ch1

ch01.lhs

@@ -0,0 +1,38 @@
+Chapter 1: Functional Programming
+===
+
+Exercise 1.2
+---
+
+Trawling through Data.List we discovered the function
+
+> uncons :: [a] -> Maybe (a, [a])
+
+of whose existence we were quite unconscious. Guess the definition of uncons.
+
+> uncons [] = Nothing
+> uncons (hd : tl) = Just (hd, tl)
+
+Exercise 1.3
+---
+
+The library Data.List does not provide functions
+
+> wrap :: a -> [a]
+> unwrap :: [a] -> a
+> single :: [a] -> Bool
+
+for wrapping a value into a singleton list, unwrapping a singleton list into
+its sole occupant, and testing a list for being a singleton. This is a pity,
+for the three functions can be very useful on occasions and will appear a
+number of times in the rest of this book. Give appropriate definitions.
+
+> -- return should work here too
+> wrap x = [x]
+>
+> -- TBH unwrap should be returning Maybe since otherwise this throws an
+> -- exception on unmatched cases
+> unwrap [x] = x
+>
+> single [x] = True
+> single _ = False