Add a link for cardinality question

antonkalinin-ml · vkpgwt · commit d3e053fc8319 · 2021-12-20T17:35:22.000+03:00
diff --git a/backend/middle-1/haskell.md b/backend/middle-1/haskell.md
@@ -58,7 +58,7 @@
 
 #### Resources
 
-TODO: add resources
+* [Cardinality of a Set](https://math24.net/cardinality-set.html)
 
 ## Polymorphic Kinds
 
