Minimal working double

src/spectator/double.cr

@@ -1,5 +1,5 @@
 module Spectator
-  abstract struct Double
+  abstract class Double
     macro stub(definition)
       def {{definition.name.id}}
         {{definition.block.body}}

src/spectator/dsl/mocks.cr

@@ -0,0 +1,13 @@
+require "../double"
+
+module Spectator::DSL
+  macro double(name, &block)
+    {% if block.is_a?(Nop) %}
+      Double{{name.id}}.new
+    {% else %}
+      class Double{{name.id}} < ::Spectator::Double
+        {{block.body}}
+      end
+    {% end %}
+  end
+end