Add one source & GHC pragma

Author: ZeppLu

README.md

@@ -5,14 +5,16 @@
 
 
 ### Sources
-- [Real World Haskell](http://book.realworldhaskell.org/read/),
+- [Real World Haskell](http://book.realworldhaskell.org/read),
   referred to as `RWH`.
-- [Learn You a Haskell for Great Good!](http://learnyouahaskell.com/chapters/),
+- [Learn You a Haskell for Great Good!](http://learnyouahaskell.com/chapters),
   referred to as `LYH`.
 - [CIS 194: Introduction to Haskell](http://www.seas.upenn.edu/~cis194),
   **TODO**.
 - [DATA 61: Functional Programming Course](https://github.com/data61/fp-course),
   **TODO**.
+- [What I Wish I Know When Learning Haskell](http://dev.stephendiehl.com/hask),
+  **TODO**.
 
 
 ### Random explorations

rwh03.hs

@@ -1,3 +1,8 @@
+{-# LANGUAGE
+    StandaloneDeriving,
+    GADTs
+#-}
+
 -- RWH Chapter 3. Defining Types, Streamlining Functions --
 
 

tying_the_knot.hs

@@ -1,3 +1,8 @@
+{-# LANGUAGE
+    StandaloneDeriving,
+    GADTs
+#-}
+
 -- From https://wiki.haskell.org/Tying_the_Knot
 data DList a where
     DLNode :: Eq a => DList a -> a -> DList a -> DList a