Use a separate DSL class for evaluation context

lib/glug.rb

@@ -10,3 +10,4 @@ module Glug # :nodoc:
 require_relative 'glug/stylesheet'
 require_relative 'glug/layer'
 require_relative 'glug/extensions'
+require_relative 'glug/stylesheet_dsl'

lib/glug/stylesheet.rb

@@ -11,16 +11,12 @@ def initialize(base_dir: nil, params: nil, &block)
       @layers = []
       @base_dir = base_dir || ''
       @params = params || {}
-      instance_eval(&block)
-    end
-
-    # Arbitrary properties can be defined, e.g. "foo :bar" results in a top-level "foo":"bar" in the style
-    def respond_to_missing?(*)
-      true
+      @dsl = StylesheetDSL.new(self)
+      @dsl.instance_eval(&block)
     end
 
     # Set a property, e.g. 'bearing 29'
-    def method_missing(method_sym, *args)
+    def add_property(method_sym, *args)
       @kv[method_sym] = args[0]
     end
 
@@ -60,7 +56,7 @@ def _add_layer(layer)
 
     # Load file
     def include_file(filename)
-      instance_eval(File.read(File.join(@base_dir, filename)))
+      @dsl.instance_eval(File.read(File.join(@base_dir, filename)))
     end
   end
 end

lib/glug/stylesheet_dsl.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+module Glug
+  # Defines the methods available to the DSL
+  class StylesheetDSL
+    def initialize(impl)
+      @__impl = impl
+    end
+
+    def layer(id, opts = {}, &block)
+      @__impl.layer(id, opts, &block)
+    end
+
+    def source(source_name, opts = {})
+      @__impl.source(source_name, opts)
+    end
+
+    def include_file(filename)
+      @__impl.include_file(filename)
+    end
+
+    # Arbitrary properties can be defined, e.g. "foo :bar" results in a top-level "foo":"bar" in the style
+    def respond_to_missing?(*)
+      true
+    end
+
+    # Set a property, e.g. 'bearing 29'
+    def method_missing(method_sym, *args)
+      @__impl.add_property(method_sym, *args)
+    end
+  end
+end