 using REPL, Dates
 using Random
 import JSONSchema
+using Setfield: Setfield, PropertyLens, @lens, @set
 
 # This import can eventually be removed, it currently just makes sure
 # that the iterable tables integration for DataFrames and friends

src/spec_utils.jl

@@ -32,3 +32,15 @@ function Base.getproperty(spec::ObjectLike, name::Symbol)
         return value
     end
 end
+
+Setfield.set(spec::ObjectLike, ::typeof(@lens getparams(_)), params) =
+    typeof(spec)(params)
+
+function Setfield.set(spec::ObjectLike, ::PropertyLens{name}, value) where name
+    params = copy(getparams(spec))
+    params[keytype(params)(name)] = _maybeparams(value)
+    @set getparams(spec) = params
+end
+
+_maybeparams(value) = value
+_maybeparams(value::ObjectLike) = getparams(value)

test/test_properties.jl

@@ -1,5 +1,6 @@
 using Test
 using VegaLite
+using Setfield
 
 @testset "properties" begin
 
@@ -14,4 +15,7 @@ vlp = getvlplot()
 @test vlp.encoding.x.field isa String
 @test_deprecated vlp.params["mark"] isa String
 
+@test (@set vlp.mark = :point).mark == :point
+@test vlp.mark == "bar"  # not mutated
+
 end