Fix polymorphism since the ADT section of VL1 is gone

VL2/content/VL2_polymorphism1.tex

@@ -2,19 +2,4 @@ So when we said that haskell is good for abstraction, what did we actually mean?
 \vspace{\baselineskip}
 \\
 \pause
-Even better! Haskell supports polymorphism for both data types and functions.\\
+Even better! Haskell supports polymorphism for both data types and functions.
-Let's start with a polymorphic data type:
-\begin{haskellcode}
-data List t = Empty | Cons t (List t)
-\end{haskellcode}
-So we just implemented our own singly-linked List. For any type! Let's use it:
-\pause
-\begin{haskellcode}
-intList :: List Int
-intList = Cons 3 (Cons 5 (Cons 2 Empty))
-
-charList :: List Char
-charList = Cons 'x' (Cons 'y' (Cons 'z' Empty))
-
--- whatever you can imagine goes here
-\end{haskellcode}

VL2/content/VL2_polymorphism2.tex

@@ -1,20 +1,19 @@
-And now we are going to write functions to use it:
+Let's start with a polymorphic function that you already know:
-\begin{haskellcode}
-isListEmpty :: List t -> Bool
-isListEmpty Emtpy = True
-isListEmpty x     = False
-\end{haskellcode}
-\pause
-We can even have more generic stuff like:
-\begin{haskellcode}
-f :: a -> b
-\end{haskellcode}
-Whatever the function does... it has something of one type and returns something of another type (it could be the same type, but doesn't need to). That's all we know.
-\vspace{\baselineskip}
-\\
-\pause
-Similarly, remember the function \hinline{head} which gives us the first element of a list? The type signature actually looks like this:
 \begin{haskellcode}
 head :: [a] -> a
 \end{haskellcode}
-Makes sense?
+\pause
+So \hinline{head} takes a list of any type and returns an element which must have the \emph{exact same} type of that list.
+\vspace{\baselineskip}
+\\
+\pause
+We can also have:
+\begin{haskellcode}
+f :: a -> b
+\end{haskellcode}
+\pause
+So, whatever the function does... it has something of one type and returns something of another type. \hinline{b} \emph{could} be the same type as \hinline{a} here, but it doesn't need to! That's all we know about this function.
+\vspace{\baselineskip}
+\\
+\pause
+You don't have to use \hinline{a} or \hinline{b} here. These letters are just commonly used for generic types.