diff --git a/ruby/lib/lox/function.rb b/ruby/lib/lox/function.rb
new file mode 100644
index 0000000..298a1a0
--- /dev/null
+++ b/ruby/lib/lox/function.rb
@@ -0,0 +1,23 @@
+require_relative "environment"
+
+module Lox
+  class Function
+    def initialize(decl)
+      @decl = decl
+    end
+
+    def arity = @decl.params.size
+
+    def call(interpreter, args)
+      env = Environment.new(interpreter.globals)
+      @decl.params.map(&:lexeme).zip(args).each do |name, value|
+        env.define(name, value)
+      end
+
+      interpreter.execute_block(@decl.body, env)
+      nil
+    end
+
+    def to_s = "<fn #{@decl.name.lexeme}>"
+  end
+end