Support first class mixins

lib/sass/value.rb

@@ -91,6 +91,12 @@ def assert_map(name = nil)
       raise Sass::ScriptError.new("#{self} is not a map", name)
     end
 
+    # @return [Mixin]
+    # @raise [ScriptError]
+    def assert_mixin(name = nil)
+      raise Sass::ScriptError.new("#{self} is not a mixin", name)
+    end
+
     # @return [Number]
     # @raise [ScriptError]
     def assert_number(name = nil)

lib/sass/value/function.rb

@@ -8,8 +8,6 @@ module Value
     class Function
       include Value
 
-      # @overload initialize(id)
-      #   @param id [Numeric]
       # @overload initialize(signature, callback)
       #   @param signature [::String]
       #   @param callback [Proc]

lib/sass/value/mixin.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module Sass
+  module Value
+    # Sass's mixin type.
+    #
+    # @see https://sass-lang.com/documentation/js-api/classes/sassmixin/
+    class Mixin
+      include Value
+
+      # @!visibility private
+      def initialize(id)
+        @id = id
+      end
+
+      # @return [Integer, nil]
+      attr_reader :id
+
+      # @return [::Boolean]
+      def ==(other)
+        if id.nil?
+          other.equal? self
+        else
+          other.is_a?(Sass::Value::Mixin) && other.id == id
+        end
+      end
+
+      # @return [Integer]
+      def hash
+        id.hash
+      end
+
+      # @return [Mixin]
+      def assert_mixin(_name = nil)
+        self
+      end
+    end
+  end
+end