Add the haskell book

Haskell-book/11/Hutton.hs

@@ -0,0 +1,13 @@
+module Hutton where
+
+data Expr
+    = Lit Integer
+    | Add Expr Expr
+
+eval :: Expr -> Integer
+eval (Lit x)   = x
+eval (Add x y) = (eval x) + (eval y)
+
+printExpr :: Expr -> String
+printExpr (Lit x) = show x
+printExpr (Add x y) = (printExpr x) ++ " + " ++ (printExpr y)