From 28291764fa543497198ed737b109c80c79c51b2c Mon Sep 17 00:00:00 2001
From: Edoardo La Greca
Date: Sat, 21 Jun 2025 15:55:29 +0200
Subject: add lecture 2 notes

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+# Lecture 2: Algebraic data types
+
+## Enumeration types
+
+	data Thing = Shoe
+        	   | Ship
+		   | SealingWax
+		   | Cabbage
+		   | King
+	derivating Show
+
+The example above is a **type declaration**. The new type is called `Thing` and it has 5 **data constructors** (`Shoe`, `Ship`, etc.). A type's constructors are the only values for that type.
+
+The line that says `deriving Show` is a special instruction that tells the GHC to automatically generate default code for converting a `Thing` to `String`.
+
+	Shoe :: Thing
+	Shoe = Shoe
+
+	listO'Things :: [Thing]
+	listO'Things = [Show, SealingWax, King, Cabbage, King]
+
+Functions on enumeration types can be written using pattern-matching.
+
+	isSmall :: Thing -> Bool
+	isSmall Show		= True
+	isSmall Ship		= False
+	isSmall SealingWax	= True
+	isSmall Cabbage		= True
+	isSmall King		= False
+
+Since function clauses are checked from top to bottom, `isSmall` can be rewritten like this:
+
+	isSmall2 :: Thing -> Bool
+	isSmall2 Ship	= False
+	isSmall2 King	= False
+	isSmall2 _	= True
+
+## Beyond enumeration
-- 
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